7.1.1 Pick-off mirror contamination and systematic pointing: practical information
There is some contamination on the pick-off mirror (POM) that feeds MIPS. These dark spots appear to move with celestial objects during scan mirror moves (dithers). The darkest, most well-defined spots are 3-4 pixels across and are ~15-20% dimmer than surrounding pixels; they are located primarily in the upper right of the 24 µm array; see Figure 7.1, Figure 7.3, and Figure 7.4. There are also diffuse features that have a ~1-5% effect. A map of all of the spots seen during normal scan mirror moves based on 24 micron observations is in Figure 7.5. These spots appear to be stable; there is no evidence for ''growing'' or ''jumping'' spots.
The standard MIPS pipeline measures the position of the darkest spots in each BCD to select appropriate flats from our database. Unfortunately, when point sources or bright diffuse structure affect the dark spot positions, the match can be bad. Thus, some individual BCDs may show spot residuals. For BCDs where the processing was unable to find spot centroids, flattening will have been done using a ''fallback'' flat without any spot correction. The pipeline also delivers enhanced BCDs (EBCDs) that use all the frames at a given mirror position within each AOR to determine spot positions. This AOR-based flattening algorithm generally provides flats with fewer cosmetic defects from spot position mismatches, although there are exceptions for a small number of BCDs where complicated backgrounds confuse the AOR-based flattening algorithm.
The fact that the scan mirror does not exactly reproduce its position has implications for the pointing; the pointing for any one campaign can be systematically offset by about 0.5 arcseconds in either direction. You can fix this effect by manually comparing a source list to 2MASS sources and editing the CRVAL1 and 2 header keywords. Note that a list of 2MASS sources is one of the products served to you by the Archive.
Figure 7.3: Two MIPS-24 flat fields at two different scan mirror positions. Note bad pixels (masked out and white) and dark spots from pick-off mirror contamination. The dark spots have a depth of about 20% and move when the scan mirror moves; see text. The gradient in the flat varies from 1.1 in the top left corner to 0.85 in the bottom right.
Figure 7.4: Pick-off mirror spots before (left, circled in red), and after (right) using scan mirror position dependent flats on a calibration star observation.
Figure 7.5: Map of the spot locations based on 24 micron observations.
Figure 7.6: Results of dividing mismatched position-dependent flat from the data - zoom in on just one spot.