In-situ interplanetary science missions constantly push the spacecraft communications systems to support successively higher downlink rates. However, the highly restrictive mass and power constraints placed on interplanetary spacecraft significantly limit the desired bandwidth increases in going forward with current radio frequency (RF) technology. To overcome these limitations, we have evaluated the ability of free-space optical communications systems to make substantial gains in downlink bandwidth, while holding to the mass and power limits allocated to current state-of-the-art Ka-band communications systems.
A primary component of such an optical communications system is the optical assembly, comprised of the optical support structure, optical elements, baffles and outer enclosure. We wish to estimate the total mass that such an optical assembly might require, and assess what form it might take. Finally, to ground this generalized study, we should produce a conceptual design, and use that to verify its ability to achieve the required downlink gain, estimate it’s specific optical and opto-mechanical requirements, and evaluate the feasibility of producing the assembly.
Silicon Carbide (SiC) optical materials are being applied widely for both space based and ground based optical
telescopes. The material provides a superior weight to stiffness ratio, which is an important metric for the design and
fabrication of lightweight space telescopes. The material also has superior thermal properties with a low coefficient of
thermal expansion, and a high thermal conductivity. The thermal properties advantages are important for both space
based and ground based systems, which typically need to operate under stressing thermal conditions. The paper will
review L-3 Integrated Optical Systems – SSG’s (L-3 SSG) work in developing SiC optics and SiC optical systems for
astronomical observing systems. L-3 SSG has been fielding SiC optical components and systems for over 25 years.
Space systems described will emphasize the recently launched Long Range Reconnaissance Imager (LORRI) developed
for JHU-APL and NASA-GSFC. Review of ground based applications of SiC will include supporting L-3 IOS-Brashear’s
current contract to provide the 0.65 meter diameter, aspheric SiC secondary mirror for the Advanced
Technology Solar Telescope (ATST).
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.
The LOng-Range Reconnaissance Imager (LORRI) is an instrument that was designed, fabricated, and qualified for the New Horizons mission to the outermost planet Pluto, its giant satellite Charon, and the Kuiper Belt, which is the vast belt of icy bodies extending roughly from Neptune's orbit out to 50 astronomical units (AU). New Horizons is being prepared for launch in January 2006 as the inaugural mission in NASA's New Frontiers program. This paper provides an overview of the efforts to produce LORRI. LORRI is a narrow angle (field of view=0.29°), high resolution (instantaneous field of view = 4.94 μrad), Ritchey-Chretien telescope with a 20.8 cm diameter primary mirror, a focal length of 263 cm, and a three lens field-flattening assembly. A 1024 x 1024 pixel (optically active region), back-thinned, backside-illuminated charge-coupled device (CCD) detector (model CCD 47-20 from E2V Technologies) is located at the telescope focal plane and is operated in standard frame-transfer mode. LORRI does not have any color filters; it provides panchromatic imaging over a wide bandpass that extends approximately from 350 nm to 850 nm. A unique aspect of LORRI is the extreme thermal environment, as the instrument is situated inside a near room temperature spacecraft, while pointing primarily at cold
space. This environment forced the use of a silicon carbide optical system, which is designed to maintain focus over the operating temperature range without a focus adjustment mechanism. Another challenging aspect of the design is that the spacecraft will be thruster stabilized (no reaction wheels), which places stringent limits on the available exposure time and the optical throughput needed to accomplish the high-resolution observations required.
LORRI was designed and fabricated by a combined effort of The Johns Hopkins University Applied Physics Laboratory (APL) and SSG Precision Optronics Incorporated (SSG).
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.
A filtered imager, the CONTOUR Forward Imager (CFI), was designed, fabricated, and qualified for the Comet Nucleus Tour (CONTOUR) Discovery class mission. The CONTOUR spacecraft was launched July 3, 2002, and failed during injection to heliocentric orbit on August 15, 2002. This paper provides an overview of the efforts to produce CFI.
The CFI imager was designed to perform optical navigation, comet nucleus imaging, and comet coma imaging. CFI was complemented in the CONTOUR payload by the CONTOUR Remote Imager and Spectrometer (CRISP). The emphasis in the CFI design was on high sensitivity at moderate to long ranges from the comet nucleus, while CRISP was designed for high-speed observations in close to the nucleus. A unique aspect of CFI was the requirement to image multiple comets after being exposed to high-velocity cometary dust on the
previous comet flybys (which damages and contaminates the forward looking optics). The first optical surface was replaceable between comet encounters, using a mirror "cube" mechanism, to alleviate the dust damage. Another challenging aspect of the design is that the spacecraft was thruster stabilized (no reaction wheels), placing limits on the available exposure time to accomplish the high sensitivity observations required.
CFI utilized ten filters covering from 300 to 920 nm to image onto a backthinned 1024 by 1024 element CCD. The Ritchie-Chrietien telescope provided a clear aperture of 62 mm, a full field of view of 2.5 degrees, and a pixel field of view of 43 microradians. CFI was designed and fabricated by a combined effort of the Johns Hopkins University Applied Physics Laboratory and SSG Precision Optronics. The CONTOUR mission was lost prior to CFI being powered on in flight.
The CONTOUR Remote Imager and Spectrometer (CRISP) was a multi-function optical instrument developed for the Comet Nucleus Tour Spacecraft (CONTOUR). CONTOUR was a NASA Discovery class mission launched on July 3, 2002. This paper describes the design, fabrication, and testing of CRISP. Unfortunately, the CONTOUR spacecraft was destroyed on August 15, 2002 during the firing of the solid rocket motor that injected it into heliocentric orbit. CRISP was designed to return high quality science data from the solid nucleus at the heart of a comet. To do this during close range (order 100 km) and high speed (order 30 km/sec) flybys, it had an autonomous nucleus acquisition and tracking system which included a one axis tracking mirror mechanism and the ability to control the rotation of the spacecraft through a closed loop interface to the guidance and control system. The track loop was closed using the same images obtained for scientific investigations. A filter imaging system was designed to obtain multispectral and broadband images at resolutions as good as 4 meters per pixel. A near IR imaging
spectrometer (or hyperspectral imager) was designed to obtain spectral signatures out to 2.5 micrometers with resolution of better than 100 meters spatially. Because of the high flyby speeds, CRISP was designed as a highly automated instrument with close coupling to the spacecraft, and was intended to obtain its best data in a very short period around closest approach. CRISP was accompanied in the CONTOUR science payload by CFI, the CONTOUR Forward Imager. CFI was optimized for highly sensitive observations at greater ranges. The two instruments provided highly complementary optical capabilities, while providing some degree of functional redundancy.