On-Orbit Performance of the MIPS Instrument
Multiband Imaging Photometer for Spitzer — first far-IR space instrument with true detector arrays
G. H. Rieke, E. T. Young, J. Cadien, et al. — Steward Observatory, University of Arizona & Ball Aerospace
Overview
The Multiband Imaging Photometer for Spitzer (MIPS) provides long-wavelength capability for the Spitzer mission, with imaging bands at 24, 70, and 160 µm and spectral energy distribution measurements from 52 to 100 µm. It is the first far-infrared space astronomy instrument to use true detector arrays, achieving simultaneously high sensitivity, a large imaging field, and maximum angular resolution through critical sampling of the point spread function.
Key Highlights
True Detector Arrays
128×128 at 24 µm, 32×32 at 70 µm, 2×20 at 160 µm
Telescope-Limited Resolution
6", 18", and 40" at 24, 70, and 160 µm respectively
Scan Mirror Modulation
Rapid signal modulation on germanium arrays for stable photometry
On-Board Stimulators
Relative calibration approximately every two minutes
5σ Detection Limits
0.11, 6, and 15 mJy in 500 s at 24, 70, and 160 µm
1% Photometry
Si:As BIB 24 µm array achieves rms relative errors of 1% or better
Overall Instrument Design
Light from the telescope is relayed by pickoff mirrors to field mirrors at the back of the instrument, which form pupils on facets of a scan mirror. From the scan mirror's center position, the 70 µm channel is directed to the large-scale imaging detector array, while the 24 and 160 µm channels are relayed to a second image plane. Nominal fields are 5×5 arcmin at 24 and 70 µm and 0.75×5 arcmin at 160 µm. The scan mirror enables rapid offsets on the sky without the inefficiency of moving and stabilizing the spacecraft.
Four Operating Modes
- Scan Map — observatory scans the sky while the scan mirror executes a reverse sawtooth to freeze images on all three arrays, then jumps forward for the next integration
- Photometry — spacecraft points at the target; scan mirror motions and small spacecraft offsets dither the image across the arrays (minimum 12 measurements)
- SED — object image placed on the 70 µm spectrometer slit; scan mirror modulates the spectrum on and off the array
- Total Power — scan mirror blocks sky view to measure offset level, then exposes sky to measure total flux
Photometric Performance
The Si:As BIB 24 µm array has excellent photometric properties, achieving rms relative errors of 1% or better. The longer-wavelength Ge:Ga detectors have poorer photometric stability, but three techniques overcome this: (1) a scan mirror modulates signals rapidly on the arrays, (2) on-board stimulators provide relative calibration approximately every two minutes, and (3) specialized reduction software. The result is good photometry with rms relative errors under 10% even for the germanium arrays.
On-Orbit Performance of the MIPS Instrument — PDF
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