IRAC Instrument Handbook - IRSA - California Institute of Technology

4.9 Astrometry and Pixel Scales
4.9.1 Optical Distortion
IRAC Instrument Handbook
Optical distortion is a significant (measurable) effect in IRAC data. The ~ 1% distortion in all channels is
due principally to being offset from the optical axis of Spitzer, with additional components from the
telescope and camera optics. In addition to varying the effective pixel size, there are also higher-order
terms such as skew (the two axes are not exactly perpendicular) and a difference in the pixel scales
between the two axes. Failure to account for the distortion will lead not only to errors in photometry
(described below), but also shifts in astrometric position approaching 1" near the array corners.
Optical distortion in each of the IRAC FOVs is described in the headers using a standard method
described by Shupe et al. (2005, [24]). This method places the center of the distortion at the center of
each detector array, in particular at CRPIX1 and CRPIX2. The linear terms and any skew are represented
in the CD matrix header keywords (CD1_1, CD1_2, CD2_1, and CD2_2), while the distortion keywords
provide the second and higher order terms. Importantly, these distortion corrections apply to the array
coordinates, prior to the transformation to sky coordinates. This means that all IRAC data for a given
detector share the same distortion keywords. In addition we also provide a separate set of keywords
representing the “reverse” transformation from sky to array coordinates.
The form of the optical distortion that is encoded in the (C)BCDs is read properly by several “standard”
tools available to the general astronomical community: (1) the Spitzer mosaicker (MOPEX), (2)
WCSTOOLS by Doug Mink (SAO) and (3) DS9 (except for grid overlays).
The optical distortion is fit independently for each IRAC detector. Originally a second-order fit was used,
but an improved fit to third order was derived from the GOODS data by S. Casertano. The (C)BCD
coefficients remove the distortion to 0.1″ accuracy.
4.9.2 Pixel Solid Angles
As a result of the optical distortion described above, the detector pixels do not all subtend the same
projected solid angle on the sky. The variation in projected pixel solid angle is roughly 1.5%.
This size variation is accounted for in the flat-fielding process because the flats are derived from actual
sky measurements. As a result, after flat-fielding, the (C)BCD images are calibrated in units of true
surface brightness (MJy/sr). This poses a difficulty because virtually all software assumes that the pixels
are in units of flux per pixel, and simply sum the pixel values. In order to properly measure fluxes from an
image in surface brightness units, one must multiply the pixel value by the pixel size. Failure to do so
could induce photometric errors at the 1% level, depending on location on the array. Unfortunately, only
the newest photometry software can read the new FITS-standard WCS distortion keywords written in the
(C)BCD headers and properly account for the sizes of the pixels.
Calibration 52 Astrometry and Pixel Scales