Multiband Imaging Photometer for SIRTF
Instrument design, performance, and cryogenic architecture of MIPS
G. B. Heim, M. L. Henderson, K. MacFeely, T. J. McMahon, et al. — Ball Aerospace & Technologies Corp & Steward Observatory, University of Arizona
Overview
This paper presents the full instrument design of MIPS — the Multiband Imaging Photometer for SIRTF — covering mechanical, optical, thermal, electrical, software, and radiometric aspects. A single-axis cryogenic scan mirror mechanism (CSMM) derived from the ISO design provides source modulation, beam switching, and an efficient scan-map mode. The instrument operates below 3 K with focal plane cooling to 1.5 K.
Key Highlights
Single Mechanism Design
Cryogenic Scan Mirror Mechanism — the only moving part — enables four operational modes
Flex Pivot Supported
CSMM uses flex pivots for frictionless rotation at cryogenic temperatures
Diffraction-Limited
Pixel sizes oversample the Airy disk for superresolution beyond the Rayleigh criterion
1.5 K Focal Planes
Challenging cryogenic end-to-end testing at flight operating temperatures
Three Detector Arrays
128×128 Si:As, 32×32 Ge:Ga, and 2×20 stressed Ge:Ga covering 24–160 µm
Scan Map Mode
Telescope scans continuously while scan mirror freezes image — 100% duty cycle
Instrument Design Overview
Light from three zones in the telescope focal plane is directed by a single axis scan mirror (placed at a pupil) to three detector arrays. The scan mirror enables rapid field-of-view motion for source modulation on germanium arrays, selects different optical trains, and enables scan-map mode in which the telescope scans while the mirror freezes image motion on the detectors.
Flex Pivot Scan Mirror Mechanism
The CSMM is the only moving part in the MIPS instrument and is supported by Lucas Free-Flex® Pivots. These flexures allow frictionless, wear-free rotation in the cryogenic vacuum environment without outgassing. The CSMM design was derived from the Infrared Space Observatory scan mirror and modified to meet MIPS cryogenic and power requirements.
Operational Modes
Scan Map mode enables large area surveys with continuous telescope scanning; the scan mirror freezes image motion on all arrays during integration and jumps to the next position. Photometry mode uses scan mirror dithering for sub-pixel sampling. SED mode places the source on a grating spectrometer covering 50–95 µm. Total Power mode chops the sky against the dark instrument interior to measure extended surface brightness.
Cryogenic Challenges
All detector arrays and optics operate below 3 K, with the germanium focal planes requiring 1.5 K. The design challenge was operating silicon CTIA readout circuitry at these temperatures — silicon suffers anomalies at cryogenic temperatures, requiring specially designed CRC-696 readouts. The stressed 160 µm Ge:Ga array uses an Aermet 100 high-strength leaf spring to maintain ~500 MPa uniaxial pressure from room temperature to 1.5 K.
Multiband Imaging Photometer for SIRTF — PDF
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