As with other NASA observatory data, there are many, many header keywords in BCD data, many of which pertain to instrument telemetry rather than astronomy, per se. This section highlights the most important header keywords for BCD MIPS data. A comprehensive list of the BCD header keywords are available in Appendix C. Most of the usual FITS header keywords you expect appear in both the BCD and post-BCD products (CRVAL1 and 2, CRPIX1 and 2, BUNIT, etc.). Here, we highlight some Spitzer-specific keywords of interest.
AORLABEL is the name of the AOR as defined by the observer requesting the observations. This name is unique for each AOR within the program, but it is not unique to the entire mission (which is why we have the AORKEY as well, see next paragraph below). OBJECT is the observer-defined target name, and may or may not relate directly to the AORLABEL. (See section 6.2 above for how to determine the AOR label and target for any given AOR.) PROGID is the program identifier for the program that contains the AOR that was used to obtain the data in question; see section 6.2 above for why this can be useful.
Each AOR has a 10-digit number called the AORKEY, which is, in fact, unique to each observation for the entire Spitzer project. This value is mostly designed to be only computer-readable, but may be of use for, e.g., corresponding with the SSC regarding a specific AOR. The AORKEY is a header keyword, and is also part of the filename for each BCD. Other elements of the filename (CHNLNUM, EXPID, DCENUM) are also header keywords.
AOT_TYPE tells you if the data were taken in scan or photometry mode (or in any of the other MIPS modes). This is particularly useful for double-checking the pixel scale of your 70 micron observations (fine or default). FOVNAME is the name of the field of view used for the observations; see Table 2.1 for a comprehensive list.
PRIMEARR tells you if the current data is 'prime' or not; see section 6.3.1 for more on these issues.
CREATOR includes the SSC software version, e.g., S13 (see sections 5.1.2 and 5.2.3).
DATE_OBS is the time at the start of the AOR, good to within at least 1 second. There are many other times in the header, but if the specific time of the start of your AOR matters, this is the one to use.
ANNLTIME is time since the last Ge anneal.
FILTERS gives the information about how the *.fbcd files were produced.
WARMFLAG indicates whether data were taken during a COLD or WARM 160 micron campaign.
STMFL_70 and STMFL_160 tell whether the image (70 micron or 160 micron, depending on keyword) is a calibration stim flash (see section 4.1.1). Any number greater than 0 indicates that it is a stim flash frame.
CSM_PRED is the predicted mirror start position. Flattening at 24 micron of each BCD depends on the scan mirror position. The exact postion is a bit uncertain (and that is why the SSC developed a special flattening procedure for 24um), but the best indicator is the "CSM_PRED" header keyword. Indeed the individual flat frames at 24um delivered in the CAL subdirectory are named using this value (or close to this value) as an indicator. The file "mips24spotmap_0._1864.75_x116.501_y126.310.fits" was applied for a CSM_PRED = 1864.75.
Under the PHOTOMETRY section of the header are the keywords BUNIT, the units of the image data, FLUXCONV, the flux conversion factor in units of DN/sec to BUNIT, and GAIN, the electron/DN conversion. In addition there are three keywords that provide an estimate of the infrared background three main components: ZODY_EST for Zodiacal Light , ISM_EST for Interstellar Medium and CIB_EST for Cosmic Background. For example:
BUNIT = 'MJy/sr ' / Units of image data
FLUXCONV= 0.0447 / Flux Conv. factor (DN/s to BUNIT)
ISM_EST = 0.1186434 / [MJy/sr] Interstellar Medium Estimate
CIB_EST = 0. / [MJy/sr] Cosmic IR Background Estimate
These estimates are per BCD and can be useful when mixing multiple epochs observations
to produce a final mosaic.
Distortion in Spitzer images is represented through header keywords consistent with the latest FITS standard, a CD Matrix using header keywords CD1_1, CD1_2, CD2_1, and CD2_2. The most versions of your favorite image display programs should be able to handle this, but it is possible some older programs may not.
The coordinates and pointing as represented in the RA_HMS and DEC_DMS and related keywords are the actual pointing (in sexigesimal), reconstructed on the ground from telescope and scan mirror telemetry. If you prefer decimal degrees, look at CRVAL1 and CRVAL2. There are also header keywords for predicted and/or requested positions, but these values reflect when the observation was scheduled, not necessarily when (or where) it was actually executed.
Somewhat confusingly, some of the keywords that appear in the BCD are redefined in the post-BCD stage. In the BCD, PA is the position angle of axis 2, in degrees E of N, so positive values are counter-clockwise. PXSCAL1 and PXSCAL2 are the scale for axes 1 and 2, respectively, in arcsec/px, at position CRPIX1, CRPIX2. In the Post-BCD, however, CROTA2 is the orientation of axis 2, in degrees W of N, so positive values are clockwise. CDELT1 and CDELT2 are the scales for axes 1 and 2, in degrees per pixel.
Note that the predicted pointing header keywords may not necessarily be correct. The spacecraft really is pointing in the correct location, but the header keywords are not necessarily correct.
