For most applications, MIPS can be taken to be purely diffraction-limited. The predicted wavefront error introduced by the instrument optics at the center wavelength of each photometric band is listed along with the requirements in Table 2.3. Only in the 24 micron band is the predicted wavefront distortion significant and the quoted value holds only near corners of the field of view; the wavefront error is significantly smaller over most of the array. The distortion is measured to be exactly as predicted at 24 micron.
Because the MIPS optical train is made up purely of off-axis reflective elements, some degree of scale change across the re-imaged focal planes is inevitable. All optical trains meet the requirement that this distortion should not exceed 10% (defined as 100 x [(max scale)/ (min scale) - 1]). Most bands meet the requirement comfortably (see Table 2.3). Nonetheless, to coadd images taken at differing places on the array, and for other science applications, it is important to correct the data in all bands for the image scale changes.
Nominal corrections of optical distortions are performed in pipeline processing of the MIPS images and maps, and necessary data can be supplied to the observer to perform the corrections on the Basic Calibrated Data (BCD) images if desired (see Chapter 6, Data Products).
Table 2.3: Optical distortion in MIPS.
RMS wavefront req./performance
Distortion (req. 10%)
70 micron survey
70 micron super resolution
70 micron SED
Scan Map Mode operation requires the use of the MIPS scan mirror to compensate for image motion during Spitzer spacecraft continuous scan motion. The desired result is the simultaneous delivery of a succession of stationary sky images to all MIPS Focal Plane Arrays (FPAs). Ideally the spacecraft slews its view vector along a straight line in object space (able to be oriented to MIPS needs) at a constant angular velocity, while the MIPS scan mirror moves at a constant angular velocity (to which the spacecraft rate can be matched). Given an ideal optical system, the full-field images will remain fixed on the FPAs for the duration of the integrations.
The actual instrument departs slightly from these ideal conditions. There are two general classes of image smear due to the scan mirror motions:
1. Image smear sources in individual bands
a. Departures from constant angular velocity.
b. Changes in field distortion or magnification during scan motion.
2. Image smear sources as a result of a mismatch between bands
a. Angular magnification mismatch between the sides of the instrument at the scan mirror.
b. Direction of image motion mismatch between sides of the instrument.
Analysis of scan mirror operation during instrument tests indicates that all of these sources of image smear should lie well within the FWHM of the diffraction limited image, and for most observing modes should be virtually undetectable in the observed shape of the PSF. The one minor exception is in the scan map mode at the fast scan rate where a slight elongation of the images may occur due to sources of type 2, which cannot be reconciled through adjusting the spacecraft motion or the scan mirror action. The mismatch may become apparent in this mode because of the relatively long throw over which the scan mirror is used to freeze the image motion.
Reflections from filters in front of focal plane arrays frequently cause ghost images. In the case of the 160 micron band, it has been possible to tilt the bandpass filter far enough so that no ghosts will occur, but at 24 and 70 micron very faint, slightly out-of-focus ghost images do appear. The intensity of the ghost images measured during tests of the integrated MIPS is 0.5% of the brightness of the primary image at 24 micron, and is 2% of the brightness of the primary image at 70 micron. The single ghost image formed on the 70 micron array is 6 pixels (in wide-FOV mode) from the primary image.