IRAC Instrument Handbook - IRSA - California Institute of Technology

COMMENT 1 blank line
BUNIT = 'MJy/sr ' / Units of image data
FLUXCONV= 0.2008 / Flux Conv. factor (MJy/sr per DN/sec)
GAIN = 3.8 / e/DN conversion.
5.1.20 Pointing Transfer (calculation of pointing information)
IRAC Instrument Handbook
The Pointing Transfer pipeline is a separate script from the actual data reduction pipeline script, designed
to insert raw-pointing and distortion information into the FITS headers of BCDs. Like the reduction
pipeline, it executes on a per-BCD basis. Pointing data was acquired by the spacecraft star-tracker at a
rate of 2 Hz, transferred to the boresight onboard, and down-linked every 12 hours as a Boresight
Pointing History File (BPHF). The BPHF is received via a separate telemetry stream from the science
data. The first step in pointing transfer is to aquire the portion of the BPHF which spans the integration
time of the BCD (getPH module). The 2 Hz sampled data are then transferred to the specific channeldependent
science FOV using a set of Euler transformations handled by the "BORESIGHTTRAN
module". The Euler angles relating the boresight and FOV positions have been determined in-flight and
are stored in a configuration file.
The pointing samples are then averaged and combined by the "ANGLEAVG" module to compute the
raw-pointing for the BCD: CRVAL1 (RA), CRVAL2 (Dec) and PA (position angle). These, along with
uncertainties and reference pixel coordinates (CRPIX1, CRPIX2), are inserted as keywords into the FITS
header of the BCD. The module also computes a CD matrix and transfers distortion coefficients
(represented in the pixel coordinate frame) from a calibration file to the FITS header. The default
projection type for the celestial reference system (CTYPE keyword) is "TAN-SIP". This is a tangent
(TAN) projection modified to make use of the Spitzer Imaging (distortion) Polynomials (SIP) in
coordinate mappings.
The Final Product Generator (FPG) is executed at the end. This reformats the FITS header and adds
additional keywords, which are most useful to the user, from the database. An example of a BCD header
containing pointing and distortion information is given below.
SIMPLE = T / Fits standard
BITPIX = -32 / FOUR-BYTE SINGLE PRECISION FLOATING POINT
NAXIS = 2 / STANDARD FITS FORMAT
NAXIS1 = 256 / STANDARD FITS FORMAT
NAXIS2 = 256 / STANDARD FITS FORMAT
ORIGIN = 'Spitzer Science Center' / Organization generating this FITS file
CREATOR = 'S18.7.0 ' / SW version used to create this FITS file
TELESCOP= 'Spitzer ' / SPITZER Space Telescope
INSTRUME= 'IRAC ' / SPITZER Space Telescope instrument ID
/ TARGET AND POINTING INFORMATION
OBJECT = 'NGC7479 ' / Target Name
OBJTYPE = 'TargetFixedSingle' / Object Type
CRPIX1 = 128. / Reference pixel along axis 1
Pipeline Processing 86 Level 1 (BCD) Pipeline