Download By the time astronomers got a big telescope into orbit, they

OPTICAL ASTRONOMY
by Ann Finkbeiner
By the time astronomers got a big telescope into
orbit, they had figured out ways to make it less
necessary. That the future, in space or on the
ground, appears to be up in the air
may be as literal as it is figurative.
12 MOSAIC Volume 22 Number 4 Winter 1991
Virtually the only way to learn
anything directly about the universe is through the wavelengths of light: Everything visible shines by its own or by reflected
light. Unfortunately, light keeps as many
secrets as it reveals: It leaves stars and
galaxies and radiates outward for millions or hundreds of millions or billions
of years. By the time it reaches the earth,
the image it carries of a star or galaxy
has dimmed drastically, sometimes to
near imperceptibility. And before light
can reach an astronomer's telescope, it
encounters earth's shimmering atmosphere and the image is either blocked
out completely or scrambled. As a result,
astronomers on earth are trying to see
objects whose light can be no more than
a few photons per second per square
meter and whose images are distorted,
blurred, and dancing around.
For nearly 400 years astronomers
have worked around the problems,
building larger and larger mirrors to
catch more of the light, siting their telescopes up higher and higher mountains
in thinner, stiller air. In 1990 astronomers tried what was billed as the bestyet solution, a telescope above the atmosphere, in space.
But the Hubble Space Telescope is
not the best yet One reason is human
error in its construction and the nearimpossibility of making repairs. Another
reason is that, in the 30 years between
the first studies for the Hubble and its
launch, ground-based astronomers
learned to build even larger mirrors to
gather even more light, invented detectors that catch nearly every photon, and
attached to their telescopes devices that
compensate for the effects of the atmosphere and restore images to precision
and stability. By the time space astronomers got the Hubble in place, groundbased astronomers were giving it a run
for its money.
Catching photons
The things astronomers want to look
at are faint That is, though the things
themselves are often uproariously
bright, their light, like all electromag-
MOSAIC Volume 22 Number 4 Winter 1991 13
netlc radiation, falls off as the square of
their distance. So an object at twice the
distance delivers a quarter of the light
and an object at ten times the distance,
a hundreth of the light.
An extreme case, a star-like quasar
with the light of ten thousand galaxies
at a distance of ten billion light-years,
might send into a mirror four meters
across only two photons a second. Humans cannot see two photons. Although
neurons in the retina will fire on impact
from one photon, the brain does not
think it sees anything until the neurons
have detected 100 photons. The retina,
moreover, does not record photons and
does not accumulate them, so to see
faint objects, astronomers attach cameras to the telescopes and expose photographic plates for hours. Photographic
plates build up images and improve on
eyesight 100-fold.
But astronomers are always, in their
words, photon-hungry. In the 1970s they
began using solid-state detectors, called
charge-coupled devices, that were more
sensitive than photographic plates.
From any given object photographic
plates mounted on telescopes could detect approximately one percent of impinging photons. Charge-coupled devices, says Sidney Wolff, director of the
National Optical Astronomy Observatories, "detect 70 or 80 or 90 percent. That
means we're detecting all the light there
is." In fact, Wolff adds, "we're coming to
the limit of what we can do by improving
detectors."
Fortunately, capturing faint light can
be done not only with sensitive detectors
but also with larger mirrors. Seventy
years after Galileo built his first telescope in 1609, Isaac Newton built a version that replaced the curved lens with
a curved mirror. Since Newton's time,
mirrors have replaced lenses in most
telescopes. The mirrors are curved in
such a way that they reflect all the photons hitting them to a single focus point.
By the late 1980s large telescopes usually had mirrors four meters across,
"whose surfaces are so controlled," says
Roger Angel, mirror maker at the University of Arizona, "that all the rays from
Finkbeiner is a Baltimore-based freelance science journalist and coauthor
with John Bartlett of The Guide to Living
with HIV Infection. Her most recent article for Mosaic was "Mapmaking on the
Cosmic Scale" in Volume 21 Number 3
Fall 19m
14 MOSAIC Volume 22 Number 4 Winter 1991
an undistorted star are aimed at one spot:
a few microns across."
Moreover, the larger the mirror's surface, the more photons it can capture; a
four-meter mirror can routinely see objects as dim as 25th magnitude. (Magnitude, an arbitrary measure of an object's
brightness, is on a logarithmic scale on
which each step up in magnitude is twoand-a-half times as bright as the previous
step. Objects of 25th magnitude are 50
million times as faint as the faintest stars
the naked eye can register.)
But, like detectors, mirrors are reaching the limits of the technology. Though
the celebrated Hale telescope on Mount
Palomar isfivemeters across, for a number of reasons a four-meter mirror is
about as big as a conventionally built
mirror can realistically get. A larger mirror built the same way as the usual fourmeter would weigh so much more that
gravity would affect the perfection of its
surface. "No further scaling up of the
standard design is practical," wrote
Roger Angel.
In short, as of the late 1980s, the detectors and mirrors of telescopes on the
ground were capturing all the photons
they could.
Seeing through the atmosphere
A second fundamental problem that
prevents astronomers from Osee
V-Ollli;
everything they want to is the hundredodd-mile depth of the earth's atmosphere. The atmosphere does unspeakable things to light.
The things astronomers want to look
at emit light in all wavelengths; the atmosphere blocks out most of them. It
blocks out all wavelengths shorter than
about 0.3 millionth of a meter, or 0.3
micron (far-ultraviolet wavelengths, x
rays, and gamma rays). It lets through
wavelengths between a few millimeters
and tens of meters (the so-called radio
window) and blocks out everything
above 100 meters (long radio waves).
The tiny fraction that does get through
and that human eyes can see (between
0.3 micron and 10 microns, the near-infrared, visible, and near-ultraviolet wavelengths) is named for the Greek word
for eye, ops, for optical.
The atmosphere is not only choosy; it
glows. It has what Angel calls "a sort of
phosphorescence that goes on all the
time." Various molecules give off energy
in infrared wavelengths, and extracting
a star's signal from the atmosphere's
noise is often impossible. In addition,
much of the light that cities emit gets
scattered off the atmosphere so that,
near a city, the seemingly dark night sky
can be a hundred times as bright as the
faintest galaxies.
The wavelengths that the atmosphere
does not block or swamp with noise are
scrambled. To an optical astronomer the
atmosphere is a collection of blobs or
patches of warmer and colder air, each
about ten centimeters across. Light trying to get through the atmosphere is
deflected off each patch differently. The
amount of deflection depends on the
patch's temperature. Cold patches are
denser than hot ones and accordingly
deflect light more. And if all that were
not enough, the patches flit through the
light's path and change the deflection
with time. A light wave from a star hits
a hot patch and deflects one way; the
patch scoots out of the light path. A
colder patch scoots into the path, and
the light deflects another way. The upshot is that in a telescope images twinkle. They shimmer, jitter, and change
hundreds of times a second, stretching
an image out by 10 to 20 times.
Astronomers call the effect on the
image of those changes in size seeing.
Any given night can have seeing that
changes little or fluctuates dramatically.
Astronomers say that looking through
the atmosphere is like looking through
a river or the air rising from a bonfire.
Images have no clear edges and sometimes no clear distinction between
neighboring objects. That is, they have
far less resolution.
Resolution
Resolution is fundamentally a measure of the sharpness of detail in an
image; the higher the resolution, the
greater the detail. Astronomers define
resolution in various ways, one of which
MOSAIC Volume 22 Number 4 Winter 1991 15
is the distance over which one can see
that two points are separate. (Human
eyes can resolve two points three centimeters apart at 100 meters.)
Astronomers measure resolution in
angles, in units called minutes of arc or
seconds of arc; 60 seconds is one minute, and 60 minutes is one degree. In
these units human eyes can resolve two
points separated by one arc minute. For
years astronomers thought that the resolution of which an optical telescope was
capable was limited by that ten-centimeter patch on the sky, an arc second
across, or two points five millimeters
apart at a distance of one kilometer.
"People have thought of it as a one-arcsecond atmosphere," says Wolff.
One arc second of resolution lets astronomers see things that are either relatively nearby or intrinsically large: Jupiter is 40 arc seconds across; globular
star clusters around the Andromeda galaxy are one or two arc seconds across.
A quasar, however, is about 0.0001 arc
second across. Quasars are extremely
old, appear to be pointlike, like stars,
and can have the brightness of ten thousand galaxies. Astronomers would like
to find out how quasars work, but the
0.0001 arc second image gets smeared
over one arc second. "A whole lot of detail is lost," says Holland Ford of the
Space Telescope Science Institute and
Johns Hopkins University. "You know
if s in there and you can't see it."
Telescopes in space
One solution to the problem caused
by the atmosphere has been to put telescopes in the still air at the tops of high
mountains located in the middle of
oceans or at the edges of continents. The
seeing on Mauna Kea, Hawaii, can be as
good as 0.4 arc second.
A better solution ought to be to put a
telescope above the atmosphere, in
space. Satellites in space have been successfully detecting wavelengths other
than the optical for years. The Hubble
Space Telescope, with a 2.4-meter mirror, operates in optical (and ultraviolet)
wavelengths at an intended resolution of
better than 0.1 arc second. The observations planned included the shapes of galaxies ten billion light-years away, black
holes in the centers of galaxies, the
clouds out of which stars form, and the
evidence for a scale by which to measure
distances in the universe, known now
only to within a factor of two.
Among other problems, unfortu-
16 MOSAIC Volume 22 Number 4 Winter 1991
nately, the Bubble's mirror was curved
wrong; instead of 70 percent of the light
hitting a focal point of 0.1 arc second,
only 15 percent of the light does. The
other 85 percent of the light is spread in
a smudge three arc seconds across. A
star seen through the Hubble is a large
bright cloud with a small brighter core.
Few of the planned observations dependent on sharp resolution can be done.
For the time being astronomers can
work around some of the Bubble's problem, observing bright objects and rectifying images by computer. They hope
in a few years to be able to correct the
problem by adding 'a mirror that curves
with the equal but opposite wrongness.
The Bubble's problems—in control
mechanisms and materials as well as in
optics - were engineering and management problems; they do not reflect on
the feasibility of telescopes in space.
What may limit the feasibility of space
telescopes, besides accessibility, is the
time they take from planning to launch.
The Hubble was originally suggested in
1946. Studies were done between 1962
and 1971; the first call for design of the
detectors on the telescope came in 1977,
and the telescope was launched in June
1990. During the 15 years between the
Bubble's design and launch, groundbased detectors became more sensitive
and larger, computers became smarter,
astronomers figured out how to make
mirrors larger, and a technology developed for military purposes went a long
way toward canceling the atmosphere's
shimmer. "I think the ground has
caught up with a lot of the things the
Hubble was supposed to do," says Wolff.
"Not all of the things—but our technology has come to the point where we are
pushing the Hubble hard."
Pushing the Hubble
One of the most dramatic changes in
optical telescopes on the ground has
been in the mirrors. Until the late 1980s
each larger mirror was scaled up from
its predecessor until mirrors were routinely half a meter thick, three and four
meters in diameter, and weighed in at
around 15 tons.
But these mirrors, if scaled up to eight
meters, would weigh 120 tons—so
much, says Angel, that gravity would
cause them to "droop." Warping of any
kind messes up the resolution.
Other than gravity, says Angel, the
enemy is temperature change. Massive
mirrors heat up and cool down slowly;
the air around them changes temperature much more quickly. So inevitably
the mirrors and the air surrounding
them will have different temperatures.
The difference causes much the same
shimmer in images as atmospheric seeing does. For each difference of one degree Celsius between mirror and air, the
shimmer adds to the image a blurring
of 0.4 arc second.
The solutions to the problems with
relative temperature and gravity have
been ingenious new designs, and for the
first time mirrors eight and ten meters
across have become feasible. With
eight-meter mirrors, astronomers can
see objects four times as faint and take
spectra on objects 10 to 20 times as faint
as they can with a four-meter mirror.
The solution for the heat problem has
been to keep a telescope at the temperature of the air around it. In the New
Technology Telescope, or NTT—a threeand-a-half-meter telescope the European
Southern Observatory built in La Silla,
Chile, to test several new technologies—the temperature of the building
housing the telescope is kept as cool as
the ambient air, as are the temperature
of the floor and the air around the mirror.
The solution to the gravity problem is
to make mirrors lighter. Astronomers
have several ways of doing this. Angel
builds one-piece mirrors not as thick
slabs but as glass sandwiches whose
"bread" Is two sheets of glass, each 25
to 28 millimeters thick, and whose "filling" is a lightweight glass honeycomb
up to 80 centimeters thick. An eightmeter mirror built the traditional way
would weigh 120 tons and sag four times
as much as a four-meter mirror does. An
eight-meter mirror built from Angel's
honeycombs would weigh only 14 tons
and sag as little as would a four-meter.
Angel's mirror laboratory will cast two
eight-meter mirrors for telescopes
owned in part by NOAG, the National Optical Astronomical Observatories,
among the largest single mirrors ever
to be cast. One NOAO eight-meter mirror,
sited high on Hawaii's Mauna Kea for
its excellent seeing, will be specialized
to work in both optical and near-infrared.
The other, sited on Cerro Pachon in
Chile, will be for observing the skies,
historically neglected, above the southern hemisphere. Together, the twin
eight-meter telescopes, being called the
Gemini, will provide coverage of the
whole sky. 'The telescope on Mauna
Kea ought to be the best Imaging
ground-based telescope in the world,"
says Wolff. Gemini is an international
project, funded by the National Science
Foundation, Great Britain's Science and
Engineering Research Council, and
other International partners.
Another way to make mirrors lighter
is to make them in pieces. The Keck
telescope on Mauna Kea will have a mirror pieced together out of 36 hexagons,
each 1.8 meters across. The segments
are positioned to act as a single mirror
ten meters across. The Keck, a private
gift from the W. M. Keck Foundation to
the University of California and the
California Institute of Technology, is
planned to go into operation In 1992.
Still another way to make mirrors
lighter Is to make them thinner. The mirror for the planned Japan National Large
Telescope, or JNLT, to be sited on Mauna
Kea by 1999, will be eight meters across
and 200 millimeters thick. The four mirrors of the Very Large Telescope, or VLT,
planned by the European Southern Observatory to be sited in Chile by 1998,
will each be eight meters across and 175
millimeters thick. (The ratio of the diameter to the thickness of the mirror in
a traditional telescope Is around six to
one; that ratio for the VLT mirrors is 46
to one.)
On the JNLT and the VLT, the
mirrors if unsupported would lop.
The floppy mirrors and the Keek's
segmented mirrors are not only lighter,
they can also be adjusted to counter
gravity's effect on the mirror. The technology for adjusting mirrors is called active optics. The Keek's segments are independently controlled: Behind each
MOSAIC Volume 22 Number 4 Winter 1991 17
segment are three little rods, or actuators, that move the segment side to side,
front to back, or up and down. "Each
motion of the actuators is five to ten
nanometers," says Keek's designer,
Jerry Nelson, an astronomer at the University of California at Berkeley, "and
the whole system is updated twice a second." The JNLT and the VLT are also
backed with actuators; the VLT'S technology is being tested on the NTT, which
also has an unusually thin mirror.
Active optics works. The NTT on its
first night in 1989 achieved a resolution
of 0.33 arc second, three times that of a
conventional telescope of the same size
at the same site on the same night. In
its first year of operation the NTT resolved for the first time the individual
stars in a cluster in the nearby Fornax
galaxy and found at optical wavelengths
some objects that had been seen only in
the radio. These may be a dense cluster
of stars and a black hole at the center of
the Milky Way. The Keek's resolution
in the near-infrared should be even better, around 0.25 arc second. (See "Buildings That Behave Like Machines" by
Kendrick Frazier, Mosaic Volume 11
Number 1 January/February 1980, and
"Astronomy From the Ground Up" by
Sandra Blakeslee, Mosaic Volume 17
Number 2 Summer 1986.)
Sharpening resolution
Mirrors are not the only technology
enabling ground-based telescopes to
challenge space telescopes. The VLT has
four mirrors because it is meant to be
used as an interferometer. An interferometer is an array of two or more telescopes, positioned so that the light from
all mirrors converges precisely into one
image. Interferometers, which have so
Galileo Galilei's spyglass
Galileo Galilei, born in Florence, Italy, taught mathematics at the University of Pisa and supplemented a faculty
income by making scientific instruments. In 1610, after he
built the first telescope, he wrote about the instrument
and its applications in a brief classic, Siderius Nuncius,
translated as The Sidereal (or Starry) Messenger. His preoccupations and methods sound remarkably familiar.
Early in 1609, Galileo wrote, he had heard rumors of a
spyglass "by means of which visible objects, though far
removed from the observer, were distinctly perceived as
though nearby." The spyglass had been made of eyeglass
lenses by a Dutch spectacle maker named Hans Lipperhey, whose patent application was denied because the
spyglass was too easy to copy. Meanwhile word spread
and other spectacle makers throughout Europe made and
sold other spyglasses, probably of poor quality. Using the
rumors, the "science of refraction," and "inspired by divine
grace," Galileo wrote, he re-invented the spyglass.
With his first instrument objects were three times as
close and nine times as large; with his second, 60 times
as large. "Finally, sparing no labor or expense," he wrote,
"I progressed so far that I constructed for myself an instrument so excellent that things seen through it appear
about a thousand times larger and more than thirty times
closer than when observed with the natural faculty only."
The following year, with his spyglass, Galileo detailed
the surface of the moon, found around faint stars "such a
crowd of others that escape natural sight that it is hardly
believable," and announced that the Milky Way is "nothing else than a congeries of innumerable stars. To whatever region of it you direct your spyglass, an immense
number of stars immediately offer themselves to view, of
which... the multitude of small ones is truly unfathomable." Most important, he said, he found four "little stars"
circling Jupiter, "never seen from the beginning of the
18 MOSAIC Volume 22 Number 4 Winter 1991
world right up to our day."
With fine grantsmanship, Galileo had given the doge
and senate of Venice the rights to manufacture the spyglass, and in return Galileo's contract at the University of
Padua was to be renewed for life and his salary increased.
When he found out the contract also precluded any further
salary increases, he changed allegiance and named the
moons of Jupiter the Medicea Sidera after the ruler of
Florence, Cosimo II de Medici. Cosimo gave Galileo a
court position and moved him to the nearby University of
Pisa as principal mathematician, a job with no duties. Galileo's spyglass was first called a telescope at a banquet
given in his honor by the Florentine Academia dei Lyncei.
Galileo understood almost immediately the implications
of what he observed through the telescope, and subsequent observations of Venus confirmed it: that Copernicus
was right, and the earth is not the center of the solar
system. In 1632 he wrote up the argument in Dialogue on
the Two Great World Systems. The two world systems were
the Copernican and the non-Copernican.
In those days the church regarded astronomy as a
branch of theology, and the theology was non-Copernican.
Galileo was tried by the Roman Inquisition for contradicting the scriptures, was forced to recant, and was exiled to
his own home in Arcetri on the hills above Florence.
But back In 1610 Johannes Kepler had written a letter
to Galileo, published as "Conversations with the Sidereal
Messenger." Galileo's discoveries, Kepler wrote, led him
to suspect that God gradually leads man "step by step
from one stage of knowledge to another." The "smug
philosophers" who think nothing is new under the sun,
Kepler wrote, should look back and reflect: "How far has
the knowledge of nature progressed, how much is left,
and what may the men of the future expect?" Optical astronomers could not agree more.
•
A. F.
MOSAIC Volume 22 Number 4 Winter 1991 19
Tony Redhead, Palomar Observatory
far been used primarily as radio telescopes, have one particularly important
advantage: Resolution goes way up. The
larger the mirror compared with the
wavelength of light, the smaller the size
of the image. Because the size of the
image is another definition of resolution,
larger mirrors have better resolution.
The four mirrors of the VLT will be
spread out over 100 meters and will have
the resolution of a 100-meter mirror.
The VLT mirrors, used as an interferometer, should have a resolution measured
in thousandths of an arc second. (See
"Optical Interferometry" by Marcia
Bartusiak, Mosaic Volume 14 Number
2; "Mapping the Sky" by Derral Mulholland, Mosaic Volume 20 Number 1
Spring 1989; and "Opening Another
Window" by Frederic Golden, Mosaic
Volume 21 Number 2 Summer 1990.)
"We've only recently started thinking
about optical interferometry seriously,"
says Robert Wilson of AT&T Bell Laboratories at Holmdel, New Jersey. One
20 MOSAIC Volume 22 Number 4 Winter 1991
early example is the Multi-Mirror Telescope (MMT) on Mount Hopkins in Arizona, built in the early 1980s. The MMT
has six 1.8-meter mirrors designed to be
used together as an interferometer. But
because the MMT'S effective diameter is
only around six meters, its owners, the
University of Arizona and the Smithsonian Institution, are replacing it with
a single, 6.4-meter honeycomb mirror
that will collect more light and have the
same resolution. Other optical interferometers have been tested on bright objects using the amateur's telescopes: Interferometers built out of off-the-shelf
Questar telescopes, says Wilson, "have
mirrors a few inches in diameter placed
10 meters apart, and you get information
corresponding to a 10-meter mirror in
resolution."
Interferometry is one of optical
astronomy's two highest priorities for
the next ten years. The decade report
on astronomy's goals by a National Research Council panel, published in
March 1991, recommended that, of the
moderately priced ground-based technologies, optical interferometry be given
second-highest priority.
Interferometry with the VLT awaits the
end of the century. Interferometry with
a second Keck, funded partly by the National Aeronautics and Space Administration, is in the works for some time
even later. "Well copy Keck I to make
maintenance easy," says Nelson, "and a
few years later, well do interferometry
with both of them." Also in the works is
a telescope called Columbus, an interferometer built of two eight-meter mirrors and so far funded by the University
of Arizona, through the Arcetri Astrophysical Observatory, and the government of Italy. Columbus will be built on
Mount Graham in Arizona and should
"see first light" in 1996. "Optical interferometers with resolutions of 0.001 arc
second," says Wolff. "It would blow your
mind. You ought to be able to see the
structure around black holes at the centers of quasars."
Compensating for the atmosphere
The NRC'S report accords its highest
priority to a new technology called adaptive optics—which, like interferometry,
drastically improves resolution; unlike
interferometry, it is a device that can be
attached to any optical telescope. While
active optics compensates for the effects
of gravity, adaptive optics compensates
for the shimmering atmosphere.
A wavefront of light, just before it hits
the atmosphere, is a plane wave, a nice,
lat wave. As it moves through the atmosphere and encounters hotter and
colder patches of air, the flat wavefront
develops tilts in different directions and
at different angles. The wavefront that
hits the telescope mirror, then, is a series of tilts, each of which changes every
hundredth of a second.
Adaptive optics sends this messy
wavefront from the mirror to a device
that senses the tilts in the wavefront.
That sensor in turn sends the pattern of
tilts to a computer. The computer controls the positions of a group of actuators
attached to a second, or adaptive, mirror, "a little tiny guy, at most several
inches across and an eighth of an inch
thick," says Angel. Depending on the
pattern of tilts in the wavefront, the actuators change the' curvature of the
adaptive mirror so that it reflects the
equal but opposite pattern of tilts. As a
result the wavefront that the adaptive
mirror sends on to the astronomer is
again a flat plane wave. The whole process repeats a hundred times every second. "The mirrors move with the response of a loudspeaker," says Angel.
The wavefront sensor, however,
needs a lot of light to make its compu-
tations and can therefore work only with
unusually bright objects. To observe
faint objects astronomers would need to
choose those that are near some unusually bright object, make adaptive corrections based on the bright object, then
apply the corrections to the faint object.
That means that the faint object needs
to be within a few arc seconds of a bright
object. "The chances of this working
out," says Sam Durrance, adaptive optics researcher at Johns Hopkins University, "are probably infinitesimal, and certainly impractical."
The solution is straight out of technology developed for missile defense:
Make a bright star artificially. Do this
by shooting a laser beam into the-upper,
less-turbulent layers of the atmosphere.
There it reflects off molecules or makes
atoms fluoresce. In either case it makes
an artificial star. If it works the laser
flashes, the artificial star shines, the
adaptive mirror vibrates, and the wavefront straightens out, all hundreds of
times a second. The astronomer sees a
sharp, distinct, steady star.
How well adaptive optics works in
practice is still a little unclear. The Strategic Defense Initiative Office of the Department of Defense has built the only
complete system-—wavefront sensor,
adaptive mirror, and laser. The purpose
was military; the specifics of the system
have since been released to NSF for use
by astronomers. The system SDIO tested
had a laser that placed an artificial star
six kilometers high and corrected an
adaptive mirror 3,000 times a second.
Optical astronomy and the human eye
The visible colors, from red to violet, are a tiny fraction
of all wavelengths of the electromagnetic spectrum. Optical astronomy includes the visible and those wavelengths
just beyond the visible and a little way into the infrared
and ultraviolet. It is still a tiny fraction, but it has an outsized importance in astronomy.
Optical astronomy is disproportionately important for
three reasons: physical, historical, and psychological. The
physical reason, says Garth Illingworth, astronomer at the
University of California at Santa Cruz, "is that a substantial
fraction of the matter in the universe—stars, galaxies,
gas—emits radiation in optical wavelengths." That includes the sun. Humans evolved to see in wavelengths
that the sun emits and earth admits and that, happily, cany
much of the information about the rest of the universe.
"If you could set up only one telescope," says Illingworth,
"your best choice would be [the optical]."
The historical reason is that the optical region has what
Illingworth calls a huge historical database. From 1.609
and Galileo's first observations until the 1940s, when the
first radio telescopes were built, astronomers took most
of their data in the optical.
The psychological reason, says Sidney Wolff, director
of the National Optical Astronomical Observatories, is that
we have "an obvious prejudice." As a result of the prejudice, says Illingworth, "if people make observations in x
rays or radio waves, they don't talk about identification of
the source until they go to an optical map to find out what
the hell it is." Wolff quotes Sandra Faber, Illingworth's
colleague at Santa Cruz: "You can have unidentified objects in other wavelengths, but never in the optical."
None of this is to say that observations at nonoptical
wavelengths are not important or fascinating, only that
somehow, to people, optical makes more sense.
A. F.
MOSAIC Volume 22 Number 4 Winter 1991 21
22 MOSAIC Volume 22 Number 4 Winter 1991
When tested on a telescope with a 60centimeter mirror, the setup took pictures in visible wavelengths of bright
stars with resolutions (0.2 arc second)
limited only by the size of the mirror.
Before SDIO systems can be retrofitted
routinely to working telescopes, the systems will have to work on dimmer objects. That means they would need to be
used with three- or four-meter mirrors,
which in turn means thousands of adaptive actuators instead of SDIO'S 241. The
systems would also need to place artificial stars above more of the turbulence,
nearer the top of the atmosphere 90 kilometers up. Researchers at the Massachusetts Institute of Technology's Lincoln Laboratory who helped develop
and so can be used only on bright stars.
Another is that their systems can correct
images taken only in infrared wavelengths, which are longer than visible
wavelengths. (The atmosphere affects
longer wavelengths less, so infrared images are easier to correct.) The European Southern Observatory's prototype
adaptive optics system recorded images
of bright stars with a resolution of 0.18
in infrared wavelengths.
Compromises notwithstanding, some
of the astronomers' systems are ingenious. Sam Durrance is building a system whose adaptive mirror is a celluloid
film and is moved not mechanically but
by changing voltages across it. Francois
Roddier at the University of Hawaii is
working on a system that may not need
a laser, because the wavefront sensor
itself is sensitive enough to detect faint
objects. Angel has tested on the MMT a
system that substitutes for a wavefront
sensor a neural network computer program trained to relate the distortion in
the image directly to the corrections
made to the adaptive mirror. (See 'The
Brain as Template" by Ann Finkbeiner,
Mosaic Volume 19 Number 2.)
But how good will the systems be?
"Five years ago there was a lot of skepticism about adaptive optics," says Durrance. "Now people just assume it's going
to happen, and it is." But no one thinks
that adaptive optics is the final solution.
Even when the systems work at visible
wavelengths, have sodium lasers and
more actuators, and cost a reasonable
fraction of the whole telescope, they will
still be able to correct only the patch of
sky near the artificial star. That means
that astronomers wanting to observe extended objects, like clusters of galaxies,
with high resolution will have to resort
to telescopes in space. "I'm not in the
foregone-conclusion camp about adaptive optics," says Sandra Faber, astronomer at the University of California at
Santa Cruz. "At the very least, it will be
maybe 20 years before they're a routine,
trouble-free part of the arsenal."
in space or on the ground?
Do bigger mirrors with fancier designs, better light-gathering capabilities,
and better resolution then mean the
foreseeable future of optical astronomy
is probably on the ground? The answer
is unanimous, obvious, and unspectacular: yes and no. What can be done
on the ground should be; what cannot
should be done in space.
The advantage of ground-based telescopes turns out to be a negative:
They're not in space. In principle ground
telescopes can do no observations better
than the space telescopes can. "But
space is going to be given a real ran for
its money by the ground-based observators," says Riccardo Giacconi, director
of the Space Telescope Science Institute. "Because," he observes, "in space
we have problems."
One problem is that space telescopes,
SDIO'S system say the problem is not
technology but who will pay for developing the technology. Aram Mooradian
at Lincoln Laboratory says he has already made an artificial star with a laser
tuned to make sodium atoms fluoresce
and has placed it in a sodium-rich layer
of the atmosphere 90 kilometers high.
Meanwhile astronomers have been
tinkering with their own adaptive optics
systems. Because most astronomers
lack SDIO'S resources, they have had to
make compromises. One compromise is
that most of their systems have wavefront sensors and mirrors but no lasers
MOSAIC Volume 22 Number 4 Winter 1991 23
once launched, are all but inaccessible:
"Gradually, it's dawned on everyone that
when it's up there, it's up there," says
Garth Illingworth, astronomer at Santa
Cruz. Correcting the mirror would have
been an annoying but easily fixable
problem in a ground telescope; in the
Hubble, correction will wait three-and-ahalf years, unless more pressing repairs—to gyros and solar panels—take
precedence. Another problem is that
telescopes in space cost 100 times as
much as comparable telescopes on the
ground. The reason, says John Bahcall,
astronomer at the Institute for Advanced
Study in Princeton, New jersey, who
chaired the NRC'S decade report, is "the
necessity of making things so well that
you never need to take a screwdriver or
Scotch tape to them and that they can
withstand the rigors of the launch."
The overriding problem is that space
telescopes, regardless of wavelength,
take so much time to design and build.
In 1978 Giacconi chaired the science
working group planning an x-ray space
telescope called AXAF and, he says, "we
are now planning launch in '98." That 20
years has affected the whole field of xray astronomy, not the least because it
is a significant fraction of an astronomer's research lifetime. "We have
burned a whole generation," says Giacconi. In the x-ray region, astronomers,
Giacconi says, "have been living off the
archives of Einstein [x-ray satellite],
which finished flying in '81, or going
around begging a little data from the
Japanese or now the Germans.
"In this country, if you're not in AXAF,
you're not in x-ray astronomy," he continues." On the other hand, if you're in
AXAF, you're not in x-ray astronomy either. It's virtual science, virtual data; nothing is actually happening. Today, if a
student asked me whether to go into
space astronomy, I would say no. Because the time schedules are so long, it
doesn't make any sense." By comparison, Illingworth points out, "the Keck
has come from nowhere to being virtually operational in a decade."
That 20 years also means, says Wolff,
that "what you launch into space is usually ten-year-old technology." On the
ground, says Illingworth, "you can
readily take advantage of new instrumentation. We will be able to put things
on Keck you can't even buy now. We
will do upgrades that aren't available
now." The upshot is, what with ground
astronomy's new technologies and
space astronomy's impracticalities, says
Faber, "if the future of optical astronomy
is anywhere, it's on the ground."
The NRC'S decade report recommended some changes that would make
optical astronomy in space more feasible. One recommendation was that NASA,
Access: a nagging problem
Most of the observing that optical astronomers want to
do must be done on one or another of the 15 major (three
meters and up) working telescopes in the world. Access
to these telescopes is a problem, one way or the other,
for the whole astronomical community.
The foundation of the problem is that telescopes are
expensive. New ones are costing anywhere from $80 million for the one Gemini on Mauna Kea, to $94 million for
the Keck, to $250 million for the Very Large Telescope,
or VLT. The United States, through the National Optical
Astronomical Observatories, or NOAO, built two telescopes
in the 1970s and will fund two more (the Gemini).
The national telescopes are open to all U.S. astronomers. (National telescopes can give time to only one of
four astronomers who apply. Of the 4,200 professional
astronomers in the United States, roughly three-quarters
may be dependent on publicly owned telescopes.)
The rest of the country's optical telescopes are privately
owned, by universities or university consortia, which restrict access pretty much to their own scientists. (See 'The
instruments: whose and where" accompanying this article.) The private telescopes allot roughly 90 percent of
observing time to astronomers in their universities and 10
percent to outsiders.
An astronomer with access to a private telescope can
observe somewhere in the range of 3 to 15 nights a year.
An astronomer with access to a national telescope, says
Sidney Wolff, director of NOAO, typically gets three nights
per semester. Nights are counted as 10 or 11 hours long.
"If your typical exposure time is a couple of hours, which
is common," says Wolff, "you can see you're not going to
get many observations [at a public instrument] or much
data. Or it could be cloudy and you're out of luck until the
next time."
24 MOSAIC Volume 22 Number 4 Winter 1991
Faber. Telescope time—a thorny issue.
which backs all space astronomy,
should increasingly back smaller, lesscomplex projects, put those projects on
a shorter time scale, and launch the projects with expendable launch vehicles,
not the often-delayed and risky shuttle.
In the meantime, says Giacconi, "on the
ground, the Keck, the Keck II, and the
Gemini are going to keep [astronomers]
busy for quite a while."
What cannot be done on the ground
The observations that can be done
only from space are those the atmosphere blocks out entirely: long radio
waves, long infrared waves, the far-ultraviolet, x rays, and gamma rays. 'You
can't do those problems on the ground,"
says Sidney Wolff. "Space will always
give you wavelengths you'll never reach
from the ground."
In addition, the atmosphere, earth,
and even the telescope all glow in the
infrared, making observations from the
ground in those wavelengths difficult.
"Infrared telescopes in space," says
Wolff, "would be as cold as outer space/'
Telescopes for all blocked-out wavelengths are either now in space or are
going there soon.
And for the further future, some astronomers think that a telescope on the
ground will never have the resolution or
the wide field of view of the same telescope in space: "If you want to look at
faint, fuzzy stuff," says Bahcall, "you'll
always have to do that from space."
Illingworth agrees: "Ground-based telescopes allow you to meet some goals,
but you don't get 100 percent there.
These new techniques are only a halfway step to where you really want to be."
The ideal telescope, they say, is a large
telescope in space.
Astronomers are already planning for
telescopes in space that sound fairly
Although many private telescopes allot observing time
for outsiders, most outside astronomers either do not get
time or do not even apply for it. "It's a tough business to
be an astronomer in a university with no telescope," says
Wolff. "It's even tougher to run graduate programs in
astronomy where graduate students need access to complete their work."
For all astronomers the result is a split in the community. Says Sandra Faber of the University of California at
Santa Cruz (who will have access to the Keck), "any graduate who's good, serious, and committed will fight tooth
and nail to get into a university with a telescope.
"Of all the astronomers who are making the greatest
contributions, how many rely on public telescopes? I'd say
a quarter," says Faber.
One solution: The National Science Foundation, which
funds NOAO, has also begun to cosponsor (and secure access to) private telescopes. The first such is a 3.5- meter
telescope on Kitt Peak called WIYN telescope for its participants: Wisconsin, Indiana, Yale, NOAO. The NOAO'S two
new telescopes may or may not ease the problem; Bahcall
says new telescopes cause "population explosions."
Another solution: Astronomers get out of optical astronomy and into space astronomy at other wavelengths.
A third solution: Many astronomers use data in archives
collected at NOAO or by space astronomers. But only data
collected at public installations is archived; data collected
by astronomers using private telescopes can remain private property. Archiving data collected privately sounds
like a solution and was recommended by the National
Research Council; but because each instrument must have
its separate archive, says Peter Boyce of the American
Astronomical Society, "archiving for private instruments
is unrealistic."
blue-sky. Illingworth and others are doing the preliminary planning for a tenmeter telescope in space, called the
Next Generation Space Telescope. The
NGST could see things as faint as 32d
magnitude, and could resolve things as
small as 0.007 arc second. "We could
resolve structures in galaxies at any red
shift," Illingworth writes, "with the resolution with which we now study the
nearest clusters of galaxies." Angel
wants a 16-meter mirror in space: uWe
asked ourselves what to put in space to
see if solar systems have planets like
earth," Angel says. Infrared satellites,
looking back at the earth's atmosphere,
clearly see evidence of ozone in the atmosphere's spectrum; and the ultimate
origin of ozone, says Angel, is life. "A
16-meter in space would be capable of
finding that kind of evidence of life on
planets around other stars."
Angel also wants to build on the
" I t s a tough business, a very tough business,"
says Wolff. Vera Rubin of the Carnegie Institution of
Washington, which owns telescopes, agrees: "Can you
imagine doing physics with access to a lab five days
a year?"
•
A. F.
Current instruments: whose and where?
/ elescope
Keck
Mirror Size
(meters)
10
Country
Access*
public private
USA
X
6
USSR
Hale
5
USA
Multi-Mirror Telescope
4.5
USA
William Herschel
4.2
Britain
X
NOAO Cerro Tololo
4
USA
X
Anglo-Australian
3.9
Britain,
Australia
X
NOAO Mayall
3.8
USA
X
U.K. Infrared
3.8
Britain
X
Canada/France/Hawaii
3.6
Canada,
France,
USA
European Southern
Observatory
3.6
ESO**
X
New Technology Telescope
3.6
ESO**
X
Max Planck Institute
3.5
Germany
X
Lick Observatory
3
USA
Bolshoi Alt-Azimuth
X
X
X
X
X
*No telescope is internationally public—that is, open al! the time to any astronomer in the
world. Both U.S. and European public telescopes are open to astronomers from the
countries that own the telescopes, but they may or may not offer small amounts of
telescope time to foreign astronomers. The private telescopes in the United States are
open primarily to the institutions that own them, but do open small amounts of time to
any astronomer in the world.
"Italy, Germany, Switzerland, Belgium, Denmark, France, Sweden, Netherlands
MOSAIC Volume 22 Number 4 Winter 1991 25
ground a 32-meter telescope that relies
on adaptive optics. Present adaptive mirrors are small enough to have their surfaces changed quickly. Angel would
build his 32-meter like the Keck, only
out of smaller pieces. 'They might be a
half-inch thick," he says, "and ten inches
or so across." The pieces would be supported by actuators but would be small
enough that they could be moved not
on the long time scale of active optics
but on the minuscule time scales of
adaptive optics. "If you made a big mirror out of little, little pieces," he says,
"you could move them fast enough to
make [adaptive] corrections. Nobody's
done that"
Probably the airiest of blue-sky space
projects is an interferometer on the
moon, NASA plans ultimately to station
astronauts on the moon. Astronomers
say that if NASA does create a lunar station, the moon would be a great place
to do astronomy, particularly interferometry. But most astronomers, and the
NRC'S decade report, agree with Wolff: "I
don't think astronomy is a reason to go
to the moon. It's difficult to imagine a
problem I want to solve so badly it's
A change of pace
The pace at which new telescopes are being built on
the ground appears to be slowing. Such telescopes are
usually built by national science-funding agencies or partnerships of universities. Since the spring of 1991, three
different partners, citing money problems, have pulled out
of three different telescope projects. The future of groundbased optical astronomy still looks promising, but the
promise will take longer to fulfill
In the spring of 1991, Johns Hopkins University backed
out of a $10 million pledge to the eight-meter Magellan
telescope. The following June the National Research
Council of Canada announced it would not add its $44
million to the two eight-meter Gemini telescopes, one each
in Hawaii and Chile. And in September Ohio State University abruptly reneged on a promise of $15 million for the
two eight-meter mirrors of the Columbus interferometer.
(See main article).
The defaults leave the telescopes alive but behind schedule and maybe a little maimed. The partners left on the
Magellan—the University of Arizona and the Carnegie
Institution of Washington—will proceed with the telescope, but they may scale it down to 6.5 meters and in
any case are looking for another partner. The partners left
on the Gemini—the National Optical Astronomy Observatories, or NOAO, and Great Britain's Science and Engineering Research Council—will build the telescope on
Mauna Kea as planned and are looking for another partner
or two before building the companion instrument in Chile.
26 MOSAIC Volume 22 Number 4 Winter 1991
The partners left on the Columbus—the University of
Arizona and Arcetri Astrophysical Observatory of Florence, Italy—face a project in stasis, at least for now. Even
with Ohio State, the partnership had enough money for
only one mirror and needed a fourth partner to build a
working interferometer. According to Roger Angel, whose
mirror laboratory at the University of Arizona at Tucson
was to build the mirrors, "Columbus is now on hold." If
more money is not forthcoming the options are to build
a one-mirror Columbus, usable as a conventional telescope until a second mirror can be added, or to redesign
the instrument entirely as a conventional single-mirror
telescope. The remaining partners, says Angel, are understandably reluctant to see that happen.
The obvious solution is to raise the number of partners
per telescope so that each partner puts up, say, $5 million
and receives, say, 15 percent of the observing time. But
the whole point of owning a private telescope is to own a
large amount of observing time. Anyone wanting only a
small amount of time is likely to get it on NOAO'S national
telescopes without cost. For an institution wanting to compute the trade-offs on a $60 million telescope like Columbus, the break-even point is probably between $5 million
and $7 million.
The need to find new partners to replace the defaulting
ones will inevitably slow ail three projects. 'There will
certainly be some reshuffling," says Angel, "but no doubt
in the long term, well get there."
•
A. F.
worth the money it would take to establish an observatory on the moon." But,
"if you're going anyway, take along the
experiments," Wolff adds. "You could do
fascinating astronomy along the way."
With an optical interferometer on the
moon, says Giacconi, "ultimately you
could hope to do microarc seconds."
A microarc second is 0.000001 arc second, enough for Vera Rubin, astronomer
at the Carnegie Institution of Washington, to test what she says is "a crazy
idea" she's been thinking about for the
past year: "to find out whether the universe has a shear." Astronomers have
known for a long time that the universe
expands away from the solar system because galaxies are traveling away, along
the line of sight. They do not know
whether the universe is also moving in
other directions, and for that they would
need to see galaxies moving across the
line of sight as well. "In the nearest large
galaxy, Andromeda," says Rubin, "the
nucleus is moving [across the line of
sight] probably at 200 kilometers a second, or one second of arc every 20,000
years. If we could get to microarc second
resolution, we could measure this in a
year. Lots of room for bright ideas here."
The future
The future of optical astronomy, like
the future of everything else, is unknowable. No one knows with certainty how
effective the new ground-based technologies will be, what the Hubble will or
will not be capable of, or to what vagaries
funding for astronomy will be subject.
But astronomers, like everyone else,
must construct the future anyway, so
they must balance ground against space.
Telescopes on the ground cost less, are
easier to maintain, and take a more reasonable amount of time to build. Telescopes in space can detect a much wider
range of wavelengths and are capable of
higher resolution and a wider field of
view. Telescopes on the ground are
more practical; telescopes in space may
allow better science.
Astronomers seem to agree that the
balance for now favors the ground. For
the near future, says Faber, astronomers
need to "work hard in the next couple
of years to test the promising new
ground-based technologies." The fruits
of this work, she continues, "will be
needed in a little while, when planning
for the next generation of space instruments begins in earnest."
The far future may belong to space or
may not. Astronomers are making plans
in case. In the last analysis, the telescopes astronomers want are those that
see fainter and In more detail, wherever
they are. "We need bigger and better
telescopes, always," says Wallace Sargent, of Caltech. "When you produce
new capabilities, you discover more."
This is not greed. The only way to get
at the universe is through telescopes,
and when the limits of a telescope are
reached, so are the limits of knowledge.
"You don't just go out and arbitrarily
build bigger and better telescopes," says
Illingworth. "You do it for a reason.
We're not here to build monuments.
We're here to do science."
And when astronomers are asked
what science they want to do, they get
a little vague and give the same answers
they gave when asked that question
about the Hubble: They want to study
quasars, faint galaxies, star formation,
other solar systems. The reason for the
vagueness Is that the universe is both
big and inaccessible.
"With the universe," says Sargent,
"we're nowhere near defining problems
narrowly. It's like you're a Victorian ex-
plorer looking for the source of the Nile.
It's a large body of water bringing fertility to all Egypt, and you naturally want
to know where it came from. And when
you ran across the Pyramids, if you have
any sense at all, you'll investigate them.
And when you notice a new kind of crocodile which seems more plentiful down
a side stream, you'll look for more of
them." In short, astronomers truly do
not know what they might find.
"I point the damn telescope at the
sky," Sargent says, "and see what's
there." Rubin answers the question another way: "I've always believed if we
had ten times as many instruments, we'd
know ten times as much." Does Sargent
agree? Yes, he does, he says, "My point
exactly."
•
The National Science Foundation has
contributed to the support of the research
described in this article principally
through its National Optical Astronomy
Observatories and Astronomical Instrumentation and Development programs.
The National Aeronautics and Space Administration is the United States' principal supporter of space astronomy.
MOSAIC Volume 22 Number 4 Winter 1991 27