A
Very Wide Field, Very Fast Telescope
Dr. Frank Melsheimer, DFM Engineering, Inc. Longmont, Colorado,
USA
Dr. Malcolm J. MacFarlane, Raytheon Co., Danbury, Connecticut,
USA
ABSTRACT
A 1.3 meter aperture Cassegrain telescope with a very wide
flat field has been completed and is now producing images.
The field of view is a stunning 1.7 degrees while the focal
ratio is a very fast F/4. The telescope is located at the
United States Naval Observatory Flagstaff Station in Arizona,
USA.
The authors discussed the concept of a fast, very wide flat
field optical system years ago, but no system was built. Now
the system has been realized using only primary and secondary
mirrors with a small fourth order (Schmidt plate) corrector.
The mirrors form a modified Ritchey Cretien' system but are
significantly more aspheric - especially the secondary mirror.
The corrector was made using the vacuum technique invented
by Bernard Schmidt. The resulting optical system is unique.
The design performance, optical fabrication techniques,
and optical test results are discussed. Images taken with
the system are also displayed.
Keywords: "Very Wide Field", "Fast",
"Unique"
1. INTRODUCTION
The United States Naval Observatory desired a very wide field
reflecting telescope to perform astrometry using a mosaic
CCD camera. The observatory staff has considerable experience
with a 1.6 meter folded prime focus telescope first using
traditional photographic plates and now using a CCD. The plate
scale of the 1.6 meter telescope limited the field based upon
the size of CCDs. The new telescope will provide a very wide
field using a realizable mosaic CCD camera with 15 micron
square pixels. The plate scale of the telescope is 25 microns
per arc second.
2. OPTICAL DESIGN
A more traditional Ritchey-Cretien' design with field corrector/flattener
was investigated by the Observatory staff and it was shown
that such a design was viable. The authors investigated the
concept of using two mirrors and a small, thin fourth order
field corrector and determined that such a system could produce
the desired results at a lower cost and provide better performance.
The detailed optical design included the effects of the
filter wheel glass and the CCD vacuum window glass. Also,
the optical testing of the primary and of the system was performed
at a slightly different optical spacing to account for not
having these additional two optical elements in the testing
path.
The optical layout of the system is shown in Figure 1. The
additional elements near the focal plane allow for the filter
and the CCD vacuum window. These elements are plano-plano
pieces of glass of the required thickness for their function.
Figure 1. Optical system layout.
Figure 2 shows the diffraction encircled energy taking into
account the central obscuration caused by the light baffles
and for polychromatic light. Even at 0.85 degrees off axis,
the design shows the system to be nearly diffraction limited-the
difference being a small fraction of the pixel size.
Figure 2. Diffraction encircled energy. (12.5 microns radius
is equivalent to 1 arc second in diameter).
| Wavelength |
600
nm |
700
nm |
800 NM |
Figure 3. Spot diagrams for (Top) on axis, (Middle) 0.6
degrees off axis, and (Bottom) 0.85 degrees off axis. The
boxes are 1 arc second square. These spot diagrams show that
the image is always smaller than 1 arc second.
3. OPTICAL FABRICATION
The optical fabrication for the primary and secondary mirrors
was contracted to Kodak because of their ability to perform
an all up system test in autocollimation using a full sized
optical flat. The fourth order field corrector was fabricated
by DFM Engineering.
The primary mirror was processed using a combination of
pitch polishing and ion milling. Some edge roughness consisting
of up-down-up-down zones was hand worked using a pitch "T"
lap as the spacing of the features was too close together
to reduce with ion milling. The hand polishing required less
than 60 minutes. The substrate material is Corning ULE.
The secondary mirror was shaped by Kodak and then subcontracted
to The Optical Corporation of America as they have perfected
a method to "stitch" together interferograms taken of a secondary
mirror using an undersized Hindle sphere. We investigated
the availability of a full size Hindle sphere within the United
States, and found no suitable Hindle sphere for this large
secondary mirror (64 cm diameter). The substrate material
is Corning ULE.
The fourth order corrector plate (diameter 230 mm) was fabricated
by DFM Engineering using the vacuum technique invented by
Bernard Schmidt. The corrector blank is supported by a narrow
ledge located just inside of the anti-chip bevel. The blank
is then deformed a calculated amount and the first surface
is ground and polished to a calculated long radius sphere.
The second surface is then worked in a similar manner.
It is generally accepted that the vacuum technique works
well for corrector plates that fully correct a sphere of F
ratio slower than about F/2.5. The limiting factor is the
stress induced in the glass when the blank is deformed under
vacuum. This corrector is faster than F/2. The vacuum technique
was successfully used because the corrector was made from
fused silica which is considerably stronger than crown, the
correction was divided between the two sides, and special
procedures were incorporated in the processing of the blank.
The corrector blank was ground to the desired mechanical
thickness using loose abrasives with a total 0f 0.7 mm removed
from each side. One side was then polished to a long radius
convex spherical surface. The polished surface was placed
in tension in the vacuum chuck while the other surface was
fine ground and polished. The corrector was then reversed
and the second side ground and polished under the vacuum induced
deformation. In this manner, a ground surface was never placed
in tension. The corrector plate was also thinner than normally
used to reduce the stress in the material when deformed under
the vacuum and to allow deforming the corrector plate sufficiently
with the atmospheric pressure available here in Colorado (1530
meters altitude)!
The vacuum technique typically allows a very smooth surface
to be polished as the polishing is performed with a full sized
lap. We have found that some control over the pitch is necessary
to provide the proper figure. Usually the central part of
the aperture needs to be raised. This was polished in using
a ring lap. The total final grinding, polishing, and figuring
time was less than 40 hours for the corrector plate.
4. OPTICAL TESTING
The primary mirror was interferometrically tested using an
Offner type null lens. The secondary mirror was interferometrically
tested using a sub aperture technique with an undersized Hindle
sphere. The secondary mirror individual Interferograms were
then "stitched" together.
The corrector plate was null tested in double pass by placing
the corrector very close to an accurate concave spherical
mirror. The test would have required testing at two widely
different conjugate foci similar to testing an ellipse, but
we chose to use the spherical mirror and a null lens. The
null lens allowed the test to be performed at the equivalent
center of curvature of the system. The testing was performed
at an equivalent focal ratio of about F/1.
The corrector plate was tested using a knife edge and Ronchi
grating. The diameter of the corrector plate was 230 mm, but
the beam size is only 50 mm in diameter. The corrector plate
is close to the focus requiring a smooth figure, but the overall
correction is not very critical. The double pass through the
corrector plate allowed Ronchi testing to provide sufficient
accuracy. A small CCD camera was used to view the Ronchi bands
and knife edge cutoff.
The overall system was interferometrically tested in double
pass autocollimation. The focal plane was moved slightly to
account for not testing with the filter glass or vacuum window.
All three optical elements were polished by different optical
shops, but when the system test was performed, no additional
correction was required to meet the specifications.
5. TELESCOPE
The telescope mount is an equatorial fork mount with
friction drives on both axes. The primary and secondary
mirrors are spaced with Invar rods to minimize focus
shift with temperature.
Additionally there are bimetallic temperature compensation
rods within the focus housing to further temperature
compensate the optical spacing. For a slowly changing
temperature, the optical tube assembly has an essentially
zero temperature coefficient.
The tracking performance of the telescope is excellent.
Open loop tracking with a total error of less than 1
arc second in 20 minutes was demonstrated. Particular
care was exercised to accurately align the telescope
on the refracted celestial pole to minimize field rotation.
|
|
6. FIRST LIGHT IMAGES
Figure 4 shows a 17 by 17 arc minute field of view image
recorded using a 1K by 1K CCD camera with 24 micron square
pixels. The seeing was about 1.8 arc seconds FWHM. Since the
first light images, additional images have been taken using
a camera containing one of the 2K by 4K CCD chips that will
be expanded into the mosaic camera. The single chip camera
provides a field 20 by 40 arc minutes.
Figure 4. A first light 17 arc minute by 17 arc minute image
of Chi Persus.
Figure 5. The Orion Nebula taken with the 1 K by 1 K CCD
camera.
REFERENCES
1. "Model Optical Systems for Eight-Metre Telescopes", Bingham,
Richard G. SPIE Vol. 1236 Advanced Technology Optical Telescopes
IV (1990)
2. "The Optical Design of the 40-In. Telescope and of the
Irenee DuPont Telescope at Las Campanas Observatory, Chile",
Bowen, I. S. and Vaughan, A. H. Jr. Applied Optics, Vol. 12(7)
pp 1430-1434 (July 1973)
3. "The Vacuum Technique of Making Corrector Plates", Cox,
Robert F. Sky & Telescope Magazine, P388, (June 1972)
|
