By starting with established and flown hardware (high Technology Readiness Level (TRL)), and implementing a
concurrent engineering environment and seamless team, a mission architect can achieve high reliability and high
performance while operating under constrained cost and short implementation schedule. We will describe methods,
including those used by the telescope team on the recent Wide-field Infrared Survey Explorer (WISE) mission, to
manage cost and realize aggressive schedules. These lessons may be evoked for telescopes addressing defense, security
and sensing, as well as those for NASA science.
L-3 Integrated Optical Systems/SSG designed and built the telescope, aft imager, and scanner for the Widefield Infrared
Survey Explorer (WISE) under subcontract to Utah State University/Space Dynamics Laboratory. The WISE mission
and collection scheme imparted several driving requirements on the telescope and scanner, including the need for low
cost implementation, <11 Kelvin operation, and the need to back-scan by half a degree during detector integration in
order to freeze the line of sight on the sky as the spacecraft pitched in orbit. These requirements led to several unique
design and implementation choices for the telescope and scanner. In this paper we highlight several of those design
choices as well as lessons learned from the telescope and scanner design, fabrication, and test. WISE, a NASA MIDEX
mission within the Explorers program, was managed by the Jet Propulsion Laboratory. WISE launched on December
14, 2009 and is currently operating successfully.
On December 14, 2009 NASA launched the Wide-field Infrared Survey Explorer (WISE), a NASA MIDEX mission
within the Explorers program that is currently performing an all-sky survey in four infrared bands. L-3 Integrated
Optical Systems/SSG designed, built, and tested the telescope, scanner, and aft imaging optical system for WISE under
contract to the Space Dynamics Laboratory. Hardware and test results for those subsystems are presented, as well as an
on-orbit status of their imaging performance. The WISE payload includes a 40 cm afocal telescope, a scan mirror for
back-scan during integration, and an aft optics imager assembly. All modules operate below 17 Kelvin. The allreflective
system uses aluminum mirrors and metering structures. The afocal telescope provides distortion control to
better than two parts in a thousand to prevent image blur during internal scanning. The one-axis scan mirror at the exit
pupil scans the detectors' field-of-view across the telescope field-of-regard, countering the orbital motion and freezing
the line of sight during the multi-second exposure period. The five-mirror imaging optics module follows the scan
mirror and feeds dichroic beamsplitters that separate the energy into four channels between 2.8 and 26 microns. Once
initial on-orbit checkout and calibration was completed, WISE began a 6-month mission performing an all-sky survey in
the four infrared bands, which is over 80% complete as of June 2010.
SSG Precision Optronics, Inc. has delivered a silicon carbide (SiC) pointing mirror and telescope for NASA's Geostationary Imaging Fourier Transform Spectrometer (GIFTS) project. The 28 x 45 cm SiC pointing mirror is part of SSG's two-axis gimbaled mirror assembly that will provide object-space pointing and jitter control. The 24 cm aperture telescope is an off-axis afocal three mirror anastigmat that is the collection aperture for the GIFTS instrument. Silicon carbide was selected for the GIFTS pointing mirror and telescope in order to minimize weight, provide athermal optical performance from room temperature to 190 Kelvin, and maintain image quality and line-of-sight stability in the presence of partial or full solar loading (minimizing solar outages). Both subsystems were successfully designed, fabricated, and subjected to testing prior to being delivered to Utah State University's Space Dynamics Laboratory for integration. This paper describes the pointing mirror and telescope design and hardware results.
The Wide-field Infrared Survey Explorer (WISE) instrument includes a cryogenic telescope, scanner, and imaging optics module that provides four channels of infrared imaging between 2.8 and 26 microns. The telescope is a 40 cm aperture reflecting five-mirror imager/collimator relay that provides 8X demagnification, a 47 x 86 arcminute field of regard, and a real exit pupil for scanning. It also provides distortion control to better than one part in a thousand to prevent image blur during internal scanning. A one-axis scan mirror at the exit pupil scans the detectors' field-of-view across the telescope field-of-regard, countering the orbital motion and freezing the line of sight during the multi-second exposure period. The imaging optics module is a five-mirror re-imager with dichroic beamsplitters that separate the energy into four channels. All modules operate below 17 Kelvin. The all-reflective system uses aluminum mirrors and metering structures. The scanner is a derivative of the SPIRIT III scanner flown previously. WISE has been selected by NASA for Phase B design.
Every space-borne optical system has a unique set of requirements and challenges. In the end, all need to achieve a delicate balance of performance, cost, and schedule to be successful. Principal to achieving this goal is the correct selection of the mirror materials from among a variety of common options: glass, aluminum, beryllium, and silicon carbide. Many engineering disciplines -- optical, mechanical, thermal, and materials -- are required to design an optical system. The intent of this paper is to describe the strengths and limitations of each mirror material from the perspective of an optical systems engineer, who must balance all disciplines in order to select the best material combination for a specific application. Several topics are discussed, including: (1) Mechanical and thermal figures of merit; (2) stability and design considerations (K/α, ΔL/L, and cryogenic test data); (3) fabrication issues: schedules, relative costs, achievable figures, and finishes; and (4) stray light rejection and achievable BRDF's. Data is presented from many recent projects in support of these topics.
An overview of silicon carbide (SiC) materials is provided, focusing on the optical properties required for space-based earth observing applications. NASA’s SiC Advanced Land Imager (ALI), produced by SSGPO and flown under the New Millennium Program, is described in order to illustrate the suitability of SiC to provide high-quality optics for these critical applications. The manufacturing processes used to produce SiC optics are described and recent improvements in the surface figure, surface finish, and stray light performance associated with SiC optics are reported. The two critical optical properties associated with the ALI instrument are surface figure and Bi-directional Reflectance Distribution Function (BRDF). In the results reported here, we demonstrate the ability to exceed these requirements by an order of magnitude using mature and repeatable processes.
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