MIPS : Optics

The telescope point spread function (PSF) size ( lambda / D ) is 6, 18, and 40 arcseconds full width at half maximum, at 24, 70 and 160 microns 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 lambda / 2D is required. This criterion is met by the pixels of the 24 micron and 160 micron arrays, which have angular sizes of 2.55 arcsec and 16x18 arcsec respectively. The pixels of the 70 micron array are somewhat larger than optimal in the wide-FOV mode, having an angular size of 9.98 arcsec. This pixel scale was chosen to provide the highest possible 70 micron sensitivity given the expected rate of cosmic ray hits, at the cost of some spatial resolution. In the narrow-FOV mode the 70 micron array pixels have an angular size of 5.2 arcsec, small enough to provide excellent performance for photometry and super-resolution, but at the cost of losing some sensitivity.

All three arrays simultaneously view non-overlapping fields on the sky. The 24 micron array has a ~5 arcmin square field of view; roughly 8 arcmin away is the 70 micron array. The 70 micron array suffers from thermally-activated high resistance in a cable connection that is external to the instrument, but feeds it. This has resulted in significantly increased noise on side B of the array, rendering that side essentially unusable. Also, one readout on side A (located in the lower right corner in data oriented as observers receive it, next to side B) is inoperative; the resulting shape (roughly 2.5x5 arcmin) can be seen in Spot visualizations.

In a single pointing, the 160 micron array views two 5.3 by 16 strips separated by a 16 wide blank strip giving a 5.3x0.75 arcmin unfilled FOV. The central blank strip of the 160 micron array is filled by taking images at interleaved scan positions. One block of 5 contiguous pixels in the 160 micron array exhibits anomalous behavior attributable to a thermally-activated short in cabling external to MIPS but well inside the CTA. In data oriented as observers receive it, this block is located in the bottom row; it is the third group of 5 pixels in from the left. The "bite" taken out of the array is visible in Spot. These pixels are not quite square at 16x18 arcsec. Occasionally for brevity, we refer to these pixels as 16 square with the knowledge that they are slightly rectangular.

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 micron 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 micron array in TP mode is required because of the multiple 70 micron optical paths which might allow light to contaminate the dark measurement. Unfortunately, this region falls on side B of the 70 micron array, i.e., the noisier side. Further work on this issue is pending.

For more information, see the MIPS chapter of the SOM

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