Cryogenic Scan Mirror Mechanism for SIRTF/MIPS
Flexural pivots in a cryogenic space telescope scan mirror
Robert M. Warden & Gerald B. Heim — Ball Aerospace & Technologies, Boulder, Colorado
Overview
A Cryogenic Scan Mirror Mechanism (CSMM) was developed for the Multi-band Imaging Photometer for SIRTF (MIPS), one of three scientific instruments in the focal plane of the Space Infrared Telescope Facility. The design is based on the ISO Short Wave Spectrometer scan mirror, modified to satisfy MIPS requirements. The CSMM is a limited angle rotation device (±7.5°) using steel flexures to support the mirror while allowing rotation without wear or friction.
Key Highlights
Cryogenic Operation
Launched and operated at approximately 1.5 Kelvin
Flex Pivot Support
Two 3/16" diameter Lucas Flex Pivots support the rotating mirror
Ultra-Low Power
Superconducting voice coil wire reduces dissipation to practically unmeasurable levels
3 Million Cycles
Designed for ~3 million cycles over a 2.6-year mission lifetime
Redundant Sensing
Dual differential impedance transducers replace the LVDT (<100 µW vs 350 µW)
Infinite-Life Flexures
Beryllium Copper electrical flexures analyzed for infinite fatigue life
Mechanism Overview
The CSMM is the only moving part in the MIPS instrument and serves three functions: mapping a region of sky by compensating for SIRTF scan motion, performing beam switching between a celestial source and nearby blank sky, and holding the mirror in specified positions to direct light into the appropriate optics for each observing mode. A linear voice coil motor offset from the center of rotation provides actuation force. The closed-loop bandwidth requirement is only about 10 Hz — this is not a fast steering mirror.
Power Dissipation Reduction
Operational lifetime is closely tied to power dissipation, which raises cryogen boil-off. The ISO design used standard copper wire dissipating 0.65 mW at maximum deflection. The MIPS team substituted superconducting wire (54 filaments of niobium titanium manufactured by Supercon), reducing dissipation at cryogenic temperature to practically unmeasurable levels while retaining ambient-temperature operability. The position sensor was also redesigned: the ISO LVDT dissipated ~350 µW, while the new differential impedance transducer dissipates less than 100 µW.
Flexural Elements
The CSMM has only two moving parts, both flexural by design, avoiding all friction and debris problems of sliding surfaces. The mechanical flexures are two 3/16" diameter Lucas Flex Pivots rated at about ten times the estimated launch load and analyzed to survive ten times the required life. The electrical flexures (service loops powering the voice coil) were changed from 0.01 mm stainless steel to Beryllium Copper, which has much greater fatigue strength and is easier to solder — analysis showed infinite life for this material in this application.
Life Testing
The CSMM must survive approximately 3 million cycles during its 2.6-year lifetime: 100,000 large-scale deflection changes (~7.5°) and 2.9 million small-angle operations (~1°). A 2× safety factor was applied. Impact loading from the rotating mass hitting end-of-travel stops was analyzed and tested — confirmed not to be an issue provided the rotating mass is well balanced.
Cryogenic Scan Mirror Mechanism for SIRTF/MIPS — PDF
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