The Multiband Imaging Photometer for Spitzer (MIPS) provides the Spitzer Space Telescope with capabilities for imaging and photometry in broad spectral bands centered nominally at 24, 70, and 160 µm, and for low-resolution spectroscopy between 55 and 95 µm. The instrument contains 3 separate detector arrays each of which resolves the telescope Airy disk with pixels of size or smaller. All three arrays view the sky simultaneously; multiband imaging at a given point is provided via telescope motions. The 24 μm camera provides roughly a 5´ square field of view (FOV). The 70 μm camera was designed to have a 5´ square FOV, but a cabling problem compromised the outputs of half the array; the remaining side (''side A'') provides a FOV that is roughly 2.5´ by 5´. The 160 µm array projects to the equivalent of a 0.5´ by 5´ FOV and fills in a 2´ by 5´ image by multiple exposures. The 70 µm array also has a narrow FOV/higher magnification mode, and is additionally used in a spectroscopic mode.
The MIPS cryogenic scan mirror mechanism (CSMM) is integral to all observational operations, allowing selection of different optical trains, image motion compensation during scanned imaging, and one dimensional image dithering. A brief, high-level summary of MIPS for astronomers appears in the ApJS Spitzer Special Issue, specifically the paper by Rieke et al. (2004, ApJS, 154, 25) entitled ''The Multiband Imaging Spectrometer for Spitzer.'' A copy of this paper is available on the website.
2.2 Description of Optics
MIPS employs three distinct array detectors (128x128 pixel Si:As BIB; 32x32 pixel Ge:Ga; 2x20 pixel stressed Ge:Ga) to provide high-sensitivity, low-noise, diffraction-limited performance from roughly 20 µm to 180 µm. The short-wavelength Si:As array is combined with a permanent filter to give a bandpass from 20.8 to 26.1 µm with a weighted average wavelength of 23.68 µm. The Ge:Ga array is combined with a filter to give a bandpass from 61 to 80 µm with a weighted average wavelength of 71.42 µm for imaging. This array can also be used in conjunction with a slit and grating to provide low-resolution () spectroscopy from 55-95 µm in Spectral Energy Distribution (SED) mode. The 2x20 stressed Ge:Ga array is combined with a permanent filter to give a bandpass from 140 to 174 µm with a weighted average wavelength of 155.9 µm. The pixel size for all 3 arrays is roughly to sample the telescope Airy pattern fully.
In addition, a fine pixel scale/narrow FOV can be selected for use with the 70 µm band to provide higher spatial resolution than is possible with the nominal pixel scale, but at a somewhat degraded sensitivity. Selection between the various bands and modes of operation is accomplished through the cryogenic scan mirror, which deflects incoming light into the desired optical path. The scan mirror is also used for image motion compensation during mapping operations, and provides chopping between source and sky to improve the accuracy of the MIPS long wavelength photometry (Rieke, 2004).
2.2.1 Spatial and Spectral Resolutions
Figure 2.1 schematically illustrates the physical layout of the major optical elements (24 µm Si:As, 70 µm Ge:Ga, and 160 µm stressed Ge:Ga focal plane arrays (FPAs), movable scan mirror, and fixed mirrors and grating) within the MIPS cold assembly. Spitzer's central axis and the telescope focal plane are to the right in this view. Two plane mirrors in the telescope focal plane deflect light into the instrument where it is reflected back by 2 mirrors to form pupils at the two facets of the Cryogenic Scan Mirror Mechanism (CSMM). The CSMM provides chopping at about 0.1 Hz, linear ramp motions, and also deflects the light into the desired optical train. Light is simultaneously sent into the 3 wide-field optical trains, or into the 70 µm narrow FOV train, or into the 55-95 µm SED optical train.
The telescope point spread function (PSF) size () is 6'', 18´´, and 40´´ full width at half maximum, at 24, 70 and 160 µm respectively. To achieve adequate sampling of the PSF for accurate photometry and for super-resolution imagery (to be produced via post-processing), a pixel size somewhat finer than the conventional Nyquist limit of is required. This criterion is met by the pixels of the 24 µm and 160 µm arrays, which have angular sizes of 2.55´´ and 16´´x18´´ respectively. The pixels of the 70 µm array are somewhat larger than optimal in the wide-FOV mode, having an angular size of 9.98´´. This pixel scale was chosen to provide the highest possible 70 µm sensitivity given the expected rate of cosmic ray hits, at the cost of some spatial resolution. In the narrow-FOV mode the 70 µm array pixels have an angular size of 5.2´´, small enough to provide excellent performance for photometry and super-resolution, but at the cost of losing some sensitivity.
Figure 2.1: Schematic diagram of the MIPS optical train, detectors (Focal Plane Arrays - FPAs), and optical paths. Two mirror facets are attached to the Cryogenic Scan Mirror Mechanism (CSMM): one mirror feeds the 70 µm optical train (normal FOV, narrow FOV and spectrometer/SED), while the second mirror feeds both the 24 µm and 160 µm optical trains. The CSMM provides chopping and one-dimensional dithering for all 3 arrays as well as selecting among the various bands and modes.
The Spectral Energy Distribution (SED) mode utilizes a 3.8´x0.32´ slit to produce low-resolution (R=15-25) spectroscopy covering a wavelength range of 52-97 μm. As half of the original array has significantly higher noise, the usable length of the spectroscopic slit is actually about 2´. The SED slit was originally designed to be offset with respect to the array such that an 8x32 portion of the array is unilluminated in SED mode. This strip was supposed to provide a dark measurement for the 70 µm array when taking measurements in TP mode; the other two arrays can be put in the dark by suitable positioning of the scan mirror. This special treatment for the 70 µm array in TP mode is required because of the multiple 70 µm optical paths which might allow light to contaminate the dark measurement. Unfortunately, this region falls on side B of the 70 µm array, i.e., the noisier side. Therefire the TP mode used side A only, chopping between the sky position and an internal dark position.