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A HISTORY OF THE MCMATH-PIERCE SOLAR TELESCOPE
CLAUDE PLYMATE
[email protected]
01 June 2001
ABSTRACT
A history of the McMath-Pierce Solar Telescope is presented. The National Science
Foundation (NSF) began what would become the Kitt Peak National Observatory in the 1950’s.
Dr. Robert R. McMath, a successful businessman turned solar astronomer, chaired an advisory
committee to the NSF on the National Observatory. McMath’s interest in solar astronomy was a
large factor in the National Observatory including the world’s largest solar telescope among the
first telescopes constructed on Kitt Peak. The telescope was named in honor of McMath at its
dedication in 1962. It was renamed the McMath-Pierce on the 30th anniversary of the dedication
to honor Dr. Keith Pierce, who headed the development of the telescope.
The telescope is of a unique design that is proving valuable even four decades after its
construction. It remains the largest aperture solar telescope in existence. The 1.6-meter diameter
aperture all-reflecting design of the McMath-Pierce telescope makes it an excellent optical
system for the new science of infrared solar astronomy. The McMath-Pierce telescope will likely
remain an active research center at the forefront of infrared solar astronomy for many years, until
another instrument can surpass its capabilities.
INDEX
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
INTRODUCTION
AURA
2.1
Site Selection
Dr. ROBERT R. MCMATH
3.1
The McMath-Hulbert Observatory
3.2
R. R. McMath’s Later Years
DR. A. KEITH PIERCE
OPTICAL CONFIGURATION
5.1
Coude Focus Cassegrain
5.2
Coelostat
5.3
Heliostat
Unobstructed Optical Design
5.4
BUILDING DESIGN
6.1
6.1
Final Design
6.2
6.2
Heliostat Tower
TELESCOPE COOLING
7.1
Optical Tunnel Cooling System
Aboveground Structure Cooling
7.2
HELIOSTAT WINDSCREEN
TELESCOPE OPTICS
9.1
Metal Mirrors
9.2
New Quartz Mirrors
East And West Auxiliary Heliostats
9.3
9.4
The Number 2 Mirror Incident
9.5
Upgrading To Cervit
9.6
Return Of The Quartz Number 2 Mirror
9.7
Integrated Light Feed
MIRROR MOUNTS
10.1 Heliostat Mount
10.2 Number 2 Mirror Mount
10.3
10.3
Number 3 Mirror Mount
COATING LAB
INSTRUMENTATION
12.1 Main Spectrograph
12.2 Spectroheliograph
12.3 Solar-Stellar Spectrograph
12.4 Fourier Transform Spectrometer
INFRARED CAPABILITY
EXAMPLES OF CURRENT PROGRAMS
14.1 Molecular Clouds Over Sunspot Umbrae
14.2 Sodium Emission From Mercury
14.3 Zeeman Splitting In OH At 12 Microns
THE FUTURE
15.1 A New Infrared Array Detector
15.2 Low Order Adaptive Optics
16.
CONCLUSION
REFERENCES
APPENDIX I
APPENDIX II
1. INTRODUCTION
The Sun - nothing can be of a more basic importance. It is the core of our solar system, the
life giver. Worshiped as divine throughout history. So basic to our existence that it merits a
single-syllable name in many cultures - Ra, Sol, and by us, Sun.
Some may feel that the Sun has lost some of its status in the eyes of modern astronomers.
Some talk of it as nothing more than an average star, middle-aged, lost, drifting undistinguished
amongst billions of others in our galaxy. They forget how fortunate we truly are to ride roundand-round about such a stable, well-behaved star, that’s nurtured our earth for some 4 ½ billion
years, and should continue to watch over us for at least that long again.
How does the Sun do this? How can it remain stably bathing the Earth in warmth over
periods measured in billions of years? What can it be like internally where common descriptive
terms and ideas are hopelessly inadequate? These questions spawned solar astronomy as a
branch of astronomy dedicated to the Sun. The modern era of solar astronomy began around the
start of the 20th century with the construction of the solar telescopes at Mount Wilson. These, and
other solar telescopes that followed, did an admirable job of furthering our understanding of the
inner workings of our star. By mid-century, new questions had arisen, questions that would
require probing deeper with greater detail. To probe for answers to these questions, an instrument
of an entirely new scale was needed.
For this, we built the McMath-Pierce
(McM-P) Solar Telescope (Figure 1).
The McM-P was dedicated in 1962.
It was built as part of the fledgling US
National Observatory at Kitt Peak in
Arizona. The National Observatory was
created with the goal of making worldclass astronomical facilities available to
the entire astronomical community. The
McM-P has remained, by far, the largest
aperture solar telescope in existence for
the past four decades. Only now are
plans being formed for a solar telescope to surpass its 1.6-meter aperture.
An unusual all-reflecting optical system was chosen for the McM-P. A 2-meter heliostat
feeds a beam to an open-air, unobstructed mirror system that produces an image of the Sun that’s
roughly 0.8-meters in diameter. This optical design would come to be considered obsolete by
many until the 1990’s when infrared array detectors began to become available for astronomical
research. The McM-P’s all-reflecting optical path became a major asset due to its unique ability
to observe the Sun in the infrared. This opened solar physics to the newly available infrared
bandpasses. The facility continues to be upgraded with new instrumentation to meet the everexpanding need of the solar astronomical community. The McM-P will remain a vital center for
optical and infrared solar astronomy until a new generation solar telescope that can surpass its
capabilities replaces it.
Plans are now underway at the National Solar Observatory to develop the Advanced
Technology Solar Telescope (ATST). The ATST is to be a 4-meter class, all-reflecting solar
telescope that will utilize all of the latest advances in telescope design to achieve the highest
possible resolution images of the Sun in both the optical and infrared. Until ATST becomes a
reality, the McM-P is expected to remain as the premier ground-based facility for infrared solar
research and as a primary test bed for ATST technology development.
2. AURA
By the middle of the 20th century, most astronomers began to find themselves in an ever
increasingly difficult situation. The world had struggled through two world wars that had created
a highly technical society. This new society seemed to have complete faith in the ability of
science to solve the world’s problems. Optimism for technology spawned a thriving economy
that produced an enormous increase in the numbers of scientists coming out of the universities in
all sciences including astronomy.
At the same time, astronomy was becoming truly big science. The size of telescopes had
mushroomed through the later 19th and first half of the 20th century. Philanthropists, searching to
immortalize themselves, financed ever-larger telescopes to carry their names. This left the
handful of largest telescope in the exclusive possession of a few private institutions spread
mostly across the US. Yerkes had its 40” refractor, Lick observatory had a 0.9-meter reflector,
Mount Wilson housed the 100-inch Hooker telescope and Palomar had the great 5-meter
telescope. To compete in cutting-edge science, the astronomer needed access to one of these
leading telescopes. If not on the staff of one of the few institutions that had one of the large
telescopes of the time, the astronomer would often be without access and at a severe
disadvantage.
President Truman signed a bill creating the US National Science Foundation (NSF) on 10
May 1950. The NSF was created to promote basic scientific research in the US. The NSF would
act as a conduit for public funding of research grants. In August 1952, the NSF formed what
would come to be named the Advisory Panel for Astronomy. The astronomical community was
quick to recognize this as a potential source of funding for astronomical research and facilities.
Two conferences on Radio Astronomy were held in Washington, D. C., the first in January
1954 and the second in July of 1956. Out of these conferences grew a proposal to the NSF to
fund a National Radio Observatory. The proposal called for universities with an interest in radio
astronomy to form a non-profit corporation to construct and manage the facility for the NSF. The
proposed name, the Association of Universities for Radio Astronomy (AURA), was changed to
the National Radio Observatory and led to the construction of the facility at Green Bank, West
Virginia.
Optical astronomers followed a similar path. A proposal was submitted to the NSF to create
an astronomical observatory to be operated jointly by the Universities of Arizona, Indiana, and
Ohio State. This proposal was submitted on the 15th of May 1952. The proposal was reviewed by
the Advisory Panel for Astronomy but formally declined on the 11th of September. The concept
of a jointly operated astronomical facility was thought to be a fundamentally good idea. The NSF
Advisory Panel for Astronomy responded by establishing an ad hoc Panel on Astronomical
Facilities. Dr. Robert R. McMath was selected as chairman of the panel.
The panel arranged for the NSF to sponsor a conference “on photo-electric methods be held
to see whether a new facility for such investigations should be established” (Edmondson 1997).
The conference was held at Lowell Observatory on the 31 August 1953. All of the larger US
observatories as well as observatories of Canada, Australia, and England were represented.
McMath’s deteriorating health prevented him from attending, so he asked that Dr. Leo Goldberg
be sent in his place to represent the University of Michigan.
Dr. Goldberg believed that the concept of a photoelectric observatory was too limited. At the
conference, he made an impassioned plea. “I think what this country needs is a truly National
Observatory to which every astronomer with ability and a first class problem can come... I don’t
think we should confine our thinking to a small photoelectric telescope. Personally I would like
to see another instrument with about a 100-inch (2.54-m) aperture established at the National
Observatory... A photoelectric telescope with perhaps a 36-inch (0.9-m) aperture would provide
a very useful auxiliary instrument. I am sure that the photoelectric observers would not be
adverse occasionally to a telescope with a much larger aperture... [Located] in the southwest
partly because the weather cycle is out of phase with California” (Edmondson 1997).
Goldberg’s idea resonated with American astronomers. The NSF Advisory Panel for
Astronomy endorsed consideration of an “inter-university Astronomical Observatory” during its
meeting in 1954 and created another ad hoc committee called the NSF Advisory Panel for a
National Astronomical Observatory (NAO). Robert McMath would again agree to serve as
chairman for the new panel. The first meeting of the Advisory Panel took place on the 4th of
November 1954. The panel was tasked with studying sites, instrumentation, costs, and other
factors relating to the establishment of a NAO. The panel responded to the NSF by
recommending a five-year plan. The plan included a two-year site selection search to be
followed by the construction of three astronomical facilities, a “36-inch (0.9-meter) and an 80inch (2-meter) photoelectric stellar telescopes and the world’s largest solar instrument, which is
to have a 60-inch (1.5-meter) aperture and 300-foot (91-meter) focal length” (McMath & Pierce
1960).
The Advisory Panel originally proposed that a board appointed by the NSF should operate
the observatory. The NSF Program Director for Physics and Astronomy, Dr. Raymond J. Seeger,
responded that the NSF “can legally neither operate nor appear to operate” such a facility. The
NSF could “appropriate funds only to a properly organized institution” (Edmondson 1997). A
non-profit corporation was formed to develop and operate the NAO. The board of directors
would be made up of representatives from the seven member Universities that formed the
organization. The member universities that made up the association consisted of California,
Chicago, Harvard, Indiana, Michigan, Ohio State, and Wisconsin.
For the name of the corporation made up by the seven universities that would construct the
NAO, Leo Goldberg resurrected the acronym AURA used earlier for the National Radio
Observatory. The acronym was reworked to now stand for the Association of Universities for
Research in Astronomy. AURA was incorporated in October of 1957.
The site selection survey for the site of the NAO was started in 1955, 14 months before the
incorporation of AURA. Four months after its incorporation, AURA selected Kitt Peak as the
site for what would be known as the Kitt Peak National Observatory.
2.1 SITE SELECTION
The site survey began in 1955. The survey was limited to the US southwestern states of
California, Arizona, New Mexico, Nevada, Utah, and Texas. The first phase consisted of
examining photographs taken from Viking Rockets launched out of the White Sands Missile
Range in New Mexico to find likely candidates. Aircraft were then used to view the more
promising looking sites. This narrowed the list of possible sites to 150. Each of these were
explored from the ground with the use of jeeps, packhorse, and on foot. At the end of this phase
of the site survey, only five candidates remained.
Test towers were erected at the five remaining candidate sites, which held equipment for
measuring the sites’ sky transparency, humidity, winds, and temperature fluctuations. Two
Arizona sites emerged by the end of the tests as superior - Hualapai Mountain southeast of
Kingman, and Kitt Peak about 70 kilometers southwest of Tucson.
Twin trailer-mounted 0.4-meter photoelectric telescopes were dispatched to the two
remaining locations to further evaluate their suitability. The final report on the site selection for
the NAO was released in March 1958. The report showed that Kitt Peak was clearly preferred
over Hualapai. Both sites were similar in rain accumulation, transparency, and light pollution.
Hualapai appeared to have a slight edge in number of clear nights, but Kitt Peak was found of
have superior seeing, lower winds, better temperature stability, and fewer vapor trails from
aircraft over flights.
AURA, Inc. presented the
proposal for the “World’s Largest
Solar Astronomical Telescope” to the
NSF in August 1958 (AURA 1958),
just five months after Kitt Peak was
named as the location for the NAO.
3. Dr. ROBERT R. MCMATH
Dr. Robert R. McMath spent a
great deal of his career devoted to
solar astronomy. Tragically, he died in
1962, just one month before the
dedication of the world’s largest solar
telescope, which he worked so hard to
create. At the dedication the telescope
was named the Robert R. McMath
Solar Telescope to commemorate the
fact that its existence was largely due
to his efforts in both the fields of astronomy and politics.
McMath (Figure 2) became one of the world’s leading authorities on solar physics in his
later years. He gained great respect among his contemporaries not only as an innovative scientist
but also as one who had the ability to get things accomplished. He was known for being a
perfectionist, driven to succeed. He had little patience for delays, whether technical in nature or
due to others. This gained him the reputation of being difficult to work with. He was also known
to be fiercely loyal to his friends and respected colleagues.
Dr. McMath’s career path was anything but typical for an astronomer. McMath was born on
the 11th of May 1891 in Detroit where he spent most of his life. His father, Francis C. McMath,
had been quite successful in the bridge building industry. Planning to follow in his father’s path,
Robert studied civil engineering and received his bachelor’s degree from the University of
Michigan in 1913. During World War I, he joined the Army Corp of Engineers. In 1919, after
the war, he left the Army after gaining the rank of major. McMath spent some time working with
his father designing bridges. A bridge accident convinced McMath that the bridge building
business was not for him. McMath was working on the bridge that spanned the St. Lawrence
River when it collapsed with him still on it! He escaped the bridge collapse with his life, never to
return to that business (Pierce 2001). After leaving bridge building in 1922, he joined Motors
Metal Manufacturing Company as Assistant Manager. Francis McMath and his associate,
Willard Pope, owned controlling shares in the company. Robert’s father instructed him to
prepare the company for liquidation. Ignoring his father’s instructions, McMath turned it into a
highly profitable company. McMath would remain with the company until 1954, rising to the
position of Chairman of the Board (Edmondson 1997).
McMath became a true insider in both the worlds of government and industry. He became
friendly with the likes of Charles Lindberg, Henry Ford, and Joseph Dodge (Director of the
Bureau of the Budget for President Eisenhower). McMath knew how things worked in
government and industry and knew how to get things accomplished. If he needed to deal with a
corporation, he wouldn’t waste time speaking to anyone below its Chief Executive Officer
(Pierce 2001).
3.1 THE MCMATH-HULBERT OBSERVATORY
McMath, with his father and close friend, Judge Henry S. Hulbert, formed an idea that
motion-picture technology could be used as an effective tool in education and popularizing
astronomy. The technical challenges associated with motion photography of celestial
phenomenon proved to be daunting. McMath’s engineering talents were drawn to these
challenges and working on them first awoke his serious interest in astronomy. In 1929, the trio
built a private observatory with a 10.5-inch (0.27-m) reflector telescope to experiment with
astro-cinematography. They had some success with filming the motions of Jupiter’s atmosphere
and its moons, a stellar occultation and the motions of shadows across craters on the Moon.
Their efforts gained the attention of the Department of Astronomy at the University of
Michigan. The astronomers at the U. of M. helped their observing programs with technical
assistance and shop time for their projects. This association eventually led to the McMath-
Hulbert Observatory being donated to the University of Michigan and R. R. McMath becoming a
member of the U. of M. Astronomy Department. He was also named the observatory’s director.
McMath’s name appeared on 78 scientific publications during his astronomical career. He had
earned the respect of the astronomical community even though his title of doctor was not earned
through the normal academic route. His Doctor of Science degree was an honorary award from
the Pennsylvania Military College in 1941. “Though he was never a professional astronomer in
the ordinary sense of the word, honorary degrees were conferred by the University of Michigan,
Wayne University, and Pennsylvania Military College” (Petrie 1962).
The McMath-Hulbert Observatory grew quickly under the direction of Dr. McMath. An
instrument called a spectroheliokinemetograph was built for the observatory’s 0.27-m reflector to
extend the motion-picture studies to solar prominences. This started the move of McMath’s
career towards a specialization in solar astronomy. A 50-foot (15.24-m) solar tower telescope
was erected in 1936 with technical assistance from the Mount Wilson Observatory staff. The
telescope’s spectrograph extended another 9.4-m underground. A 0.6-m Cassegrain telescope
was added in 1940. This was designed as a general-purpose telescope to which various
instruments could be attached. The 0.6-m Cassegrain was named the F. C. McMath Memorial
Telescope in honor of Robert’s recently deceased father.
One year later a second solar tower was constructed, this one 75-feet (21.3-m) tall. It
received a new Vacuum Spectrograph in 1955 (University Lowbrow Astronomers 2000).
3.2 R. R. MCMATH’S LATER YEARS
McMath’s scientific reputation continued to grow. In 1945, he was made professor of solar
physics for the University of Michigan and in 1951 he became professor of astronomy. He
served as president of the prestigious American Astronomical Society between 1952 and 1954.
McMath’s scientific reputation along with his connectedness in industry and government made
him the natural choice in 1954 to head the NSF’s advisory committee on the National
Astronomical Observatory. It is of little surprise that the panel’s recommendation to the NSF
included construction of the world’s largest solar telescope as part of the national observatory.
McMath spent many years at the McMath-Hulbert with the dream of constructing the world’s
largest solar telescope to probe the Sun’s secrets. The solar telescope project at Kitt Peak
represented the realization of his long sought goal. He spent the last several years of his life
deeply involved with the planning and development of this new solar telescope that would
ultimately carry his name. Unfortunately, Robert McMath did not live to see its dedication. He
died on the 2nd of January 1962, ten months before the formal dedication of (as it was then
known) the McMath Solar Telescope.
McMath’s failing health prevented him from making the trip to the solar telescope project on
Kitt Peak more than a few times. He left the details of the project to his younger and trusted
colleague, Dr. A. Keith Pierce (Figure 3).
4. DR. A. KEITH PIERCE
A. Keith Pierce was a graduate student in astronomy at the University of California at
Berkeley in the early 1940’s. His education was disrupted by America’s entrance into World
War II. During the war, he worked for the Lawrence Radiation Laboratory. Pierce returned to
Berkeley after the war to finish his doctorate in 1948. Dr. A. Keith Pierce found a position with
the University of Michigan as Assistant Professor of Astronomy working as assistant to
astronomer Dr. Leo Goldberg. Dr. McMath was sufficiently impressed with Pierce to have him
transferred to the McMath-Hulbert Observatory as a staff astronomer. Dr. Pierce has always held
McMath in the highest esteem; Pierce always refered to him as Dr. McMath and never addressed
him by his first name (Pierce 2001).
It was natural for McMath to hire Dr. Pierce to help in the development of the very large
solar telescope. One of Pierce’s earliest assignments for the fledgling observatory, in October
1956, was to visit several “solar observatories in England, Germany, and France to study their
equipment, research programs, etc., but with particular emphasis on the solar seeing problem”
(Pierce & McMath 1958). He spent two weeks at Pic du Midi in the Pyrenees where he learned
about their program to improve the local seeing conditions.
Pierce became the primary scientist in charge of the development of the telescope from its
design to its construction. McMath recommended Pierce be appointed as the Associate Director
of the National Astronomical
Observatory at an AURA Executive
Committee meeting on the 13th of
September 1958. Dr. Pierce’s title was
later changed to Associate Director in
charge of the Solar Division during a
reorganization of the observatory in
March of 1960. (Dr. A. B. Meinel was
named as the Associate Director in
charge of the Stellar and Space
Divisions.)
Dr. Pierce stayed with the Solar
Telescope through the rest of his
professional career. After the dedication
of the McMath Solar Telescope, he
remained at the observatory as an active solar researcher. Though now officially retired, Pierce is
still an active observer at the telescope and holds the title of Astronomer Emeritus with the
National Solar Observatory.
A ceremony was held on the 30th anniversary of the telescope’s original November 2nd1962,
dedication to commemorate Dr. Pierce’s vital contributions to the development of the solar
facility. It was rededicated on that day as the McMath-Pierce Solar Telescope Facility.
5. OPTICAL CONFIGURATION
The design of any device should start with the final product. The end product of a solar
telescope is the solar image. The first step of designing the new solar telescope was to determine
the optimal image scale. “Work on the spectra of solar granules, on the physical structure of
sunspots and their associated magnetic fields, requires considerable image size. Past experience
has shown that the optimum image of the sun should be approximately a yard (0.91-m) across”
(McMath & Pierce 1960). Several optical designs were considered for the solar telescope design.
5.1 COUDE FOCUS CASSEGRAIN
The image scale required could easily be obtained at the Coude focus of a standard
Cassegrain type telescope. The Cassegrain had the advantage of being a well-known design, very
popular with stellar astronomers, that is compact enough to be mounted on a conventional
equatorial mounting.
The Cassegrain was quickly rejected. It was felt that the intense heat load from a short focal
length primary could distort the secondary mirror’s figure. The Cassegrain secondary mirror
obstructs the optical path. The secondary blocks only a small portion of the incoming light but
causes diffraction. The diffraction around the secondary degrades the telescope image quality.
5.2 COELOSTAT
A fairly common optical design for a solar telescope is the coelostat. A coelostat consist of a
flat mirror that rotates about the equatorial axis at half the diurnal rate. The solar beam is
reflected up to a secondary flat mirror that then diverts the beam, either horizontally or vertically,
onto the imaging optics of the telescope.
The coelostat has the advantages of an unobstructed light path since the secondary mirror is
placed above the coelostat to intercept the solar beam. Another advantage is that the image
formed by the coelostat doesn’t rotate through the day.
There are several disadvantages of the coelostat design. The secondary mirror, or its support,
will often cast a shadow across the coelostat at some time during the day losing a fair amount of
observing time. The angle of the solar beam coming off of the coelostat changes through the year
with the seasonal change of the Sun’s declination. The secondary mirror must be able to be
raised or lowered to intercept the solar beam from the coelostat as the Sun angle changes through
the seasons. This represents a complication and expense in the telescope design. The coelostat
requires two mirrors to tack the Sun. Two mirrors obviously add to the costs of the telescope as
well as decreasing throughput and contrast.
5.3 HELIOSTAT
The heliostat is the configuration that was eventually chosen for the solar telescope. Unlike
the coelostat, the heliostat uses only a single mirror to track the Sun. The mirror is mounted
equatorially and rotates with the celestial sphere, once per day. The heliostat can also move in
declination. This mirror configuration reflects the solar beam down the telescope’s equatorial
axis to the telescope’s imaging optics that are stationary and so require quite only simple
mountings.
The downside of the heliostat is that its image rotates once per day. To compensate for the
rotation, the instruments must be built in such a way that they can rotate at the same rate to
compensate. This complication was felt to be a small price to pay for the heliostat’s other virtues
- a single moving mirror system that doesn’t get shadowed during the observing day.
5.4 UNOBSTRUCTED OPTICAL DESIGN
The focal length required to produce the specified 0.9-m diameter prime focus solar image is
about 90-m. This long focal length was used as an advantage by the designers to produce a fully
unobstructed imaging system. It was determined that a1.52-m f/60 imaging mirror could be
tipped as much as 2 off-axis to feed a folding flat mirror while introducing only 1/3 of an
arcsecond of coma (Pierce & McMath 1958). The folding flat, #3 mirror, would then be
completely out of the optical path between the heliostat and #2 imaging mirror avoiding the
diffraction effects of a Cassegrain or Newtonian optical system.
Another benefit of a slow (large f/ratio) optical design is that there is no advantage to going
to the added expense of parabolizing the mirror’s figure. The imaging mirror can be figured to a
simple sphere.
The heliostat mirror must be larger than its #2, imaging mirror, if the solar beam is to fill the
entire aperture of the #2 mirror. The heliostat tips upward in declination to reflect the solar beam
down into the telescope. Tipping the heliostat back, in declination, foreshortens its projected size
is in that axis. The heliostat diameter for the solar telescope was chosen to be 2-m. This would
allow the heliostat to point as far north as -8.9 before under-filling the #2 mirror
{DeclinationMax = 2cos-1(1.52/2.00) - 90}.
The time of the year that the Sun’s declination is higher than -8.9, the full aperture of the
telescope is not utilized. This was considered to be an acceptable compromise. The unused
aperture is not terribly large in comparison to the entire surface area of the #2 mirror and a larger
heliostat mirror would have been prohibitively expensive at the time. The Sun’s declination is
above -8.9 between 26 February and 16 October each year (Nautical Almanac Office 2000) and
reaches a maximum declination of +23.44 on the summer solstice on June 21st. At least a part of
this observing time is expected to be lost to the rainy Arizona monsoon season that typically
begins in early July.
6. BUILDING DESIGN
In the fall of 1957, Mr. William F. Zabrinskie, a civil engineer from Detroit, was
commissioned to prepare three preliminary designs for the solar telescope building. All three
designs are conceptually quite similar. They all appear as very large right-triangular structures
with the heliostat mounted at the top of the vertical section. The beam from the heliostat shines
down the triangle’s hypotenuse, which is aligned along the telescope’s equatorial axis. The
imaging, #2 mirror, is located at the bottom of the hypotenuse, which reflects back up toward the
folding flat mirror, #3, located just below the heliostat. The #3 was positioned out of the way of
the beam coming down from the heliostat making for an unobstructed optical path. The #3 mirror
diverts the beam downward to an air-conditioned observing room located just below ground
level (Figure 4). Spectrographs and other instrumentation were to be mounted horizontally in the
observing area (Pierce 1957) along the bottom of the triangle.
The differences between the three designs were mostly in the style of framing that was to be
used. One design called for a structure made from H-beams and columns, another used large
steel tubes, while the third was designed to be constructed from reinforced concrete.
Mr. Zabrinskie thought that the triangular shape of the telescope structure made it appear like
the index of a gigantic sundial. Because of this, he referred to the telescope building as the
“Index Building”. Dr. McMath thought that while the structure was not “architecturally
beautiful”, it was “functionally beautiful” (Pierce 1957).
It is interesting to note how much of Zabrinskie’s preliminary design concepts survived to be
incorporated into the final telescope structure. The final telescope structure retained the overall
triangular shape. All of the optical mounts were placed on railroad-like tracks to enable them to
be moved for focusing and easy servicing. The exterior of the structure was to act as a wind
shield and was water cooled. All of these ideas from Zabrinskie’s designs can be seen in the final
telescope structure (Figure 5).
Following Zabrinskie’s preliminary design work, AURA contracted with the Chicago
engineering firm of Skidmore, Owings, and Merrill to study all possible structural designs for the
solar telescope. Their studies were to include, but not be limited, to the designs done a year
earlier by Zabrinskie.
Skidmore, Owings, and Merrill prepared ten designs for the telescope building. Of these ten,
they recommended two for final consideration by AURA. One consisted of a tapered
cantilevered tower that extended from the ground at the local latitude angle of 32. This design
has been described as appearing like an “extreme case of the leaning tower of Pisa” (Kloeppel
1983). The design may have been aesthetically beautiful but more expensive and less stable than
the simpler tower structure that was adopted by AURA in June 1959.
6.1 FINAL DESIGN
The chosen design featured a vertical tower to support the heliostat. The heliostat looks down
a long shaft that is tipped along the equatorial axis, much as in Zabrinskie’s preliminary
concepts. Unlike in the initial conceptual designs, the optical shaft extends below ground level
for roughly another 90-m to the #2, imaging mirror (Figure 5). The #2 mirror is tipped about 1
¼ off axis toward a third, folding flat, mirror that is located near the ground. The #3 mirror
diverts the beam vertically downward into the observing room, which is just below ground level.
In Zabrinskie’s designs, the solar beam is also fed into the observing room from above, but is
reflected off of a 4th mirror delivering a horizontal beam to the instruments. The final design
delivers the image vertically, eliminating the need for a #4 mirror. This meant that the
spectrograph tanks and other equipment had to be built vertically into the ground and had to be
able to rotate around their vertical axes to compensate for the daily image rotation.
6.2 HELIOSTAT TOWER
Seeing is caused by light passing through air masses of differing temperature. The Sun heats
the ground during the day creating thermals that deteriorate image quality. (This is obvious to
anyone who’s gazed down a hot asphalt covered road.) It had been know for some time that
temperature fluctuations decrease with height above ground. It was not known at what rate they
decrease or how high one would need to go before the fluctuations are no longer of concern.
“After light travels through the atmosphere the quality of the seeing should not be destroyed in
the last few hundred feet near the focus” (McMath & Pierce 1960).
Staff astronomer William Livingston designed and
constructed equipment to test temperature fluctuations at
Kitt Peak as a function of height. Measurements taken
with Livingston’s equipment indicated that temperature
fluctuations decrease exponentially with height.
Temperature fluctuations at ground level were
around 3 C. At a height of 55 feet (16.8-m) the
temperature fluctuations were less than 0.4 and about
0.2 at 55-m. The temperature data indicated that the
heliostat should be mounted as high as possible. The
engineers wanted the heliostat tower to remain as short
as possible for minimum costs and maximum rigidity.
Based on Livingston’s data and the costs estimates of
building a tower, the height for the heliostat tower was
chosen to be100 feet (30.5-m). The tower is a vertical
concrete cylinder 7.9-meters in diameter with a wall
thickness of 1.2-meters (Figure 6). The tower is
protected from the wind by external cooling panels that
are mounted to the structure’s framework. The framing of the outer structure is acoustically
isolated from the tower. It was calculated that the tower deflection would be less than 0.4 mm in
a 40 Kilometers/hour wind, which would move telescope’s primary image by less than 1/3 of a
second of arc.
7. TELESCOPE COOLING
Great effort was taken in the design of the McM-P to minimize the effects on the telescope's
image quality by thermals crossing the optical path inside the telescope. Air heated by the solar
beam will rise and break into turbulent flows in the telescope's optical tunnel. Since air masses at
different temperatures have differing refractive indexes, a warm turbulent flow will distort the
optical wavefront and blur the resulting image. The distortion due to the atmosphere inside the
telescope is what is referred to as “telescope seeing”.
The telescope was designed to stabilize the seeing by carefully controlling the air
temperature in the optical path. The system strives to get the bottom of the optical tunnel to
around 10 - 12 F (5.6 - 6.7 C) below the outside ambient temperature. The temperature would
then be allowed to rise steadily, to within a few degrees of ambient, towards the top of the
telescope. It was believed that this steady temperature gradient would allow the air to rise very
slowly through the optical path in a more-or-less laminar flow. “Irregular air temperature
fluctuations in the optical tunnel, it should be noted, are ‘doubly troublesome’ because they
influence not one but two light beams: both the beam coming down from the heliostat to Mirror
No. 2 and the beam coming back up the tunnel from Mirror No. 2 to Mirror No. 3.” (Donovan &
Bliss 1962). By looking through a smooth, laminar flow and by having only very small
variations in the air mass temperature, the telescope's image quality would be maintained. The
strategy for dealing with the telescope's excess heat load was separated into two independent
cooling systems, one for the underground optical tunnel and the other for the portion of the
structure that is above ground.
7.1 OPTICAL TUNNEL COOLING SYSTEM
About two-fifths of the telescope’s roughly 150-m optical path was excavated below ground.
(The ratio of the above to below ground portions of the telescope varies depending on which port
and instrument the focus is delivered to.) The optical tunnel was cut through the south side of the
mountain downward at an angle of 32 degrees (the local latitude) so that the tunnel points up
toward the north celestial pole. The bottom, south end, opens out onto the southern slope of the
mountain. Fans were installed at the bottom opening to allow outside air to be pulled into the
telescope through the top and exhausted out of the south end opening.
The rock surrounding the optical tunnel stays at a fairly constant temperature of around 13 C
throughout the year. This temperature was found to be low enough so that no additional cooling
would be needed to stabilize the seeing in the warm Arizona summers. The ambient temperatures
in winter can drop well below freezing for extended periods and so a chiller system was designed
and installed.
The walls of the belowground optical tunnel are lined with aluminum panels, about 20,000
square feet (1858 m2), that are attached to a grid of over 7500 meters of water pipe, 2.5 cm in
diameter. An insulating blank 3” (7.6 cm) thick was installed between the cooling pipes and the
tunnel’s rock wall. The water pipes circulate a chilled 42% glycol and 58% water solution that is
chilled by a 3-cylinder compressor rated to dissipate 160,000 Btu (1.688 x 10^8 joules). The
glycol/water solution enters the grid of pipes at the bottom of the optical tunnel and exits through
the top. The temperature of the liquid will warm steadily as it rises up the telescope optical path
absorbing heat from the surrounding rock. This steady increase in temperature is just what is
desired to create the stable air mass in the optical path. The chiller system is left off during the
summer months. During the winter, there are inside and outside temperature gauges that regulate
the compressor to keep the tunnel walls at the proper temperature.
7.2 ABOVEGROUND STRUCTURE COOLING
The aboveground structure is covered by roughly 30,000 square-feet (2787 m2) of panels.
Several materials and paints were evaluated to test their absorption of heat from sunlight. A
surface painted with white titanium dioxide pigment paint was found to warm by only 10 to 15
F (5.6 - 8.3 C) in full sunlight (McMath & Pierce 1960). Copper was chosen for the panel
material due to its very high heat conductivity. Still, it was calculated that during midday, the
Sun will heat the skin panels with about 80,000 Btu/hour (23.45 kilowatt-hours) in the winter
and closer to 140,000 Btu/hour (41 kilowatt-hours) during the summer months (Donovan & Bliss
1962).
The copper panels used to cover the telescope include built-in 1.27 cm diameter water lines
known as “Tube-in-Strip” (Figure 7). The exterior “Tube-in-Strip” panels were covered with the
white titanium dioxide pigment paint to minimize the absorption of solar heating. A mixture of
glycol and water is continuously circulated
through the Tube-in-Strip panels. The glycol
cooling mixture is pumped to a 16,000 gallon
(60,566 liter) holding tank. This large holding
tank acts as a heat-mass for the telescope. The
warm liquid returning from the telescope skin
mixes with the water and glycol in the tank
and slowly warms the tank liquid through the
day. The temperature of the glycol coolant that
flows from the tank lags behind the climbing
temperature by a few degrees through most of
the day. The cool glycol mixture enters the
telescope at ground level and warms as it makes it way upward through the skin and returns to
the tank a few degrees above ambient. This continues the steady climb in the temperature inside
the telescope all the way to the top where the air from the telescope mixes with outside air with a
very small temperature differential. The rate at which the cooling liquid flows through the Tubein-Strip panels is regulated by temperature gauges that read the temperature difference between
the liquid flowing into and returning from the telescope.
The circulation system continues to operate through the night. This allows the cooling liquid
that was heated during the day to re-radiate its excess heat back to the cool nighttime sky. By
morning, the temperature in the tank is back to ambient and the process is ready to begin once
again. If the outside temperature climbs unexpectedly quickly, the fans at the bottom of the
tunnel can be turned on to draw outside air into the telescope through the opening at the top.
8. HELIOSTAT WINDSCREEN
The telescope's 2-m heliostat was completely exposed to the winds at the top of the telescope
tower. The telescope's best seeing was found during periods of light winds of less than about 7
m/s. It is believed that these light winds kept a hot layer of air from forming over the heliostat
mirror surface. (A hot air layer would tend to distort the wavefront moving through it.)
Somewhat stronger winds, 7 to 15 m/s, disturb the seeing only slightly. High wind speeds of 15
to 25 m/s were found to shake the heliostat causing motion at the image plane. Image motion has
a similar effect to poor seeing on the image quality. The image motion due to high winds was
found to be relatively minor in the right ascension, 2.5 to 5 seconds of arc. The declination
motion was much larger, up to 40 seconds of arc.
A novel and inexpensive solution to this problem was found. A windscreen was erected
around the heliostat. The windscreen was modeled on a design developed for temporary
telescope installations such as for site surveys. The design is simple and only required a few days
of down time to install.
The design consists of vertical
wooden slats, around 0.2-m in width,
with gaps between that are the same
width as the slats (Figure 8). Wind
flowing through the vertical slats form
eddies on the leeward side. The kinetic
energy in the wind is dissipated in the
eddies. The slight rise in air temperature
due to the energy dissipation in the
eddies was calculated to be trivially
small, 0.05 C with a wind speed of 10
m/sec. A solid wind fence would
produce a stagnant air mass around the telescope that would heat and would mix with the cooler
air above the fence in a turbulent flow (Hammerschlag & Zwaan 1973).
The windscreen was installed at the McM-P in 1986. The widths of the slats and gaps were
made to be 1/40 the distance to the heliostat mirror. The windscreen wraps around the heliostat’s
south side running from east to west forming a U-shape. It is tallest to the south as the prevailing
wind comes from the south or southwest. It tapers in height to the east and west so that it never
obstructs the solar beam. The windscreen has been found to decrease the heliostat declination
windshake by at least a factor of ten (Pierce 1986).
9. TELESCOPE OPTICS
Two of the original optics used in the solar telescope had a long, unique history. The General
Electric Co. (GE) went to great deal of effort and expense trying to produce a quartz mirror blank
for the Palomar 5-m telescope project in 1931. Quartz has many desirable characteristics as a
telescope optical material. It is hard, relatively easily to figure, will accept a standard aluminum
or silver coating, and has a very low coefficient of expansion compared to standard glass. The
project was successful at producing three quartz disks, each about 1.6-meters in diameter, but
was unable to scale their techniques up to produce a 5-meter diameter disk. GE ultimately
abandoned the project and Corning was given the contract to produce a disk made from Pyrex
that became the mirror for the great Palomar telescope.
The GE quartz disks were put into storage at their Lynn facility. Around 1950, GE decided to
dispose of the disks. As far as they were concerned the old disks were useless. GE contacted
Palomar Observatory to see if they wanted the blanks. Palomar was not interested but passed the
information along to Dr. McMath at the McMath-Hulbert Observatory. McMath was able to get
the blanks for the cost of their transport from GE to the McMath-Hulbert Observatory. There,
they went back into storage until McMath offered them up to the Kitt Peak solar telescope
project. McMath realized that it would take several years to get a 2-m mirror cast and polished
for the telescope’s heliostat. In the meantime, one his quartz mirrors could be polished and used
temporarily for the telescope’s heliostat. Although smaller in diameter than the planned 2-m
heliostat, its use would allow the telescope to go into operation years earlier. The undersized
heliostat would be replaced with a 2-m mirror when it became available (Kloeppel 1983).
Another of the quartz disks, 1.73-m diameter, was cracked during the annealing process. GE
engineer, A. E. Ellis, was able to repair the cracked disk by “welding” it back together with a
blowtorch. Ellis heated the mirror and filled the crack with molten quartz. This could have never
been attempted with a normal glass disk. Glass would surely shatter with such non-uniform
heating. Pierce & McMath (1958) stated that “its back surface, particularly near the edges, is in
poor shape and rather large chunks can be removed by hand.” The disk was eventually cut down
to 1.2-m in diameter to remove the worst sections by a monument works in Pasadena, California.
The now 1.2-m disk was polished and put into service as the first #3 mirror in the McM-P. Both
of the quartz flat mirrors were polished in the optical shop of Mount Wilson and Palomar
Observatories (AURA 1961).
The third quartz disk was full of bubbles and had a brick pattern from its mold in it. It was
determined to be unsuitable for an astronomical mirror. Some other option would need to be
found for the telescope’s #2 mirror - the imaging mirror.
9.1 METAL MIRRORS
Early astronomical reflecting telescopes used metal mirrors, normally made of speculum.
These mirrors would tarnish quickly and require laborious repolishing. Since the discovery of the
process to silver coat optics in the mid-1800’s, nearly all astronomical telescope optics have been
made of glass, quartz or one of the ultra-low expansion ceramics such as Cervit.
A major concern in the design of the solar telescope was the heating of the mirror surfaces.
At the elevation of Kitt Peak, a mirror is exposed to roughly 1.6 calories/cm2/min of solar
radiation. The mirror’s thin reflective coating will reflect around 85% of this energy. The other
15% is absorbed into the coating. The energy is dissipated by convection and radiation away
from the surface and conduction into the substrate. This non-uniform heating of the mirror will
cause expansion of the substrate, distorting its figure and destroying the image quality (Pierce &
McMath 1958).
Dr. McMath had a theory that a highly conductive substrate would keep the temperature
gradient throughout the mirror quite small and preserved the optical figure. A program was
undertaken to develop metal mirrors and to evaluate their viability for use as solar telescope
optics.
Metal is not an amorphous material like glass. It tends to have internal stresses. There was
concern that these stresses would not allow a metal mirror to maintain an optical figure. In 1960,
the solar program purchased a 0.9-m aluminum mirror blank from Kaiser Aluminum & Chemical
Sales, Inc. in Los Angeles, California. It was ground and polished at the AURA optical shop in
Tucson and set up in the courtyard of the Tucson offices to evaluate the stability of its figure and
how it coped with exposure to the Sun and the elements (AURA 1960). Aluminum is not a
material that can be optically polished, so the mirror was coated with a 0.005-inch (0.13 mm)
thick layer of Kanigen before the polishing. Kanigen is a chemically deposited amorphous nickel
phosphide developed by General American Transportation Corporation of Chicago, Illinois in
the late 1950’s. Kanigen is “exceptionally homogeneous and hard enough for polishing... The
material appears to be well suited for a mirror base as far as scattering properties are concerned”
(Pierce 1957).
The mirror was periodically tested. After eight months of sitting, it still held its figure. This
success led to placing an order for a 1.52-m aluminum test mirror in May 1961 (AURA 1961a)
from L & F Machine Co. in Los Angeles, California.
This was used as the first imaging mirror (#2) in the McM-P telescope. The long-term
performance of this aluminum mirror would determine whether the final 2-m heliostat mirror
would be made from metal or from a more conventional material such as quartz.
The Tucson optical shop completed work on a 1.22-m flat mirror made of beryllium in 1966.
This was used to replace the quartz #3 mirror of the same size that had been welded back
together after cracking in the annealing oven at GE. The figure of the original mirror began to
develop a ridge along the weld. This was apparently caused by stress in the weld. The mirror
could have been refigured, but there was no guarantee that it wouldn’t again lose its shape. This
was used as an opportunity to try beryllium as a mirror material.
9.2 NEW QUARTZ MIRRORS
The observatory received the first
of two quartz mirror blanks ordered
from Corning in August 1965. The
1.6-m mirror was polished by the
AURA optical shop and became the
second #2 mirror for the McMath
telescope. The other blank ordered
from Corning was destined to become
the 2-m heliostat to replace the
temporary 1.6-m quartz heliostat. The
telescope’s #2 mirror mount was
modified to hold both the aluminum
and the quartz 1.6-m #2 mirrors on a
turntable. The turntable would allow
the mirrors to be switched in about
three minutes so that the relative
merits of each could be evaluated
(AURA 1965a). The contract to modify the #2 mirror mount went to Keystone Engineering of
Los Angeles, California. The mirror turntable was installed in January of 1966 (AURA 1966).
The temporary 1.6-m heliostat was not retired after being replaced by the 2-m mirror. The
Kitt Peak 2.1-m telescope coude spectrograph was redesigned in 1971 with its own telescopic
optical system (Figure 9). This allowed the coude spectrograph to be operated independently of
the main 2.1-m telescope. The old 1.6-m quartz mirror was used in an altazimuth mounting to
feed starlight to a 0.9-m image-forming mirror mounted in a tower just south of the 2.1-m
telescope. The Coude spectrograph, used in its stand-alone mode, is about as efficient as it was
when used with the 2.1-m telescope due its fewer number of reflections (AURA 1971a). The 1.6m flat continues its long and varied career and is still making valuable contributions to
astronomy.
9.3 EAST AND WEST AUXILIARY HELIOSTATS
An often-overlooked feature of the McMath telescope is that it was not only designed to be
the world’s largest solar telescope, but at the time, it was to be the world’s three largest solar
telescopes. On either side of the main optical path, two smaller heliostats were added to the
telescope. Known as the East Auxiliary and West Auxiliary Telescopes, each has a 0.9-m
heliostat and a 0.9-m imaging (#2) mirror with an f/ratio of approximately f/50. The auxiliary
telescopes were designed to have roughly the same f/ratio as the main McMath telescope but
with about half the focal length (and image scale). Instrumentation designed to work with the
McMath’s beam should also work with the auxiliary telescopes.
An aperture stop in the optical paths limited the effective aperture of the telescopes to 0.8-m.
At 0.8-m the twin telescopes were, for many years, tied as the second largest aperture solar
telescopes in the world. The same engineering firm that designed the main heliostat, Charles W.
Jones, designed the auxiliary heliostats.
C. W. Jones
designed them to be
mirror images of one
another. Each heliostat
would be mounted on a
fork mount. The fork
would point upward
toward the north
celestial pole. The solar
beams would then be
reflected down the
optical tunnel toward
the south celestial pole
where their #2 mirrors
would reflect the beam
back up the tunnel and
form an image in the
observing room after being reflected downward by their #3 mirrors. Each optical system looks
much like the main optical system but roughly half scale.
The heliostats were mounted above and to the two sides of the main heliostat carriage (Figure
10). Their appearance was described as looking like “ears on the main heliostat mounting”
(AURA 1963). The heliostat forks are attached on their south ends to hollow polar shafts that are
0.8-m inside diameter. It is this that restricts the telescopes’ effective apertures. The shafts are
hollow to allow the solar beams to pass through to the #2 mirrors to the south. The hollow polar
shafts carry the right ascension bearings and worm wheels.
The request for bids from companies to manufacture the two heliostat mounts and drives was
announced on 17 July 1961 and the bids were due by 11 September 1961. At $85,883.00, the
well-known telescope manufacturer Boller & Chivens, Inc. was the low bidder and was awarded
the construction contract. Boller & Chivens delivered the completed heliostats to AURA on 13
August 1963 (AURA 1963). The two heliostats were mounted onto the main heliostat carriage
during the McMath telescope’s first annual maintenance shutdown in September 1963 (AURA
1963a).
The East Auxiliary went into operation with a 0.92-m diameter fused quartz heliostat mirror.
The AURA optical shop completed the polishing of the quartz flat in January 1965 (AURA
1965). The heliostat fed the solar beam down to a 0.92-m Kanigen-coated aluminum spherical
mirror with a 39-m focal length (Pierce 1969) and a 0.92-m aluminum folding flat mirror. The
author believes, but hasn’t been able to document, that the 0.92-m folding flat mirror was the
metal mirror that was polished and used to test the stability of metal mirrors before the 1.6-m
aluminum disk was purchased for the main #2 mirror. All of the mirrors for the West Auxiliary
would be made from the ultra-low expansion ceramic, Cervit (Pierce 1969) but wouldn’t become
available until 1970. The West Auxiliary #2 and #3 mirror mounts were installed in the telescope
in the fall of 1969 (AURA 1969). The West #2 Cervit mirror was figured 1.5 off axis to match
the off axis angle of the #3 mirror. The system was found to deliver “very fine images, free of
coma and astigmatism” (AURA 1970).
The East and West Auxiliaries have their own optical ports in the ceiling of the McM-P
Observing room so that their images can feed independent instruments in the observing room.
All three of the McM-P telescopes can be used fully independently of one another.
9.4 THE NUMBER 2 MIRROR INCIDENT
The 3rd of November 1970 was scheduled to be a routine monthly maintenance shutdown.
The East Auxiliary #2 mirror was being lowered down the telescope when its hoist cable slipped
off of the drum. The carriage raced down the tunnel incline about 30-meters before impacting the
main #2 mirror carriage! The East Auxiliary’s 0.9-m aluminum mirror broke free of its mounting
and fell another 5-meters onto the main #2 carriage. The 1.6-m main imaging mirror was seen to
“jump about 2 inches”. When the mirror settled back into its support, the clips at the mirror edge
broke a roughly 15-cm round chip from the front edge (Figure 11).
The East #2 was destroyed,
but the figure on the main #2
quartz mirror was unaffected.
The telescope’s pupil image
over-fills the diffraction grating
in the Main Spectrograph so a
chip at the mirror’s edge didn’t
affect its data (AURA 1970b).
9.5 UPGRADING TO
CERVIT
The debate over the relative
merits of metal verses quartz
mirrors had gone on for years
with no clear winner between them. Kanigen-coated metal mirrors were found to be somewhat
more difficult to maintain than glass. The reflective aluminum coating adhered quite well to
Kanigen and was difficult to strip off for recoating without damaging the Kanigen (Harvey
2001). By the time of the incident that damaged the McM-P #2 mirror, Corning had developed
Cervit. Cervit was an ultra-low expansion glass-like ceramic that was clearly superior as a mirror
material to both metal and quartz. Cervit had already been tried and found to work extremely
well in the West Auxiliary telescope (AURA 1970). The decision was made to make the best of
the situation by treating this as an “opportunity” to upgrade to Cervit. Cervit blanks were ordered
to replace all three of the optics in the McM-P main beam.
Cutting the cassegrain hole in the Cerro Tololo 4-m telescope’s primary mirror left a 1.06-m
diameter Cervit disk. This disk was used to replace the destroyed East Auxiliary #2 mirror
(AURA 1970b).
9.6 RETURN OF THE QUARTZ NUMBER 2 MIRROR
Several observers at the observatory felt that the Cervit #2 mirror didn’t deliver images with
the quality they remembered from the earlier quartz mirror. It was believed that the Cervit mirror
had a turned edge and a mask was sometimes put over the mirror to reduce the effective aperture
by several cm. Some users believed that this would improve the image quality.
In the late 1980’s the mirror was pointed out of the door of the coatings lab while suspended
from the lab’s ceiling-mounted crane. One of the telescope engineers set up a knife-edge tester
on the roof of one of the residence houses on the mountain that happened to be located at the
center of curvature of the 87-m focal length mirror. The knife-edge test revealed that the mirror
did have a rolled edge. It also showed that the mirror had a central raised area.
The old quartz mirror was then tested in the same fashion and was found to have, aside from
the large chipped area, a much better figure. The quartz mirror was reinstalled in the telescope
and the Cervit mirror went into storage. The quartz mirror, chip and all, is still the imaging
mirror being used in the McM-P. Some observers have even commented that the chip is helpful
when focusing the telescope’s pupil image inside of an optical setup. They find the chip is easier
to focus on than a large, round, featureless image of a mirror.
9.7 INTEGRATED LIGHT FEED
The McM-P produces an image of the Sun that varies between 822.7mm and 795.5mm
depending on our distance from the Sun. The slit for the telescope’s Main Spectrograph is about
90mm long and is typically around 200 microns wide. The spectrograph only sees about
0.0036% of the solar disk at a time! The spectra from the Main Spectrograph represent only that
very small area. Stellar astronomers never resolve the stellar disk of the star they’re looking at
and so the light that enters their spectrographs represents the average of the entire star. To
directly compare the solar spectrum to stellar, the astronomer would need to take spectrum at
every position on the disk and average them together. This is clearly not a practical approach.
Solar staff astronomer Dr. William C. Livingston had an idea in 1967 for a simple approach
that would allow the spectrum from the entire integrated solar disk to be taken at one time.
Livingston’s concept was to beam non-imaged sunlight into the spectrograph. To accomplish
this, Livingston mounted a couple of 0.25-meter flat mirrors onto the #3 mirror carriage. One
mirror reflected the solar beam coming down the telescope from the heliostat up to the other. The
second mirror then folded the light path down into the observing room. The light path went
roughly in the shape of a “4”. This optical arrangement was referred to as the “Integrated Light”
feed and the data with it was called “Integrated Light” or “Sun as a Star” spectra (Livingston
2001).
Observing with this simple mirror arrangement was not convenient. Livingston’s Integrated
Light pick-off mirror was not located along the equatorial axis of the main light path. The
heliostat had to be tipped a bit off-axis to position the solar beam on to it. This caused the
telescope not to track the Sun properly and required constant telescope drive corrections or use
of an autoguider.
A few years later, an improved Integrated Light Feed mirror was designed and installed in
the telescope by Kitt Peak engineers. This feed mirror used a 0.9-meter flat mirror mounted to
the roof of the optical tunnel that could be lowered, with the push of a button, into the center of
the light path in front of the normal #2 imaging mirror. The mirror mount was motorized to allow
fine alignment to be done with a handpaddle from the observing room. The Integrated Light Feed
remains a heavily requested configuration at the telescope.
10. MIRROR MOUNTS
The colossal task of designing the heliostat mounting and drive, along with the mounts and
carriages for the #2 and #3 mirrors, was contracted to Charles W. Jones Engineering of Los
Angeles, California. The firm came to the project with considerable experience in large telescope
design. C. W Jones had been involved with the design of telescopes at Mt. Wilson and Palomar
and the 2.1-m telescope at Kitt Peak (US Naval Observatory 1997). The mirror mountings and
drives were built at the Sunnyvale, California plant of the Westinghouse Electric Company. All
three mirror mountings were delivered to Tucson on railroad flat cars on the 9th of August 1962
(AURA 1962).
10.1 HELIOSTAT MOUNT
The heliostat is a flat mirror that is mounted equatorially to track the Sun. It must also be able
to move in the declination axis to follow the changing height of the Sun through the seasons. The
design chosen for the heliostat drive was a yoke style. The yoke is held by a bearing at the north
and south ends of the mount, with parallel arms running between that straddle the mirror. The
south bearing required a large opening through its center of rotation to allow the solar beam to
pass to the telescope tunnel below (Figure
12). The hour angle drive gear is also at the
south end with the solar beam passing
through its center.
The design of the lower (south) right
ascension bearing that carries most of the
yoke’s 12,000-kg weight, uses a large, 304cm diameter, oil pressure pad bearing
(Pierce 1964) similar to what was used for
the Palomar 5-meter telescope’s
“horseshoe” mount. The yoke carries the
mirror cell that attaches at the declination
bearings. Two motors are used in each axis;
large DC motors for fast, slewing motions;
and separate motors for the fine motions.
The declination slew motor uses a worm
and gear, while the fine motions are
accomplished with a tangent arm. Limit switches ensure that tangent arm can’t be driven beyond
its limit of travel. “The right ascension drive consists of a synchronous motor driven at mean
rate, which has superimposed on it, by means of a differential, incremented rotations from a
stepping motor” (AURA 1962). The stepping motor introduced rate corrections to compensate
for refraction through the Earth’s atmosphere.
Westinghouse produced a 720-tooth worm wheel gear for the right ascension drive. During
installation, an inspection revealed that the worm had failed. The material had apparently been
stressed during the case hardening process. It showed numerous hairline cracks, some extending
through the teeth. The worm was estimated to only retain 10% of its strength (AURA 1962a).
Westinghouse temporarily replaced the worm with another made of chrome-plated soft steel that
was left over from the lapping of the original worm while they worked to produce a permanent
replacement (AURA 1962b).
The heliostat tower doesn’t have a dome or other structure over the heliostat mirror to protect
the mirror from the elements. The telescope was designed so that the entire 55,000-kg heliostat
carriage could be lowered down inside the optical tunnel and a horizontal sliding door could be
closed to protect against inclement weather. The process of lowering the carriage about 15-m
down into the tunnel and closing the covers was expected to take about 20 minutes (Pierce
1964). It was quickly realized that moving the heliostat carriage induced a warping of the
structure and binding in the right ascension bearing. The warping had to be corrected with the
use a small hydraulic jack placed along the base of the carriage. Moving the heliostat carriage
often was not a practical option in practice. An innovative solution was devised. The carriage
would be left exposed on top of the tower. Nightly, the heliostat would be pointed straight
downward. A cup-shaped cover was constructed on top of a hydraulic ram mounted below the
heliostat carriage. The ram would lift the cover upward until it sealed around the edge of the
heliostat mirror to protect it from the weather.
A mercury-filled tire-like bag was installed around the heliostat mirror in the summer of
1970. The mercury bag supports the mirror around its edge (AURA 1970a). The bag was
designed such that the mirror effectively floats on the bag and is supported evenly around its
edge no matter the orientation of the heliostat. The back support of the mirror cell was later fitted
with a network of weights and levers meant to evenly support the mirror while pointing at any
position on the sky.
10.2 NUMBER 2 MIRROR MOUNT
Charles W. Jones came up with a quite simple yet adequate mounting for the #2, imaging
mirror. A carriage was welded together from 30-cm diameter heavy wall pipe. The mirror was
supported across its bottom by a simple steel sling. The mirror, leaning back at the latitude angle
of 32, is supported from behind by a six pads. The telescope is focused by moving the entire
carriage up or down the incline on the tunnel’s 12-foot (3.66-m) gauge track.
Originally, the 5,000-kg carriage was moved by a motor-driven ball screw that allowed for a
focal travel of 2-m (Pierce 1964). To move farther, such as up the tunnel to the coating lab, the
#3 mirror’s hoist was used (AURA 1962). The carriage was later given its own hoist to allow the
focus to easily be moved over longer distances to deliver the image to multiple observing ports.
Collimation can be adjusted from the telescope’s observing room with a handpaddle that
controls motors for moving the mirror horizontally and vertically.
10.3 NUMBER 3 MIRROR
MOUNT
The #3 mirror could also be of a
fairly simple design since the mirror
only needs to be held motionless
during observing. Its hoist is used to
move the folding flat between the
numerous observing ports in the
telescope and to deliver the mirror to
the aluminizing room for its periodic
servicing. The mount holds the mirror
on a fork that can be moved in two
axes for collimation from a handpaddle in the observing room. The carriage rides up and down
the telescope track on four wheels (Figure 13).
There are stops built into the side of the telescope, just inside and below the tracks, at each
observing port. Tabs on the carriage are flipped down by hand before the carriage is lowered
onto the stops at the desired port. The stops, not the hoist, hold the weight of the mirror carriage
stably during observations.
11. COATING LAB
About midway down the optical path, a bit south of the main observing room, is the
telescope’s coating facility. It is located just below the division between the above and below
ground cooling panels. The coating lab houses a large coating chamber from the F. J. Stokes
Corporation of Philadelphia, Pennsylvania. It is used for periodic re-aluminizing of the telescope
mirrors. The chamber was installed as part of the original telescope equipment and continues to
be used. The chamber (Figure 14) was designed to accommodate optics as large as 2.1-m in
diameter so that it could used for servicing the Kitt Peak 2.1-m Telescope and other smaller
stellar telescopes. The 1958 proposal for the solar telescope included a budget of $25,000 for the
coating chamber (AURA 1958).
Four large removable
ceiling panels top the section
of the optical tunnel next to the
coating lab. These panels can
be removed and open into the
coating lab. The telescope
optics can all be moved with
their winches to below the
opening into the lab. A large
roof mounted crane can then
lift the mirror into the lab.
Once in the lab, the mirrors
can be stripped of their old
coatings, cleaned and lifted
into the coating chamber. The
chamber is oriented horizontally so that the mirrors can simply hang vertically in slings inside
the chamber. A diffusion pump is used to evacuate that chamber to a few times 10^-6 torr. A
current is applied to tungsten coils in the chamber and are heated to incandescence. This
evaporates aluminum rods that were placed in the coils and deposits a thin layer of aluminum on
everything inside the chamber, including the mirror surface (Poczulp 2001).
The lab’s location mid-way down the optical path makes access to the mirrors rather
convenient. The mirrors never need leave the telescope structure. This minimizes the risk of
damage due to handling and contamination of fresh coatings. The current optical coating
technician that operates the lab, Gary Poczulp (2001), believes that “having coating facilities that
are conveniently located to the telescope is something that seems often overlooked in modern
designs”. He feels that the original designers came up with a “lab that is well laid out and is quite
user friendly.”
The telescope is normally scheduled for a shutdown period of one to two weeks per year to
remove and recoat any mirrors that have degraded due to exposure. The shutdown has normally
been done during the summer rainy season. The rainy season, or monsoon, typically occurs
between July and August.
The telescope mirrors are washed once or twice between the annual shutdowns. The
procedure for washing a telescope mirror begins with rinsing the mirror thoroughly with a soapy
water solution. The soapy water is a mixture of deionized water and a particular brand of horse
shampoo – “Orvus” brand! This first rinse is meant to loosen and remove grit on the mirror
surface. The mirror is then lightly rubbed with a natural sea sponge in circular motions, starting
at the top and working downward. It is then rinsed thoroughly with deionized water to clean
away all of the soap residue. The mirror is then dried by lightly and carefully patting with a lintfree paper towel. Great care is taken during the patting to ensure that the paper towel is not
dragged across the mirror surface, which could scar the soft aluminum coating (Poczulp 2001).
12. INSTRUMENTATION
The McM-P telescope was designed as a general-purpose telescope for investigations into
solar physics. The telescope is often used to feed a light beam into equipment brought by visiting
astronomers. There were two primary instruments that were planned as part of the facility from
the project’s beginning - a vertical spectrograph and a spectroheliograph.
12.1 MAIN SPECTROGRAPH
The Main Spectrograph (Figure 3) was planned to be the primary workhorse instrument for
the McM-P telescope. It has proved to be an extremely useful and versatile instrument that
continues to be the telescope’s main instrument. The spectrograph was modeled on the very
successful vacuum spectrograph at the McMath-Hulbert Observatory. The spectrograph is
housed in a 1.8-m diameter, 21-m long vertical steel cylinder. The cylinder rests on a bearing at
its base and is rotated with a clock-drive to match the 24-hour rotation rate of the telescope’s
image.
The spectrograph uses a Czerny-Turner optical system. The spectrograph can use a selection
of slits ranging in width from 60-microns to 300-microns depending on the required
spectrographic resolution. The light passes through the slit and down to collimating mirror 13.7m below, which reflects the beam up to the diffraction grating (Pierce 1969). The grating sends
the dispersed beam back down to a second mirror, 13.7-m down, that focuses the spectrum at the
top of the spectrograph tank where it is easily accessed by the detectors.
Horace W. Babcock, of Mt. Wilson Observatory, produced the grating. The grating has 610
grooves/mm covering a 25-cm by 15-cm area with a total number of lines equaling 155,000. The
fifth order spectrographic resolution () as been measured to be approximately 500,000 at
5641Å, which is quite close to the theoretical value of 775,000 (Pierce 1964). The spectrograph
can also be used in double pass mode. Double passing is accomplished by reflecting the exiting
spectrum back through the optical path a second time, doubling the theoretical resolution.
The grating is mounted in a rotatable drum-like fixture that is suspended from the
spectrograph’s top. Motors were installed that allow the grating to be rotated to and/or scanned
across the wavelength of interest. The grating drum was modified in the mid-1990’s to include
modern optical encoders and stepper motors that can more accurately find wavelengths under
computer control. The original diffraction grating was replaced with a 632 grooves/mm grating
produced by Milton Roy Optical. The new grating uses a Zerodur substrate and is larger than the
original, 42 by 32 cm. The new modified drum includes a second diffraction grating that is
mounted back-to-back with the original grating. This grating measures 47 by 36.8 cm with 120
grooves/mm and is optimized for the use in the infrared. Either grating can be selected by the
throw of a switch, which flips the drum. The two gratings give the Main Spectrograph a spectral
range of 0.29 - 12 microns.
12.2 SPECTROHELIOGRAPH
Dr. Keith Pierce and Dr. Robert Leighton, from the California Institute of Technology,
jointly designed a spectroheliograph for the McMath telescope. The Spectroheliograph was built
by Boller & Chivens and installed in November 1966 (AURA 1967). The instrument used a 25cm slit and a 112-mm diameter grating in a 6-m vertical tank (Pierce 1969). Like the Main
Spectrograph, the tank was made to rotate to compensate for image rotation. It was mounted
vertically under its own observing port in the ceiling of the McMath observing room, 5-m south
of the Main Spectrograph’s port.
The Spectroheliograph had a fairly short life. By the mid 1970’s, the primary staff user of the
Spectroheliograph, Dr. Neil Sheeley, had left the observatory and interest in the instrument
dwindled (Harvey 2001).
12.3 SOLAR-STELLAR SPECTROGRAPH
In 1983, the Spectroheliograph was rebuilt as a stellar spectrometer. Dr. Richard Dunn from
Sacramento Peak Observatory redesigned the system to take high-resolution spectra of stars. By
1984, a full-time nighttime operator had been hired and a program of collecting spectra of “solar
type stars” began (Giampapa 2001). Astronomers could submit proposals to the program.
Proposals accepted by the Telescope Allocations Committee would be organized into a logical
sequence by the operator who would collect the data for the astronomers. The astronomers’ data
would be mailed to them on 9-track magnetic tape.
The spectrograph is of a conventional design with the exception of the input slit; it doesn’t
use a slit. In its place is an image slicer. The image slicer is an optical device that accepts a
square image and through internal reflections, slices the image up into five or ten wide slices,
depending on which slicer is used. The slicer aligns each slice end-to-end creating the image of a
slit while preserving all of the starlight falling on the input aperture. A normal slit would reject a
large fraction of the starlight making the spectrograph less efficient.
The spectrograph began operation with a single-dimension Reticon array. In 1987, the
detector was upgraded to an 800x800 pixel liquid-nitrogen cooled CCD from Texas Instruments.
The CCD allowed spectra to be routinely taken of stars down to twelfth magnitude. A manually
rotated drum selects between two gratings, a low dispersion grating and a high dispersion
Echelle grating. Depending on grating and optics used, the spectrographic resolution () can
vary between 21,000 and 100,000 (National Solar Observatory 1998).
The stellar program at the McMath telescope was declared to not be “central to the mission
of the National Solar Observatory” and was officially closed in January 1995 to cut operations
costs. The instrument continues to be used, mostly for planetary studies, but there is no longer an
operator for the instrument on staff (Giampapa 2001).
12.4 FOURIER TRANSFORM SPECTROMETER
Dr. James Brault joined the observatory staff in the mid 1960’s. His training was as a
physicist, not as an astronomer. Brault had a strong aptitude for instrumentation and computer
programming. He applied his unique perspective and talents to improving upon the design of the
Main Spectrograph. What he proposed was a radical departure from the grating spectrometers of
the past. He pushed for and ultimately headed a program to develop and build the 1-meter path
difference Fourier Transform Spectrometer (FTS) at the McM-P Telescope.
spectrum.
An FTS is inherently
different from a standard
spectrometer. It has no
diffraction grating. Instead, it
uses a scanning Michelson
Interferometer (Figure 15) to
produce an interferogram of the
light entering the instrument.
The interferogram is the plot of
the intensity seen at the
instrument’s output verses the
path difference between the
retro-mirrors in the
interferometer. Applying the
Fourier Transform to the
interferogram reveals the
The FTS has several advantages over a grating spectrometer. The spectrum it produces is
very wide-range and has extremely high wavelength accuracy. It doesn’t suffer from any of the
inaccuracies from errors in the ruling of a grating. The FTS uses a circular input aperture (up to
1-cm diameter), which has a much larger collecting area than a grating spectrometer’s input slit
making it very efficient. The spectrographic resolution of an FTS is limited only by the
maximum path difference that the retro-mirrors can achieve (see appendix 1).
A major difficulty in designing an FTS is controlling the retro-mirrors, whose position must
be known at all times to within a fraction of a wavelength the light. This difficulty was overcome
in the Kitt Peak FTS by mounting the retro-mirrors in massive carriages that ride on oil bearings
and are moved with linear
electric motors. The positions
of the mirrors are tracked using
of two separate laser
interferometers. The entire
instrument is housed inside of
a large vacuum chamber
(Figure 16) to avoid acoustic
and microthermal disturbances
(AURA 1971). It uses four sets
of optics to cover from 250 nm
in the ultraviolet to 18 microns
in the infrared.
Brault began studies of the
theory and design of the FTS in the late 1960’s. Construction began in 1971 (AURA 1971). A
new observing room had to be constructed to house the instrument. Construction began on the
new lab in 1969, which is located to the southeast of the main observing room (AURA 1969).
The telescope’s #3 mirror is moved down the tunnel to just below what is now the Stellar
Spectrograph port and is rotated eastward to feed the solar beam horizontally into the FTS.
The FTS has been in continuous operation at the telescope since 1976. It is still a very
heavily subscribed instrument. It is used about half of the time as a solar spectrometer with the
McM-P telescope and about half of the time by physicists and chemists as a stand-alone
laboratory spectrometer.
13. INFRARED CAPABILITY
Tremendous effort in the design of the McM-P went into cooling and stabilizing the air
temperature and flow inside of the telescope to minimize its distorting effects on the image. The
astronomers at the Sacramento Peak Solar Observatory in New Mexico had a very different
concept for dealing with this problem. If air inside of the telescope is such a problem, why not
get rid of it? From this very simple concept came the design of the Sacramento Peak Vacuum
Tower Telescope (now known as the Richard B. Dunn Solar Telescope), which was constructed
in the 1960’s and dedicated in 1969 (National Solar Observatory 1996). The vacuum tower
telescope mounts all of its optics inside of a very large vertical vacuum tank, with a window at
its entrance and exit. The air is evacuated from the tank and the problem of internal seeing is
eliminated. So sharp were the images formed from this type of solar telescope, that almost every
large solar telescope built since then has been based on the vacuum tower concept (Livingston
2001).
The vacuum tower design necessitates the use of at least two transmitting optics, the entrance
and exit windows. These windows work fine in the visible spectrum but become opaque in the
infrared past around 2.5-microns. This was considered an acceptable compromise when the only
astronomical imagers available worked in the visible spectrum. Infrared array detectors began to
become available to the astronomical community in the early 1990’s and suddenly a new
wavelength band was opened to astronomical research. The vacuum solar tower telescopes,
although still performing very well in the visible spectrum, were excluded from this new
innovation.
Solar astronomers quickly realized that the McM-P was the only large all-reflecting solar
telescope available. The McM-P was suddenly thrust into the position of being the default
facility for infrared solar astronomy. A high percentage of the current programs on the telescope
emphasize its infrared capabilities.
14. EXAMPLES OF CURRENT PROGRAMS
McM-P Telescope time is allocated on a competitive basis. Astronomers from around the
world are invited to submit observing proposals, which are graded and sorted by their scientific
merit by the Telescope Allocations Committee. The telescope schedule is then arranged to
accommodate as many of the proposals as possible starting with the highest-graded proposals.
This leads to a very diverse range of programs on the telescope. Below are some examples of
current programs using the unique capabilities of the McM-P.
14.1 MOLECULAR CLOUDS OVER SUNSPOT UMBRAE
Dr. T. Alan Clark from the University of Calgary, Canada, has used the McM-P with its 256
x 256 pixel near-infrared array detector and the Main Spectrograph to create spectroheliograms
of molecular absorption lines in the 4-micron region. Molecules of particular interest include
H2O, and HCl that only form over the coolest region of the sunspot umbra (Figure 17).
Determining the temperature threshold for the formation of these molecules “should lead to a
method for defining the temperatures of sunspot umbrae more precisely” (Clark et al. 2001).
14.2 SODIUM EMISSION FROM MERCURY
Dr. Drew Potter uses the McM-P
Solar-Stellar Spectrograph to map the
distribution of sodium emission from
the planet Mercury. An image slicer,
described earlier, is used to dissect the
view into one arcsecond blocks. A total
area of 10 square arcseconds allows the
entire planet to be imaged. The
spectrograph is used to analyze the
intensity of the sodium D2 line
emission in each block. An image can
be assembled from the spectra of the
sodium distribution across the planet.
The image (Figure 18) shows the
illuminated side of the planet Mercury
on the right with the terminator cutting
across the left. The image clearly
shows a higher emission (red) on the hot illuminated side than on the cooler dark (blue) side
(Potter 2001).
14.3 ZEEMAN SPLITTING IN OH AT 12 MICRONS
Dr. Don Jennings of NASA Goddard Space Flight Center used the McM-P FTS in January
2001 to measure Zeeman splitting in sunspot spectrum of the hydroxyl radical (OH) (Figure 19).
“Splittings in these lines are of interest not only as a probe of magnetic fields but also because
the Zeeman effect is rarely seen in molecules” (Jennings & Plymate 2001). Zeeman splitting is
proportional to the
wavelength. Longer
wavelengths exhibit
greater splitting. To
measure weak
magnetic fields, it is
desirable to use
spectra taken as far
into the infrared as
is possible.
The two
Zeeman
components were
recorded separately using a quarter-wave plate and a linear polarizer setup in front of the FTS.
The difference of the two spectra are shown in the accompanying figure. (See Appendix II for a
more in-depth explanation of Zeeman splitting.)
15. THE FUTURE
Plans are progressing to build the next generation solar telescope, the Advanced Technology
Solar Telescope (ATST). ATST is planned as the successor to both the McM-P at Kitt Peak and
the Dunn Solar Telescope at Sacramento Peak. It is to be a 4-m class all reflecting infrared solar
telescope that will utilize adaptive optics and state-of-the-art instrumentation to support the
research requirements of the solar astronomical community of the 21st century (National Solar
Observatory 2001). The project is still in its infancy. What the ATST design will eventually look
like and even where it will be placed is not yet known. Conceptual designs and possible site
locations are currently being evaluated.
Over the short term, the McM-P continues to be the only available facility for infrared
investigations of the Sun. Current plans are to continue support of operations and upgrades
consistent with the funding constraints during development of ATST. This indicates that the
McM-P will continue operations through at least the first decade of the 21st century. The
telescope upgrades will be mostly aimed toward enhancing the telescope’s unique infrared
capabilities.
15.1 A NEW INFRARED ARRAY DETECTOR
The primary infrared instrument for the solar telescope is the Near Infrared Magnetograph
(NIM). The NIM uses the Main Spectrograph with some polarization optics mounted before the
input slit. The output from the spectrograph is reimaged onto an Indium Antinomide (InSb)
infrared array that is sensitive between 1 and 5 microns. The array is a commercial quality 256 x
256 imager that was purchased in the early 1990’s from Amber Engineering, Inc. The array is
rather small by modern standards; it has many bad pixels and is known to be a “noisy” array.
This is the same array that was used by Clark to produce the sunspot umbrae molecule cloud
spectroheliograms discussed earlier.
A project has been funded that will replace the old Amber array with a 1024 x 1024
“Aladdin” InSb array built by Raytheon that will have much lower noise and a shorter readout
time. The new array will require all new control and readout electronics. This new array will
greatly expand the capabilities of the NIM program and should stimulate numerous new
proposals from within the solar community. The new array should be operational in 2003 (Rabin
2001).
15.2 LOW ORDER ADAPTIVE OPTICS
Improving the quality of the image is another primary goal. The telescope scientist for the
McM-P, Dr. Christoph Keller, has begun an adaptive optics (AO) project for the telescope. The
goal of this AO program is to correct for atmospheric seeing and telescope aberrations to the
diffraction limit in the infrared. Limiting the system to the infrared simplifies the design by only
requiring correction of low order aberrations. A low order adaptive optical system requires fewer
actuators for the deformable mirror that corrects the wavefront. Fewer actuators simplify the
computational needs, etcetera. Dr. Keller believes that the entire program can be done for
$25,000 spread over a few years. The low costs are achieved by using as much off-the-shelf
technology as possible.
A proof of concept test
was performed on the
telescope using two CCD
cameras taking simultaneous
images. One camera
recorded the primary image
while the other was used to
measure the wavefront
distortion through a ShackHartmann subaperture array.
Dr. Keller was able to deconvolve the wavefront errors from the image and produce a corrected
image (Figure 20) that “is a good approximation of what an adaptive optics system with the same
wavefront sensor would deliver” (Keller & Plymate 2000). A deformable mirror has been
purchased and is being evaluated. It is hoped that a prototype system will be working at the
telescope by 2003.
16. CONCLUSION
The McM-P is about to enter its fifth decade of service. It has a long and distinguished
career, but it isn’t time to think of retirement just yet. There is still much science to be done. The
telescope can deliver its beam to any of several observing ports including both the main
observing room and southeast or FTS observing room. This allows a beam to be delivered to not
only the telescope’s primary instrumentation that was discussed earlier, but to several general
purpose observing ports. Astronomers find the telescope extremely convenient for setting up and
testing prototype instrumentation. Many think of the telescope more like an optical lab that has
as a special feature the ability to deliver a telescopic image than a conventional telescope where
all instrumentation must be designed and ready to mate to a specific instrument port.
The telescope will continue to serve the solar astronomical community as the workhorse
infrared facility until it is at last replaced by ATST. In the meantime, the McM-P will serve as
the test bed for much of the ATST development.
It is harder to guess what might become of the McM-P once the ATST in online. It is
expected that the National Solar Observatory will not be able to continue to afford the costs
associated with supporting the McM-P after that point. This does not necessarily mean that the
telescope will be left abandoned to slowly rust away into history. A more likely scenario is that
the operation of the telescope will be given over to some other institution or consortium that will
to continue its use for research and/or development. The McMath-Pierce Solar Telescope is still
a unique facility that offers unique capabilities to the scientific community. The primary mission
of the McM-P may evolve with time, but it will likely continue making contributions to science
for quite some time to come.
REFERENCES
AURA, Proposal to the National Science Foundation from Association of Universities for
Research in Astronomy, Inc. for the Construction of the World’s Largest Solar Astronomical
Telescope, AURA, August 1958
AURA, Kitt Peak National Observatory, Monthly Report, August 1960
AURA, Kitt Peak National Observatory, Monthly Report, January 1961
AURA, Kitt Peak National Observatory, Monthly Report, May 1961a
AURA, Kitt Peak National Observatory, Monthly Report, August 1962
AURA, Kitt Peak National Observatory, Monthly Report, September 1962a
AURA, Kitt Peak National Observatory, Monthly Report, October 1962b
AURA, Kitt Peak National Observatory, Monthly Report, August 1963
AURA, Kitt Peak National Observatory, Monthly Report, September 1963a
AURA, Kitt Peak National Observatory and Cerro Tololo Inter-American Observatory, Monthly
Report, January 1965
AURA, Kitt Peak National Observatory and Cerro Tololo Inter-American Observatory, Monthly
Report, July and August 1965a
AURA, Kitt Peak National Observatory and Cerro Tololo Inter-American Observatory, Monthly
Report, January 1966
AURA, Kitt Peak National Observatory and Cerro Tololo Inter-American Observatory, Monthly
Report, January and February 1967
AURA, Kitt Peak National Observatory and Cerro Tololo Inter-American Observatory, Monthly
Report, September and October 1969
AURA, Kitt Peak National Observatory and Cerro Tololo Inter-American Observatory,
Quarterly Report, April-May-June 1970
AURA, Kitt Peak National Observatory and Cerro Tololo Inter-American Observatory,
Quarterly Report, July-August-September 1970a
AURA, Kitt Peak National Observatory and Cerro Tololo Inter-American Observatory,
Quarterly Report, October-November-December 1970b
AURA, Kitt Peak National Observatory and Cerro Tololo Inter-American Observatory,
Quarterly Report, July-August-September 1971
AURA, Kitt Peak National Observatory and Cerro Tololo Inter-American Observatory,
Quarterly Report, October-November-December 1971a
Clark, T. A., et al. 2001, NOAO Newsletter, 66, In Press
Donovan & Bliss, The Cooling System of the Kitt Peak Solar Telescope, Technical Report 62-1,
15th June 1962
Edmondson, F. K., AURA and its US National Observatories, Cambridge University Press
(1997)
Giampapa, M. S., Deputy Director, National Solar Observatory, Tucson, Arizona, 2 May 2001
Hammerschlag, R. H. & Zwaan, C. 1973, PASP, 85
Harvey, J., Astronomer, National Solar Observatory, Tucson, Arizona, 3 May 2001
Jennings, D. & Plymate, C. L. 2001, NOAO Newsletter, 66, In Press
Livingston, W. C., Astronomer Emeritus, National Solar Observatory, Tucson, Arizona, 6 April
2001
Keller, C. & Plymate, C. L. 2000, NOAO Newsletter, 63
Kloeppel, J. E., Real of the Long eyes, Univelt Inc. (1983)
McMath, R. R. & Pierce, A. K. 1960, S&T, 20, 2
Nautical Almanac Office, The Astronomical Almanac for the Year 2001, U.S. Government
Printing Office (2000)
National Solar Observatory, Richard B. Dunn Solar Telescope, http://nsosp.nso.edu/dst/, 8 April
1996
National Solar Observatory, Solar Stellar Spectrograph,
http://nsokp.nso.edu/mp/aboutstarspect.html, 2 July 1998
National Solar Observatory, Advanced Technology Solar Telescope, http://atst.nso.edu/, 2 May
2001
Petrie, R. M. 1962, S&T, 23, 4
Pierce, A. K., Progress Report on the 60” Solar Telescope, Report #3, July 1957
Pierce, A. K. & McMath, R. R., Plan for a Large Solar Telescope of 60” Aperture and 300 Feet
Focal Length, AURA, Inc., Technical Report No. 1 (Solar Project), January 1958
Pierce, A. K. 1964, Applied Optics 3, 12
Pierce, A. K. 1969, Solar Physics 6
Pierce, A. K. 1986, Solar Physics 107
Pierce, A. K., Astronomer Emeritus, National Solar Observatory, Tucson, Arizona, 6 April 2001
Poczulp, G., Senior Technical Associate, National Optical Astronomy Observatory, Tucson, 4
April 2001
Potter, A. E., Visiting Scientist, National Solar Observatory, Tucson, Arizona, 24 May 2001
Rabin, D. M., Solar Branch Chief, NASA Goddard Space Flight Center, Greenbelt, Maryland, 11
May 2001
University Lowbrow Astronomers, The History of the McMath-Hulbert Observatory,
http://www.umich.edu/~lowbrows/history/mcmath-hulbert.html, May 2000
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http://ftp.nofs.navy.mil/about_NOFS/hist.html, 1997
APPENDIX I
In this section, an attempt is made to explain how a Fourier Transform Spectrometer works in
as simple to understand and as concise a form as is possible.
First, consider a Michelson interferometer that has a collimated monochromatic incoming
light source, such as a laser. The entering
wavefront will be split into two by the
beamsplitter, reflect off of the retromirrors and recombine back at the
beamsplitter. Assuming that the two retromirrors are at equal distance from the
beamsplitter, the recombining wavefronts
will be in phase and will constructively
interfere. Viewing the beamsplitter, one
would see a bright spot (figure 21).
Moving one of the retro-mirrors forward or back by a quarter of a wavelength (/4) would
increase its total optical path to and from the retro-mirror by a half wavelength (/2). The
recombining wavefronts would now be out of phase causing destructive interference and the spot
would be dark. Move another quarter wavelength and the beam would again become bright. A
plot of the spot intensity verses path difference (Figure 22) shows that the intensity will trace out
a continuous sine curve. The distance the retro-mirror moves can be deduced by simply knowing
the wavelength of the laser and counting the number of cycles of bright to dark (called fringes).
This is how a distance measuring interferometer works.
Now
assume that
the entering
beam is not
monochrom
atic, but is
made up of
two
wavelengths
, 1 and 2, that are very close to each other in
wavelength such that 2 = 1 +  (Figure 23).
Start moving the retro-mirror from its zero path difference position. Both wavelengths begin
in phase with each other. With each passing fringe, the two will become more out of phase. After
some number of fringes, depending on the size of , the 1 and 2 will become completely out
of phase and no fringes will be seen. Continue moving and the fringes will increase again in
contrast until 1 and 2 are again in phase. This is the point where the retro-mirror has moved
exactly one less 2 half wavelengths than 1 half wavelengths. Stated mathematically:
n 2 = (n+1) 1
Rearranging: 2 / 1 = (n+1)/n
Substituting: (1 + ) / 1 = (n+1)/n
1 +  / 1 = 1 + 1/n
 / 1 = 1 / n
Flipping:
 /  = n
Remember that  /  is the definition of spectrographic resolution. This equation states that
the spectrographic resolution of an FTS is determined solely by the number of wavelengths that
the path difference (equal to twice the movement of the retro-mirrors) changes.
It would be relatively trivial to determine the two wavelengths that produced the fringe
pattern above. The more wavelengths that are present makes for a fringe pattern, or
interferogram, that is ever more difficult to interpret. There is a mathematical procedure known
as the Fourier Transform (FT) that will determine all wavelengths present in the interferogram.
By using a computer to perform an FT on the interferogram, the spectrum of light entering the
FTS can be determined.
APPENDIX II
Some spectral lines are sensitive to magnetic fields. These lines, in the presence of a
magnetic field, will broaden or split into two. The width of the splitting is dependent on the
strength of the magnetic field. This is what is known as the Zeeman effect.
The two Zeeman components are circularly polarized. One side is polarized clockwise while
the other is polarized counter-clockwise. One can take the spectrum of each polarization
component independently by using a quarter wave plate to convert the circular polarization states
into orthogonal linear polarization. Then a linear polarizer can be rotated to select each of the
states, one at a time. Subtracting the two polarization spectra cancels the non-Zeeman lines and
leaves a characteristic looking down and up pattern (Figure 24). By measuring the separation
between the down and up lines, the magnetic field strength can be determined.