Here we describe some of the important header keywords (given in boldface in this section for clarity). Complete IRAC image headers are included in Appendix E.
AORLABEL is the name of the AOR as it was defined by the user in Spitzer Observation Planning Tool Spot when the observations were requested. The P.I. of a program under which the data were taken will be listed as the OBSRVR of each project. AORKEY is a unique identification number or digit sequence for each observation; it is also part of the filename for each BCD.
EXPID is an exposure counter incremented within a given AOR for each data-taking command. Most data-taking commands generate multiple files: one per array in full array mode. The DCENUM is a counter of individual frames (per wavelength) from an individual command; it can be used to separate frames generated with internal repeats. The only observations with non-zero DCENUM are channel 4 BCDs for 100/200 second frame time (taken as two/four 50 second frames). In high dynamic range mode, the long and short exposures are generated with independent commands and have different EXPIDs. Thus, for example, data from 12-second high dynamic range observations can be separated into long and short frames using the odd or even EXPIDs.
DATE_OBS is the time at the start of the AOR. Other times in the header include the time since IRAC was turned on (both for the beginning and end of the frame). FRAMTIME is the duration of the frame including Fowler sampling, and EXPTIME is the effective integration time (“exposure time”).
UTCS_OBS is the start of IRAC data taking sequence. ET_OBS is the beginning time of the observation (DCE) in barycentric dynamical time (TDB) seconds since J2000 or 2000 January 1, 12 hours (JD 2451545.0). SCLK_OBS is the Spitzer Spacecraft clock time at the beginning of the observation (DCE) since 1980, January 1, 00:00:00 UT. Modified Julian date is in keyword MJD_OBS and the corresponding heliocentric modified Julian date in HMJD_OBS. There is also a Solar System barycentric modified Julian date in BMJD_OBS. Note that BMJD_TDB = BMJD_OBS + (ET_OBS - UTCS_OBS) at the start of the DCE. In Instrument Engineering Request (IER; used by the IRAC team to perform non-standard IRAC observations) observations where several IERs are taken successively in the staring mode, only the first IER contains good requested pointing information that is used in calculations of HMJD and BMJD, and therefore these keywords are not reliable in the successive IERs. ATIMEEND is the correct time of an integration end. HDRMODE tells you if the frame was taken in the high dynamic range mode.
BUNIT gives the units (MJy/sr) of the images. For reference, 1 MJy/sr = 10–17 erg s–1 cm–2 Hz–1 sr–1. FLUXCONV is the calibration factor derived from standard star observations; its units are (MJy/sr)/(DN/s). The raw files are in DN. To convert from MJy/sr back to DN, divide by FLUXCONV and multiply by EXPTIME. To convert DN to electrons, multiply by GAIN.
The predicted background (using the same model as what was implemented in Spitzer Planning Observations Tool Spot, evaluated for the wavelength, date, and coordinates of observation) is contained in three keywords: ZODY_EST, ISM_EST, CIB_EST. These are not based on the actual data from Spitzer. SKYDRKZB is the zodiacal background prediction for the skydark that was subtracted from the science image in the reduction pipeline. Thus, the predicted background in the BCD data is ZODY_EST – SKYDRKZB.
Absolute pointing information is contained in the following keywords. ORIG_RA and ORIG_DEC give the coordinates of the image center constructed from the telemetry using the Boresight Pointing History File, as indicated by the Boolean keyword USEDBPHF, and the file is listed in BPHFNAME. When pointing telemetry is not available, due to a telemetry outage, the commanded positions are inserted instead, USEDBPHF is false, and the coordinates will be less certain. RARFND and DECRFND are the refined positions derived by matching the brightest sources in the image with the 2MASS catalog. PA is the position angle of the +y axis of the BCD image, measured east from north. CRVAL1 and CRVAL2 give the coordinates of the image center (pixel coordinates CRPIX1, CRPIX2), derived from the refined positions in channels 1 and 2 (superboresight pointing refinement), and are usually the most accurate coordinates available.
Sometimes a bad pixel value (having the value of exactly zero) can be found among the array pixels in the FITS data field. These pixels are detected and shown in raw frame header where ABADDATA assumes the value of 1. In the BCD FITS header you will then find header keyword BADTRIG set to “T” (true) and the number of zero pixels in the frame listed in header keyword ZEROPIX. If there is only one bad pixel, the pipeline fixes the problem and gives the bad pixel position in header keyword ZPIXPOS.
The BCD +x-axis (bottom, or horizontal axis) is in the direction of the telescope +Y-axis, and the BCD –y-axis (left side or vertical axis) is in the direction of the telescope +Z-axis.
DS_IDENT is a journal identification number for the Astrophysics Data System (ADS) to keep track of papers published from these data.
Next, we give an example of how an AOR file translates into final data products. A Spitzer observation is specified by a small list of parameters that are listed in the “.aor" file. This file was generated when the observation was designed (using Spot). Here is an example AOR file:
For this AOR, there are 210 files (6 columns x 7 rows x 5 dither positions) of each type for each channel. The final data products from this AOR in channel 2, provided it got assigned the AORKEY 6213376, are as follows:
SPITZER_I2_0006213376_0000_0000_01_dce.fits
SPITZER_I2_6213376_0000_0000_1_bcd.fits
SPITZER_I2_6213376_0000_0000_1_cbcd.fits
SPITZER_I2_6213376_0000_0000_1_bcd.log
SPITZER_I2_6213376_0000_0000_1_bunc.fits
SPITZER_I2_6213376_0000_0000_1_cbunc.fits
SPITZER_I2_6213376_0000_0000_1_bimsk.fits
SPITZER_I2_6213376_0000_0000_1_brmsk.fits
SPITZER_I2_6213376_0000_0000_1_ptn.log
(SPITZER_I2_6213376_0000_0000_1_bdmsk.fits: historical; replaced by bimsk.fits)
....
SPITZER_I2_0006213376_0209_0000_01_dce.fits
SPITZER_I2_6213376_0209_0000_1_bcd.fits
SPITZER_I2_6213376_0209_0000_1_cbcd.fits
SPITZER_I2_6213376_0209_0000_1_bcd.log
SPITZER_I2_6213376_0209_0000_1_bunc.fits
SPITZER_I2_6213376_0209_0000_1_cbunc.fits
(SPITZER_I2_6213376_0209_0000_1_bdmsk.fits: historical; replaced by bimsk.fits)
SPITZER_I2_6213376_0209_0000_1_bimsk.fits
SPITZER_I2_6213376_0209_0000_1_brmsk.fits
SPITZER_I2_6213376_0209_0000_1_ptn.log
After the name of the telescope, the first partition gives the instrument (“I” = IRAC), and the number after the “I” gives the channel (in this case, 2). The next part gives the AORKEY, then we have the EXPID, DCENUM, and the version number (how many times these data have been processed through the pipeline). One should generally use only the data from the highest version number, in case multiple versions have been downloaded from the archive. To verify that the data are from the latest pipeline version, check the CREATOR keyword in the header. Finally, there is a group of letters that specify what kind of data are in the file (see Table 6.1 above), and the file type (usually “fits” or “log”). The post-BCD file names include telescope name (SPITZER), “I” (for “IRAC”), the channel number, the productid (not the same as the AORKEY), the DCENUM, the (pipeline) version, “ensemble product id,” the type of the data and the suffix. In the case of an ensemble product, “DCENUM” in the filename refers to the first DCE that was used in the ensemble creation, and “version” refers to the version of that first DCE. The letter “C” stands for “calibration" product: in the case of a calibration product, “DCENUM” refers to the first DCE that was used in the calibration creation, (pipeline) “version” refers to the version of that first DCE, and the number after the “C” letter is the “calibration number.” Note that for a given AORKEY of science data being retrieved, the AORKEY for the associated calibration products is different.
A list of 2MASS sources for the field of the IRAC observation is included in the data delivery as *irsa.tbl. Note that the 2MASS magnitudes given in the *irsa.tbl file are not meant for scientific use. For scientific use of the 2MASS data, query the 2MASS catalog directly from IRSA, and take into account the flux quality flags.
