The FITS header keywords in an IRS BCD data file provide detailed information about the observation. The header is updated late in the pipeline as part of the creation of the BCD file. Thus, the bcd.fits file contains the updated header, while partially-processed products from earlier in the pipeline may have different keywords. In addition, a history of the pipeline processing is provided at the end of the FITS header. In this chapter, we review the keywords in the spectroscopic bcd.fits header. Many keywords are self-explanatory, so we highlight only those of greatest interest to the observer. An example FITS header is given as an appendix to this handbook. The meaning of most keywords is the same for Peak-Up Imaging observations.
6.6.1 Basic Keywords
CALSET indicates the version of the calibration files used to produce the data product.
CREATOR indicates the latest pipeline version used to reprocess the files.
AORKEY is a unique 7-digit identification number for each Astronomical Observation Request. It is part of the file name for each BCD.
PROGID gives the program ID under which the current data were obtained.
FOVNAME is the name of the field-of-view used for the observation. For example, a Staring Mode observation using SL1 will result in DCEs acquired with the target (given by header keywords RA RQST and DEC RQST) positioned first at "IRS Short-Lo 1st Order 1st Position", and then at "IRS Short-Lo 1st Order 2nd Position", corresponding to each of the designated nod positions. See also Section 6.6.4.
BUNIT gives the units of the data; e.g., “electrons/sec".
FLUXCON gives the conversion factors from electrons/sec to Janskys. This keyword is given for each spectral order, but pertains only to the module associated with the BCD.
VHELCORR and VLSRCORR allow correction of the spectra for the orbital velocity of Spitzer. The values of these keywords should be added to velocities derived from IRS spectra, to obtain final velocities in the Heliocentric or LSR frames respectively. These keywords will be of interest only to observers measuring velocities with high precision from Short-High or Long-High spectra.
For bookkeeping, it may be useful to search for particular targets by looking at OBJECT = Target Name.
The IRS peak-up keywords are mostly relevant to the peak-up DCEs. In spectral DCEs the only peak-up keyword of interest is APKUPCEN. If the value of this keyword is 1, then the corresponding peak-up was successful, meaning that the centroid of the brightest source in the peak-up array field-of-view was returned.