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▪ CONTENTS ◄ 14.2. ATM telescopes ▐ 14.4. Commercial telescopes ► 14.3. OBSERVATORY TELESCOPES
Unlike amateur telescopes, used for both, visual and photography,
observatory telescopes - and the subject here are those used in the
visual range of the spectrum - are generally used for photographic work. They
range in sizes from fraction of a meter to eight meters, or more
(segmented mirrors) in diameter. All large telescopes are based on
the reflecting systems, and those sub-meter can be both, reflecting and catadioptric; very few refractors are still in observatory use. Specific
uses of these telescopes vary just as much: there is an immense amount
of information coming from the Universe, and with all our observatory
arsenal, we are barely scratching the surface.
Since they are a part of professional operation, observatory telescopes
are not only larger, but also generally more sophisticated and
technologically advanced. As such, they are a special breed, puzzling
and interesting not only to amateur astronomers, but also to people at
large.
This should open a small window into the realm of big professional
telescopes, seeking to find the truth of our Universe, and our place in
it.
1 - ULTRA-FAST 8.4-METER f/1.25 WIDE-FIELD TELESCOPE
Arguably one of the best corrected simple systems is the three-mirror
system known as Paul, or Paul-Baker telescope.
So when a group of astronomers from the University of Arizona's Steward
Observatory were looking for a very large, ultra-fast widefield design
for the Dark Matter Telescope project, it led them to the Paul-Baker,
since no two-mirror system, or a Schmidt-like telescope, could achieve
the needed level of correction. The telescope would be limited only by
atmospheric seeing and sky background photon noise, which means it has
to produce star images smaller than 0.5 arc seconds, achievable
occasionally with large telescopes on best sites. The goal was the
largest telescope possible with 10m focal length, needed for sufficient
sampling with a 15-micron pixel (1 arc second=51 micron).
Their first design, a 6.5m f/1 Paul-Baker telescope with all three
mirrors aspherized in Zemax, was capable of producing required image,
but had suboptimal plate scale, and unacceptable chromatism from the
dewar window. That led to the revised design, a 8.4m f/1.25 telescope
with a curved dewar window and added correcting lens. This design was
the starting point for the 8.4m f/1.234 Rubin Observatory Simonyi Survey
Telescope (El Peñon, Cerro Pachón, Chile), with even wider, 3.5-degree
field, and a bit better correction (less than 0.3 vs. less than 1/3 arc
seconds design image). Since there is no prescription for the Rubin
telescope, the University of Arizona's design
will be used to illustrate
this, we could say unique kind of a telescope. Unfortunately, prescription
given in the paper isn't working, but a system with comparable
performance can be reconstructed from it.
The main parameter of optical performance is ensquared energy. It is
given on the bottom of every 1-arcsec square containing ray spot plots
for the 0.80 energy square all values are in meters, so the 7.77-6
for the axial e-line 0.80 energy square is 7.77 microns). At 1.5 degrees
off axis the polychromatic 0.80 energy square side is 24.9 microns, or
nearly 0.5 arc seconds. It would probably be the subject of final
optimization, generally by near-equalizing error over the field, by
allowing more of an error in the inner field. At 1-degree off, the 0.80
energy square is 1/3 of arc second, becoming significantly better toward
axis. Astigmatism is minimized, near zero at 1.5 degrees off, with the
dominant aberration there being trefoil.
The small dot in the center of each box is the Airy disc; entirely
irrelevant here, but illustrates the magnitude of aberrations in this
highly corrected large telescope.
More complete picture of the aberrations is given by the Zernike terms.
The main contributors are in red (piston is not an aberration in the
single-aperture system, just a nominal artifact, and the defocus value -
which translates to 0.33 wave RMS, i.e. 1.16 wave P-V - is evident on
the astigmatism plot (best focus is not necessarily where astigmatism is
at its minimum; in the presence of other aberrations it can shift to
another location).
The largest single contributor is trefoil (#9). The next highest one,
quadrafoil (#16) is already near negligible, adding only 15% to the
trefoil (because Zernike terms add up as square root of their squared
values), and even less considering secondary trefoil (#18). Most of the
terms are negligible individually, but do have significance as a total,
creating a higher-order aberration "noise" in this large and fact
telescope.
At 1 degree off axis dominant aberration is astigmatism, but it is
significantly smaller than what the longitudinal aberration graph
indicates (nearly 6 waves P-V, from W=L/8F2, L being
the longitudinal aberration, and F the system focal ratio). The
reason is that a mix of lower- and higher-order astigmatism of opposite
signs has up to four times lower error at the best focus for given
longitudinal aberration, and that large central obstruction also lowers
the error.
2 - SDSS TELESCOPE: 2.5m f/5 MODIFIED RITCHEY-CHRETIEN
Located at Apache Point Observatory in New Mexico, the main tool of the
Sloan Digital Sky Survay project, this telescope uses a two aspheric
lens field correctors to achieve a well-corrected 3-degree field. As described
in paper by Gunn et al.
its second corrector lens is interchangeable, allowing it to be
optimized for camera and spectroscopic mode. It is used to map the
depths of our Universe.
The two corrector lenses have their radii given indirectly, through the
value of a2 coefficient, whose full expression is
a2=d2/R[1+(1-K)(d/R)2],
with d being the the surface radial height, R the
radius of curvature and K the surface conic.
It is not clear why, because the surfaces are spherical (K=0), and (d/R)2
is negligibly small, especially for the first corrector, hence for all
practical purposes a2=d2/2R, and R could be
expressed directly (for the second corrector, it would give about 3%
longer radius, but it would have little consequence).
Raytracing the prescription for camera mode shows, perhaps, less than
expected correction-wise. The field is, as described in paper, nearly
flat up to about 80% of the 1.5-degree radius, curving inward rather
strongly after that. Color correction is good overall, but widely
separated lines show significantly different forms of aberrations toward
field edge, i.e. their best foci do not coincide. One of the
requirements was near-zero distortion; according to OSLO, it just
exceeds 0.1% at the field edge.
The outer portion of the field required adjustment of the detector
shape. It cannot be fitted with a near-spherical curve, and requires
higher order aspherics. Here, the asherics are combined with 67m
curvature, which resulted in a good, but still someone uneven fit (the
edge is for the best green focus; the red becomes roughly round at 99m
curvature, but green out of best focus).
The paper gives no specific performance criteria for the camera mode.
Final images were calculated as convolved with 0.8 arc seconds seeing
FWHM, which proved to be too optimistic. For the system focal length,
one arsecond is 60 microns. On the fitted field, the green line 80%
energy square is 18.7 microns on axis, 15.5 microns at 1-degree, and
35.8 microns at 1.5. It indicates that the seeing is limiting factor,
rather than optics.
Full spectral range of the telescope is 0.3-1 micron, but the familiar
visual 0.43-0.70 range well illustrates its chromatic correction.
3 - EELT: THE 39m, LARGEST-EVER-TO-BE OPTICAL TELESCOPE
European Extremely Large Telescope is designed as a three-mirror
anastigmtic aplanat. As described in a paper by Cayrel, the 39-meter
f/0.93 ellipsoidal primary consists of 798 hexagonal segments, each
1.45m across. The 4.2m convex secondary and 4m concave tertiary are each
a thin meniscus. All three mirrors are active: the primary to make
possible for it to conform to the proper shape, and the other two -
particularly tertiary - to correct wavefront errors, mainly those caused
by atmospheric turbulence, but also those caused by mechanical
deformations.
Two folding flat mirrors direct light beam to the side (Nasmyt focus).
The first, 2.5m in diameter, is also active, "specified
to deliver near infrared diffraction limited images with over 70%
Strehl
ratio in median
atmospheric conditions (0.85 arcsecond seeing, t0
of 2.5ms)". The second flat, 2.2 by 2.7m, corrects for tip-tilt
errors.
Field radius is nearly 5 arc minutes, limited by the central hole in the
fourth mirror (even such a small angular radius produces almost 1m image
radius). The prescription gives a final f/16.6 system, somewhat slower
than f/17.5 cited in the paper; not sure what is causing the difference,
but the design correction level is very high.
Even at the field edge, correction is still at the level of 1/12 wave
P-V of lower order spherical aberration. With the ~660m focal length,
one arc second at the detector is whooping 320 microns - about 1/3 mm.
The only significant aberration is field curvature.
4 - THE 40-INCH YERKES REFRACTOR
Keeping in the company of the largest - ever wondered how much
chromatism there is in the largest refractor ever built: the 40-inch
Clark refractor at the Yerkes observatory in Wisconsin? I was always
curious, just how much of color a quarter tone of achromat glass
generates. Using general data on the Clark doublet, and Barnard's
measurements, gives the following picture.
On axis, Airy disc is a barely visible dot in the center of defocused
C (656nm), F(486nm), r (706nm) and g (436nm)
lines. The F/C error is nearly 7 waves P-V, which puts it at the level
of a 100mm f/1.8 achromat. At the field edge, coma is nearly three times
the Airy disc - or 0.8 waves P-V - visually unnoticeable (about f/16
paraboloid level).
5 - 1m KLEVTSOV-CASSEGRAIN VARIETY (Vihorlatska Observatory,
Slovakia)
In his book "New serial telescopes and accessories"
(2014), Klevtsov gives prescription obtained from the Slovakian
observatory, as a big-scale example of a telescope of the Klevtsov type.
The telescope was made in Odessa, SSSR. Original prescription is with
BK7 glass (i.e. K8, its LZOS analog), but K7 glass reduces longitudinal error in the blue/violet to
less than half, so that this 1-m telescope easily passes the "true apo"
requirement. As with every system of this type, dominant aberration is
off axis astigmatism.
6 - 0.7m 5° HOUGHTON-TEREBIZH SURVEY TELESCOPE
One of many designs of Valery Terebizh, the Russian "master designer" as
referred to by M.R. Ackerman, is a simple Richter-Slevogt (known as
Houghton on the Western side) modification with widely separated
corrector lenses, making possible
significantly wider fields. Survey telescopes are usually of relatively
small apertures, with large corrected field being the primary concern,
but larger apertures can be needed for detecting fainter targets.
The Houghton-Terebizh offers 5-degree field at f/3.2. The system shown
is slightly tweaked to have the axial error minimized (at no expense to
off axis performance).
Color correction even at this aperture size and relative aperture
passes the "true apo" criterion. The 80% energy square is 6.6 microns on
axis, and 8 microns at 2.5 degrees off (diffraction images are for the 5
wavelengths with even sensitivity).
This design can also be used as a typical survey telescope, smaller in
size and faster. If rescaled to 350mm aperture and f/2.4 focal ratio -
parameters of a telescope of this design used in the gamma-ray burst
system observatory near Moscow, Russia (The Mobile Astronomical System of Telescope-Robot)
- it still preserves a very satisfactory performance.
Note that the actual telescope is somewhat shorter, with the corrector
lenses more widely separated and with the final image that would form
behind the rear corrector lens, if not directed to the side with a
diagonal flat).
7 - 3.8m f/2.5 UK INFRARED TELESCOPE
While designed for work outside the visual spectrum, the UKIRT is based
on unusual relay lens design which, with minor adjustments, could be
used in the visual range as well. Here is presented its Wide Field
Camera mode, as described in this online paper.
System is simplified by omitting optical window and filters, which has a
minor influence on its output. The entire field covers 0.93-degree
diameter, but the actually used are only four rectangular portions of
it, as illustrated in the paper. The telescope is optimized to operate
in four IR ranges: Y (0.97-1.07 microns), J (1.17-1.33), H(1.49-1.78)
and K(2.03-2.37). As given here, the is somewhat biased toward the lower
three, possibly the consequence of omitting mentioned elements.
Sub-aperture aspheric plate helps correct not only spherical aberration,
but also coma and astigmatism.
The ray spot plots are given for the central line of each of the four
wavebands, when refocused to their respective best focus. Above right
are plots for this same system in the visual wavelengths (focused on
e-line). Only minor optimization is needed to have it perform
satisfactory as a visual telescope.
◄
14.2. ATM telescopes
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14.4. Commercial telescopes
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