Download 11 01 04 Portable M Class Astro

Portable Meter-Class Astronomy
Developmental and Observational Programs
Russell Genet and Bruce Holenstein
Introduction
We discuss below a specialty branch of the astronomy family tree—the development and
use of portable meter-class telescopes. These telescopes trace their origins to John Dobson and
his innovative ideas for making relatively large aperture alt-az telescopes from low cost materials
with ordinary tools. These alt-az telescopes more closely resembled their large mountaintop altaz brethren than other small amateur telescopes which, at the time, were almost all of very modest
aperture and equatorially mounted.
Amateur telescope makers have continued to refine alt-az telescopes which, not
surprisingly, have come to even more closely resemble their huge mountaintop cousins. Heavy,
enclosed Sonotubes have been replaced with lightweight, open trusses. Mirrors have become
thinner and significantly larger in diameter—supported by increasingly sophisticated whiffletrees.
Computer controlled slewing and tracking have been added. Temperature compensated focusers
and instrument rotators facilitate long-exposure astro-imaging.
The Alt-Az Initiative was founded in June 2007 with the express purpose of facilitating
the evolution of portable, lightweight, low cost, meter-class alt-az telescopes. The Initiative, from
its inception, aimed at achieving much larger aperture portable telescope than currently existed,
setting 1.5 meters as its initial goal. Initiative members realized, from the outset, that it would
pay to closely examine the technologies already developed for modern mountaintop telescopes,
so they initiated a purposeful “tech transfer” program that would, at low cost, innovatively
emulate appropriate technologies such as direct drives, thin active primary mirrors, deformable
secondary mirrors, multiple mirrors, etc. Thanks to “free” ATM labor, rapid advances in low cost
electronics, innovative engineering, sharing of information, and rapid prototyping, Initiative
members are successfully transferring enabling technologies from larger to smaller alt-az
telescopes, thus advancing the era of portable meter-class astronomy.
The primary application of portable meter-class telescopes—discussed in more detail
below—has always been, and will likely continue to be, visual observations. Portability allows
telescopes to be safely stored at homes and schools in light polluted cities, yet operated just hours
away under dark skies. Large apertures allow faint objects to be observed, while low costs
facilitate widespread ownership and use. Thanks to computer-controlled slewing, tracking, and
instrument rotation, portable meter-class telescopes are beginning to be used for Astro-imaging
and, increasingly, in various areas of instrumented scientific research.
To facilitate discussion, we have divided portable meter-class telescopes into three basic
types: visual observation telescopes, general-purpose imaging telescopes, and on-axis light bucket
telescopes. Each type is discussed below.
Visual Observation Telescopes
These “eyeball at the eyepiece” telescopes require high quality optics, and need to be
easily transported and quickly set up. They can be either “pure” non-computerized “push Dobs,”
or they can be equipped with modest performance go-to/tracking systems. Image rotators,
generally, are an unnecessary and hence unwanted complication.
The main use of these telescopes is recreational observing, but they are also used for
public outreach, education, and scientific observations such as searches for supernovae, faint
visual double star astrometry, and faint visual variable star photometry. The recreational, public
outreach, educational, and scientific aspects of these telescopes meld together nicely. These
telescopes appeal to a wide and diverse set of users. Think modern large aperture Dobs.
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Emphasis is on 0.4 to 1.0 meter apertures, fast optical systems, convenient portability, light
weight, and affordability.
General Purpose Imaging Telescopes
These telescopes also require high quality optics. One of the most important features of
telescopes intended for astro-imaging is their etendue—the product of their aperture in square
meters and their well-corrected field in square degrees. The eye inherently senses the total
information content of an image, and it's a basic fact of the physics of astro-imaging that the total
information content of an image (the product of the signal-to-noise ratio and the total number of
resolution elements in the photograph) is a linear function of the etendue of the telescope and the
square root of the exposure time. Thus, the best systems for astro-imaging have large sensors
attached to large telescopes with high resolution, large angular fields-of-view, and relatively short
focal ratios.
To stay on target, telescopes intended for astro-imaging require either superb long-term
tracking or autoguiding. Because they are alt-az telescopes, their fields rotate, so accurate
instrument de-rotation is required. Operating out in the open, general purpose imaging telescopes
need to be resistant to wind gusts. This can be achieved by designing inherently stiff telescopes
with low aerodynamic cross sections and equipping them with fast response control systems and
rapid tip/tilt image stabilization.
These telescopes are not only useful for recreational astrophotography, but also for the
many science projects that utilize CCD cameras and other sensors for astrometric, photometric,
and spectrographic observations. Everyone, scientists included, appreciate large aperture, high
resolution, wide field-of-view, general purpose telescopes which they can transport to their
favorite dark sky site and set up in minutes. Think the perfect portable meter-class telescope.
As with visual telescopes, the greatest interest is between 0.4 and 1.0 meters. Larger than
this, high performance, general purpose telescopes rapidly get prohibitively heavy and expensive.
However, for some science applications, larger effective apertures can be achieved by forming an
array of several general purpose imaging telescopes. Images or data can be combined
electronically or optical fibers brought together to feed a single detector, different color bands can
be observed simultaneously, or telescopes can be spread out in a line to “map” the shape of an
asteroid. Such arrays of general purpose imaging telescopes would be adaptable to a wide
number of uses.
For the same aperture, general purpose imaging telescopes are necessarily more
expensive than visual telescopes, so they occupy a different niche. There are no free lunches.
On-Axis Light Bucket Telescopes
Light bucket telescopes are light concentrators used in those areas of astronomical
research where vast quantities of ultra low cost, on-axis photons are required. There is no more
authoritative definition of astronomical terms than the Cambridge Dictionary of Astronomy
(Mitton 2001). Mitton’s definition:
Light bucket: a colloquial expression for a flux collector.
Flux collector: a telescope designed solely to collect radiation in order to measure its intensity or
to carry out spectral analysis. No attempt is made to form an image so a flux collector can have
a more crudely figured reflective surface than a conventional telescope.
Light bucket telescopes are advantageous in those situations where—compared to the
noise from the sky background—the noise from one or more other sources is dominant. Putting it
the other way around, light bucket telescopes can be advantageous when the sky background is a
small or nearly negligible source of noise. This situation can occur when:
(1) the object being observed is very bright,
(2) the integration times are very short and hence photon arrival noise becomes important,
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(3) scintillation noise becomes a dominant noise source,
(4) the bandwidth is very narrow or the light is spread out as in spectroscopy resulting in
significant photon arrival noise, or
(5) noise from the detector is dominant (as it can be in the near infrared if the on-axis diameter of
the detector is significantly larger than the seeing spot size).
The on-axis optical performance of light bucket telescopes need not be any better than
average lowland seeing and, for many types of observation, can be significantly worse. Since
observations are on-axis, instrument rotators are not usually required. Furthermore, optical
systems for on-axis (or very narrow field-of-view) telescopes can be easier to design and fabricate
than for wide field-of-view telescopes. For example, it is very difficult, over a wide field-ofview, to correct the spherical aberration in a telescope with a large aperture, fast, spherical
primary mirror. On axis (or for a very narrow field-of-view) the correction is relatively straight
forward, thus not only allowing low cost spherical primary mirrors to be utilized. Opticians we
have talked to point out that the real work in mirror making is not obtaining a spherical surface—
the natural surface you get when you rub two pieces of glass against each other, but in forcing the
surface to become parabolic and not revert back to being a sphere. The faster a parabolic mirror
the more difficult it is to make, but this is not the case for spherical mirrors. Finally, spherical
mirrors have the advantage that they can be combined together in a multiple mirror telescope to
form a much larger spherical surface. Two of the world’s largest aperture telescopes utilize
multiple spherical mirrors—the Hobby-Eberly and South African Large Telescope (SALT)—
utilize multiple spherical surface, hexagon shaped mirrors to form 11 meter telescopes that were
built at a fraction of the cost of the 10 meter Keck telescopes.
Of course there are no free lunches, and because light buckets have, at best, narrow
fields-of-view, so they are not, essentially by definition, general purpose telescopes. Many
astronomical telescopes are specialized. The Hobby-Eberly and SALT spherical primary mirror
telescopes mentioned above are specialized spectroscopic telescopes. The robotic telescopes at
the Fairborn Observatory, which began operation in 1983, were specialized for aperture (on-axis)
high-precision photometry. In these and many other instances, the telescope itself has been
optimized for a single, dedicated function. Rather than maximizing etendue, light bucket
telescopes march to the beat a different drummer: maximizing, for a given cost, the number of
target photons that fall within a reasonable spot size. Think photon economics.
The "sweet" aperture range for portable light bucket telescopes is 1 to 2 meters, with
emphasis on the high end. Arrays of 2 meter portable light bucket telescopes would be very
useful. Seven 2 meter telescopes would have the light collecting area of a single telescope larger
than 5 meters in aperture. Such an array could be formed into an intensity interferometer that
could image details on the surfaces of large nearby stars. It is surprising that an array of on-axis
high speed photometric telescopes could produce an image the largest aperture, high resolution
telescopes cannot, but several groups are pursuing the quantum effect intensity interferometry
initiated by Hanbury-Brown many decades ago in Narrabri, Australia, when he directly measured
the diameters of many of the closest stars with a two-telescope interferometer.
Set up time for on-axis light bucket telescopes can be longer than other types, as this is
not recreation per se, just grunt-work observational science. Similarly, transportation
requirements can be more demanding (e.g. sizeable but still roadable trailers).
Portability
We hasten to clearly state at the outset that we are all in favor of fixed-location
observatories. Many folks are fortunate to live under dark night skies, and for them, a fixedlocation observatory in their back yard or on or near their campus makes lots of sense. For those
not so blessed, owning or sharing a telescope at a remote, good-weather dark site is becoming an
increasingly attractive and economically feasible possibility.
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It is also worth noting that operation of portable telescopes from a fixed-location
observatory is very common. Although the telescopes are perfectly capable of being transported
elsewhere, the owner chooses to keep them at home except perhaps for the occasional summer
star party or a move due to job relocation or retirement. Some of us are blessed with exceedingly
fine observing weather for months on end, and can simply leave a portable telescope out in the
open in our yard. When the weather turns bad or we head out for a vacation, we can simply roll
our telescope inside or take it with us.
Howard Banich’s now classic definition of portability requires that, on arrival at the
operational location, the telescope can be extracted from its transport vehicle and assembled by
no more two persons in a half hour or less (see Howard’s “Telescope Portability: Moving a Big
Telescope Doesn’t Have to Be Difficult,” in The Alt-Az Initiative: Telescope, Mirror, and
Instrument Developments). Of course a telescope can still be useful even if it doesn’t meet
Howard’s requirement, but its likelihood of frequent use will not be as great. A related issue is
the time and effort required to align a telescope once it gets dark. Computerized alt-az telescopes
with sophisticated alignment systems can generally be aligned much faster than noncomputerized or equatorial telescopes. Built-in GPS and electronic compass telemetry can shave
minutes off of the alignment process. Sidereal Technology’s control systems not only incorporate
totally automatic alignment, but mount modeling. Plate Solve XP, written by Dave Rowe handles
Sidereal Technology’s automatic field recognition and centering.
While it is obvious that if a telescope requires an 18-wheeler to move, or an industrial
crane to emplace, it isn’t portable, there are various grades of probability. We suggest, below,
four such grades.
Ultra-lightweight telescopes are designed to enable a single person to transport and set
up a sizeable aperture telescopes. Two excellent examples of ultra lightweight telescope are
those designed and built by Mel Bartels (http://www.bbastrodesigns.com/trilateral.html) and Greg
Babcock (http://www.synrgistic.com/astro/24inch.htm). Mel defines ultra-lightweight telescopes
as telescopes where the weight of the entire telescope sans primary mirror is approximately onehalf the weight of the lightweight primary mirror itself. The mirror in Mel’s 20 inch telescope
weighs 50 lbs, while his entire telescope, including the mirror, weighs only 75 lbs.
Airline check-in telescopes can be carried as “luggage” on normal domestic or
international flights. Most airlines allow, for an additional fee, overweight (70 lbs) and oversize
(80 linear inches) bags. Both ultra-lightweight and airline check-in telescopes should easily fit
within ordinary cars for transport.
SUV/pickup telescopes range from fairly lightweight telescopes easily loaded into a
small SUV by a single person in a “wheelbarrow” mode, to telescopes which weigh many
hundreds of pounds and require mechanical assistance—such as a winch—to load. This class of
telescopes is too heavy or large to transport on an airplane as check-in luggage or fit in a car.
Trailer telescopes, in turn, are too large to fit with into an SUV or even in the back of a
pickup truck, but they can be loaded on a trailer that can be towed down the road without any
special permits. Such heavy telescopes might benefit from motorized wheels or even being built
right into the trailer itself. If you can’t at least tow a telescope down the road by an ordinary
vehicle without a special permit, it isn’t a portable telescope.
Developmental Programs
Designing and building portable meter-class telescopes can be challenging, thanks not
only to constraints on their size and weight, but also to the demands of transportation and field
assembly and operation. A number of parallel developmental efforts are being pursued by the
members of the Alt-Az Initiative. Their progress had been reported in the recent book, The Alt-Az
Initiative: Telescope, Mirror, and Instrument Developments, and on their web site
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www.AltAzInitiative.org. Daily email discussions take place at the Alt-Az Initiative’s Yahoo
group.
While the technology for 0.5 to roughly 1.0 meter aperture portable telescopes that weigh
a few hundred pounds is not well established, further development would benefit ultralightweight and airline check-in telescopes. The development of technology for portable
telescopes with apertures significantly beyond 1.0 meters is only now beginning. The reasonable
upper limit to portability is probably about 2.4 meters (96 inches), the largest telescope one could
transport on a trailer without special permits. Mention tech transfers from lager to smaller
telescopes in some detail here. Visits, etc.
Mirrors, especially low cost, lightweight mirrors, are the key ingredient required for the
continued evolution of meter-class portable telescopes. Active efforts are underway to further
develop meniscus, sandwich, foam glass composite, spin cast, and layered mirrors. Forces are
just beginning to be applied to adjust the figure of lightweight thin meniscus primary mirrors in
meter-class telescopes in a manner somewhat similar to their large mountaintop relatives.
Similarly, work has just begun on low cost active secondary mirrors which can be “warped” to
counteract distortions in lightweight primary mirrors. Such active mirrors can also be tip/tilted to
reduce the effects of the wind gusts on portable telescopes. Finally—also similar to the largest
mountaintop telescopes—we anticipate that the largest meter-class telescopes will employ
multiple mirrors to reduce weight and facilitate manufacture and handling.
Other developments and tech transfers including DD motors, high res encoders (see
Ridgely email).
Observational Programs
Visual Recreational, Outreach, and Educational observational programs involve both
recreational observing for enjoyment, as well as public outreach, and education. Recreational
visual astronomers are interested in everything they have a chance to see, so the exciting
advantage of a high optical quality, one person set up, one meter telescope is that there’s much
more to observe. Visual observers marvel at seeing the real photons from objects ranging from
our solar system to billions of light years away. Many are inspired by the excellent astro-images
being made today and want to see a similar amount of detail visually. Some are excited by being
able to barely detect the faintest possible objects such as distant galaxy clusters, quasars and
gravitational arcs. Other examples:
1. Planetary detail in saturated color.
2. Hints of color in the brighter emission nebulae.
3. Spiral arms in 17th magnitude and brighter galaxies.
4. HII regions and star clusters in 16th magnitude and brighter galaxies.
5. Exotic new discoveries like Hanny’s Voorwerp.
6. Tidal distortions in Arp peculiar galaxies.
7. Seeing colors in the stars that make up the brighter globular clusters.
8. Seeing the complex internal structure of planetary nebulae, their central stars, and
extended halos.
9. 19th magnitude and brighter supernovae in distant galaxies.
10. Digging out small scale, faint details in brighter objects like Herbig-Haro objects in M20.
11. Soaking in the glorious, exquisite details of brighter objects like M51, M42, Saturn, etc…
The lowest power of a high quality one meter f/3 Newtonian that produces a 7 mm exit
pupil is 150 with a field-of-view is just under 0.6 degree using a 21mm Ethos eyepiece and the
Paracorr 2 coma corrector. This combination is large enough to see the entire Moon and big
chunks of larger deep sky objects such as the Veil Nebula. The field of view at lowest power
should be well corrected to the edge and provide an aesthetically pleasing image. That’s exciting
enough, but the sharp and detailed, high power images portable meter-class telescopes are
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capable of producing under a dark, transparent, and steady sky are their real strength and appeal.
Visual observers will appreciate the convenience of a comfortable Nasmyth focus viewing
position.
Astro-Imaging can also benefit from larger apertures and portability. As suggested by
Dave Rowe, a 1-meter f/3 telescope with a 52 mm diameter field would be marvelous instrument
for astrophotography. The focal length is perfectly matched to the pixel size and average seeing,
and the wide field allows for the greatest possible (affordable) etendue, for spectacular wide-field
imaging. The optics for such a telescope are something of a challenge.
Scientific Research programs, as is the case with visual and astrophotography programs,
can benefit from larger apertures and the dark skies opportunities that portability creates. In
addition to greater program object fluxes, scintillation noise decreases with telescope aperture, so
meter-class telescopes have superior program object signal-to-noise ratios for a given integration
length.
Although the bulk of scientific observations are now made with instruments, visual
scientific observations can still make significant contributions to the advancement of our
knowledge. Some of the visual observations that could benefit from portable meter-class
telescopes include discovery searches for supernovae in not-too-faint galaxies, visual photometric
observations of fainter variable stars such as Miras, dwarf novae (to see which stars are “up”),
and astrometric measurements with laser-etched astrometric eyepieces of fainter double stars.
There are instances where scientific observations actually require portability. For
example, the size and shape (and hence albedos) of smaller diameter trans-Neptunian objects
(TNOs) can be determined from occultations observations as a TNO passes in front of a star,
casting a narrow shadow that races across the Earth. Since such narrow shadows rarely
conveniently pass over fixed location observatories, they must be observed with portable
telescopes. With larger, meter-class telescopes, many more TNO occultations can be observed
than would otherwise be possible.
Telescopes can be moved to a location not because of some astronomical event, but
because the local observing conditions are advantageous in some way. For example, near IR
photometry, especially Ks band, benefits from high altitudes and dry skies. A number of
mountaintop observatories, blessed with good roads to the summit, wouldn’t mind sharing their
skies with portable meter-class telescopes.
Conclusions
We have described three basic types of portable meter-class telescopes: visual
observation, general purpose imaging, and on-axis light bucket. Each type has its own strengths
and weaknesses when it comes to the three types of observational programs mentioned above
(visual recreational, astro-imaging, and scientific research). Think different strokes for different
folks.
If one’s sole interest is visual, eyeball to the eyepiece, observing, then a lightweight
telescope that is easy to transport, quick to assemble, simple to operate and maintain, and has a
fast, wide field-of-view, then a visual observation telescope will give you the best value. No
sense paying a premium price or putting up with the complexities of high precision tracking and
instrument rotation needed for astro-imaging. Whether your visual observations are recreational,
scientific research, or both, KISS may apply to you. Although they are obviously specialized,
visual telescopes are currently the most numerous type of portable meter-class telescopes and are
likely to remain so for the foreseeable future. They are leading the way into meter-class
telescopeland.
On the other hand, if one is interested in both visual observing and astro-imaging, then a
general purpose imaging telescope may be your best choice. These telescopes are also excellent
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scientific work horses, as they can handle every sort of detector, be it area (cameras) or point
source (photodiodes, spectrographs, etc.). Of course for the same aperture, these telescopes are
more expensive than visual observation telescopes. They are just beginning to appear on the
market.
If one is only interested in aperture photometry, spectroscopy, or polarimetry, then an onaxis light bucket telescope may give you the most on-sensor photons for your dollar. These are
specialized telescopes—they typically don’t even feature an eyepiece! If you main interest isn’t a
specialized area of scientific research, then one of the telescopes is probably not for you. These
are relatively rare telescopes and are likely to remain so.
The era of portable meter-class astronomy has been underway now for several decades—
ever since the first large aperture alt-az Dobsonian telescopes first made their appearance. Visual
observations through Dobs have endeared an entire generation of the public to observational
astronomy. We hope and expect that the use of portable meter-class telescopes will increasingly
be extended beyond visual observation, not only to recreational astro-imaging, but also to a wide
variety of scientific research projects. The dark skies beckon the ever increasing numbers of
portable meter-class telescopes of all types.
We maintain that portability doesn’t have to be restricted to “smaller” apertures. The
only limits should be our ingenuity and what can be legally towed down the road. There is no
reason we shouldn’t see portable 2 meter telescopes in the near future, starting first with on-axis
light buckets and then spreading to visual and even general purpose imaging telescopes. There
will be many problems to overcome on the way to 2 meter portability, but we'll solve them all—
one at a time.
Acknowledgements We thank Howard Banich for his suggestions on visual observing programs
and Dave Rowe for his suggestions on astrophotography programs. We also thank Andrew
Aurigema, Mel Bartels, Tom Frey, Greg Jones, Tom Osypowski, … for their reviews and helpful
suggestions.
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