IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
`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.
UTCS_OBS is the start of IRAC data taking sequence. 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. See the IRAC FITS header in Appendix D for more
timing related keywords. 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 “data numbers” (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 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. DS_IDENT is a
journal identification number for the Astrophysics Data System (ADS) to keep track of papers published
from these data.
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_RFND is the refined position angle of the +y axis of the image, measured east from north (CROTA2
is the same position angle but measured west from north). CRVAL1 and CRVAL2 give the coordinates of
the image center, derived from the refined positions in all channels, and are usually the most accurate
coordinates available.
Sometimes a bad pixel value (zero) was inserted in the 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.
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
Data Products 100 IRAC Specific Header Keywords