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Proceedings ArticleDOI

Development of the SPIRIT III telescope: from design through test

21 Jan 1993-Vol. 1765, Iss: 1765, pp 41-52

Abstract: This paper documents the development of the SPIRIT III telescope from the design through its test activities. The SPIRIT III Instrument is the primary infrared instrument on the Mid-Course Space Experiment (MSX). The telescope is an all reflective optical system consisting of twelve mirrors. The nominal collecting apertures is 14 inches. It was designed and built to integrate with a multicolor radiometer and a Michelson interferometer built by the Space Dynamics Laboratory at Utah State University. Key performance features are discussed, and measured test data is presented. The structural/thermal trade-off issues of a satellite-based cryogenic instrument are presented along with a review of the test techniques and test equipment.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
Topics: Reflecting telescope (58%), Telescope (55%)

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Development of the Spirit III Telescope: from Design through Test Development of the Spirit III Telescope: from Design through Test
Andrew A. Mastandrea
Richard R. Glasheen
James J. Guregian
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Mastandrea, Andrew A.; Glasheen, Richard R.; and Guregian, James J., "Development of the Spirit III
Telescope: from Design through Test" (1993).
Space Dynamics Lab Publications.
Paper 89.
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Development of the Spirit III telescope: from design through test
Andrew A. Mastandrea, Richard R. Glasheen, and James J. Guregian
SSG, Inc.
150 Bear Hill Road, Waltham, Massachusetts 02154
Roy Esplin
Space Dynamics Laboratory, Utah State University
Logan, Utah 84321
This paper documents the development of the SPIRIT III telescope from the design through its test activities
at SSG, Inc. The SPIRIT III Instrument is the primary infrared instrument on the Mid-Course Space Experiment
(MSX). The telescope is an all reflective optical system consisting of twelve mirrors. It represents the largest high
straylight rejection, cryogenic telescope built by SSG to date. The nominal collecting aperture is 14 inches. It was
designed and built to integrate with a multi-color radiometer and a Michelson interferometer built by the Space
Dynamics Laboratory at Utah State University. Key performance features are discussed and measured test data is
presented. These include: an internal scan mirror assembly, low scatter mirrors and baffle assemblies, cryogenic
optical performance and contamination control. The structural/thermal trade-off issues of a satellite-based cryogenic
instrument are presented along with a review of the test techniques and test equipment utilized at SSG to qualify the
SPIRIT III telescope.
The SPIRIT III telescope configuration is shown in block diagram form in Figure 1.0. The telescope consists
of three optics modules - an
afocal high straylight rejection optics module, the radiometer optics module, and the
interferometer optics module. The afocal optics operate at 4:1 magnification and image the 14.5 inch diameter
entrance aperture to the Lyot stop which is located on the scan mirror assembly. The entrance aperture is located in
the baffle assembly approximately 32 inches from the primary mirror. The afocal optics are: a concave off-axis
parabolic primary, a convex off-axis hyperbolic secondary, a concave off-axis parabolic tertiary, and a fold flat. The
first field stop is located between the secondary and tertiary. It is at this first field stop where the telescope splits the
SPIE Vol. 1765 Cryogenic Optical Systems and instruments V (1992) /41
Figure 1.0. Spirit III telescope block diagram
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Mastandrea, Andrew A., Richard R. Glasheen, James J. Guregian, and Roy Esplin. 1993. “Development of the SPIRIT III
Telescope: From Design through Test.” Proceedings of SPIE 1765: 41–52. doi:10.1117/12.140893.

energy between the radiometer and the interferometer channels. The first field stop contains two rectangular
apertures. The radiometer aperture is 3.0 x 1.0 degrees and allows the 0.3 x 1.0 degree IFOV of the radiometer to
be scanned by 3 degrees in object space. The interferometer field is 0.86 x 0.7 degrees. The combination of low
scatter finishes on the primary and secondary mirrors, the baffle assembly, the entrance aperture, Lyot stop, and field
stops produce the required high straylight rejection performance of the SPIRIT III telescope. These issues are
discussed in section 4.3 of this paper. Figure 2.0 shows the location of the optical elements in the system.
2.0. Spirit III optical layout
The radiometer optics consist of a three mirror reimager, the scan mirror assembly and a fold fiat. The 3.5
diameter collimated output of the afocal front end feeds into the scan mirror assembly. The energy is directed
by the fold flat to the first mirror in the reimager. This element is a concave ellipse which focuses the energy
through the second field stop (03 x 1.0 deg aperture). The remaining two optical elements -
a convex sphere and a
concave sphere reimage the energy onto the focal planes. The radiometer focal planes consist of three focal
plane/dichroic modules which were built by Space Dynamics Laboratory at Utah State University (SDL\USU).
The interferometer optics split off at the first field stop as previously mentioned. These optics provide a 1.6
inch diameter collimated beam to the Space Dynamics Laboratory Michelson interferometer.
The mechanical design of the SPIRIT III telescope was based on the design concepts and techniques
developed for the SPIRIT II telescope and other previous SSG cryogenic high straylight rejection telescopes.1 Figure
3.0 shows a photograph of the completed telescope prior to shipment to the Space Dynamics Laboratory. To achieve
an athermal telescope, the entire structure, mirror substrates and their mounts are made from 6061 aluminum. The
main structural assembly consists of the telescope main plate and the optics housing. The main plate of the
telescope, shown on the right side of Figure 2.0, is the key interface point of the telescope to the SDL\USU
radiometer and the Lockheed cryostat. The main plate is 34 inches in diameter and the baffle assembly extends 50
inches off the main plate. The optics housing consists of many precision machined aluminum plates that along with
the main plate were dip brazed together to form one rigid "optical bench". The dip brazing process produces an
assembly that has the structural characteristics of a single part. It also results in an assembly that has excellent heat
/ SPIE Vol. 1765 Cryogenic Optical Systems and Instruments V (1992)
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transfer properties which minimizes thermal gradients in the telescope at its operating temperature of 20K. SSG has
utilized dip brazing on at least ten previous telescope systems including the CLAES telescope which in currently in
orbit aboard the Upper Atmospheric Research Satellite (UARS).2 The optics housing and its covers contain the
mounts for all the mirrors and the interfaces for the SDL\USU shutter assembly and interferometer.
The telescope structural design was driven by the environmental test levels required by the MSX program.
The random vibration levels produced the highest stresses into the telescope. SSG successfully tested the completed
telescope in a qualification and acceptance level random vibration testing. The thermal design requirements had the
following nominal temperature limits.
Optics & Structure: Less than 20K
Baffle Assembly: Less than 70K
Focal Plane Assemblies: 10K
To achieve these design requirements, SSG and SDL\USU utilized several thermal design techniques. The baffle
assembly is thermally isolated from the telescope structure. This aluminum assembly is supported off the main plate
by a G-10 isolator. The details of the baffle isolator are described below. The SDL\USU radiometer FPA assembly
is also thermally isolated from the telescope structure through three G-10 pedestals. In addition, the thermal design
Vol. 1765 Cryogenic Optical Systems and Instruments V (1992) /43
Figure 3.0. Completed telescope
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has the telescope structure and optics, the baffle, the radiometer, and the interferometer separately heat sunk to the
Lockheed cryogen.
The baffle isolator is an interesting example of the trade-offs required to meet the structural and thermal
requirements of a satellite-based cryogenic instrument. The mission lifetime and the high random vibration levels
were working against each other on this isolator. The telescope design required this isolator, shown in Figure 4.0, to
support the entire baffle assembly. The baffle was not allowed to contact the telescope structure at any other point
in order to minimize the thermal loads into the optical bench and therefore the cryogen assembly. An additional
complication for this isolator was imposed during the conceptual design phase of the program. In order to maximize
the collecting area of the telescope within the limits of the envelope the baffle isolator became "non-circular". The
final design had two flat sections connecting the semi-circular halves of the isolator. SSG worked with a local vendor
to produce the isolator to meet all the structural and thermal requirements. The G-1O was wound onto a precision
mandrel to a create a uniform part of approximately 0.5
inches in thickness. The mandrel was machined to meet the
tight mechanical tolerances of the ID of the isolator. It is 16.5 inches in diameter with two 1.4 inch long flat sections
at the top and bottom. The length of the completed assembly is 6.5
inches. The G-1O was then machined to a wall
thickness of 0.040 inches with three circumferential ribs and four lateral ribs. The lateral ribs were located at the
flats to strengthen the part in these high stress areas. Many iterations of the structural, mechanical, and thermal
analyses were expended to achieve the final design of this critical component. Several isolators were fabricated and
tested to insure the success of the telescope. The prototype unit was tested in static compression and tension tests to
verify the design analyses. The ifight unit baffle isolator passed the telescope random vibration tests during the
acceptance testing at SSG.
As previously mentioned, the SPIRIT III scan mirror assembly operates in collimated light at the output of
the 4:1 afocal optics. The nominal beam size is 3.5
in diameter and is limited by the Lyot stop which is
located on the scan housing. Figure 5.0
the completed flight scan mirror assembly. The scan housing and the
scan mirror are made from 6061 aluminum to be consistent with the athermal design of the telescope. The scan
mirror is a lightweighted, machined aluminum optical element that was diamond turned to its fmal optical figure.
The physical size of this mirror is 6.0 x 4.5
with a face thickness of 0.3 inches. The scan mirror was
cryogenically tested as a component and maintained a one wave peak-valley surface figure (@0.633 jim) at 94K.
/ SPIE Vol. 1765 Cryogenic Optical Systems and Instruments V (1992)
Figure 4.0. Baffle isolator
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