Beginning in pipeline S18.12, there are two versions of the BCD products for MIPS-24 Photometry data: the original BCDs (bcd.fits) and the enhanced BCDs (ebcd.fits). The BCD and EBCD pipelines differ in the method of flat fielding, as explained below. The EBCDs should, in most cases, have superior flat fielding. The delivered Post-BCD mosaics are made only from the EBCDs (for data from the photometry mode). See Table 6.1 for a complete list of EBCD files.
To properly appreciate the difference in the EBCD products, one needs to understand the three different types of flats that are used to flatfield 24 micron data:
Campaign gain flats. These are from ''campaign flats'' made from observations of relatively blank sky at the beginning of each campaign. The gainflat is either made from a dedicated flat field observation at the beginning of the observing campaign or, for early mission campaigns before appropriate latent-free flats were taken, a fallback flat. The two file names below are examples of the smooth, spot-free gain flat and mask for the BCD pipeline. Figure 6.2 is an example of a typical gain flat.
SPITZER_M1_20668928_0000_6_A31018392_fcmsk.fits gain flat mask SPITZER_M1_20668928_0000_6_C6025416_flat.fits gain flat
Figure 6.2: An example of an overall smooth (spot-free) gainflat image.
Spotmaps. The spotmap for each BCD is taken from a large library of fallback spotmaps. The spotmaps are matched independently for each individual BCD by calculating the centroid of a fiducial spot on the (pre-flattened) BCD and picking the closest match in the library to the calculated position. See Figure 6.3 for example spotmaps. For the original BCD pipeline, the spotmaps have names like:
Figure 6.3: Example spotmap images. Top corresponds to mirror position CSM_PRED=1907.5, while the bottom is CSM_PRED=2149.5.
Default flatfields if spotmatching fails. If spotmatching fails for any given BCD, a fallback flat field is used that matches the CSM_PRED but is not necessarily a good match to the spot position. This flat field is a single file with the CSM_PRED value in the filename but with no x and y position of the spot. It represents the product of a spotmap and a smooth flat field (therefore it is NOT a spotmap file). An example of such a fallback and its mask are shown below (this particular example is from a medium scan observation). Spotmatching fails more frequently for scan mode observations because the spots are streaked and it is more difficult to derive good centroids, especially for regions with bright backgrounds.
The filename convention for the default flatfield when spotmatching fails is:
Enhanced BCD (EBCD) Flat Fielding
The 24 micron flatfields are the product of a smooth, spot-free gainflat and a ''spotflat'' shifted to match the spot locations in each BCD. For the EBCDs, all of the BCDs in the AOR are used to compute a single spot offset (SPOT_DY) in the y-direction relative to a reference template spotflat file. This single offset shift is applied to all of the planes in the spotflat file. The shifted spotflat file is a cube, matching the spot position of the current AOR, with one plane for each mirror position used in MIPS-24 photometry mode, in ascending order of header keyword CSM_PRED (predicted cryogenic scan mirror position). After shifting the reference to match the current AOR, the appropriate planes are used for flat fielding. In cases of 24 micron data taken in parallel to Ge observations, the improvement in flat fielding could be quite significant. See Figure 6.6 and Figure 6.5 for examples.
The headers of these files have explanatory keywords (COMM1,COMM2, etc.) describing the nature of each file.
Figure 6.4: A small field photometry example, demonstrating the S18.12 EBCD flat fielding improvements. On the left is the S16 version with the uncorrected spots circled. On the right is the S18.12 EBCD version of the same mosaic.
Figure 6.5: An example of the significant improvement of 24 micron data taken in parallel mode. Above is a parallel mosaic taken when enhanced 160 micron is the primary AOT. On the left is the S16 version while on the right is the S18.12 EBCD version of the same mosaic. The Enhanced 160 mode had effectively no spot correction before S18.12.