IRAC Instrument Handbook - IRSA - California Institute of Technology

Point Source Fitting IRAC Images
with a PRF
153
IRAC Instrument Handbook
Appendix C. Point Source Fitting IRAC Images with a
PRF
This Appendix discusses the use of point source response functions (PRFs) for fitting sources in IRAC
data. For true point sources, it is possible to obtain agreement between PRF-fitted and aperture flux
measurements at better than the 1% level. In this Appendix, we describe validation tests on point sources
in IRAC data using the PRFs in combination with the MOPEX/APEX software. The procedure for using
the PRFs in conjunction with MOPEX/APEX is given in the form of a “How To'' description, and the
necessary corrections to the resulting flux densities are detailed.
Point source fitting is a valuable tool for characterizing images. If the image consists of true point
sources, PRF fitting can make optimal use of the information in the image, thus improving astrometric
and photometric results beyond what is achievable using other techniques. PRF fitting also allows point
sources to be subtracted from an image (for example, using the apex_qa task in MOPEX/APEX),
enabling any diffuse background emission to be more easily characterized. Point source fitting is less
useful in fields containing large numbers of partially-resolved objects (as typically seen in IRAC
extragalactic survey fields), and aperture photometry is recommended in such fields. (In principle, model
fitting could be used for extended sources by convolving a source model with the appropriate point source
realizations, but such techniques lie outside the scope of this Appendix.) For isolated point sources on
featureless backgrounds aperture photometry and point source fitting should give almost identical results.
Point source fitting to IRAC data has proven problematic as the PSF is undersampled, and, in channels 1
and 2, there is a significant variation in sensitivity within pixels. Techniques for dealing with these
problems were developed for the WFPC2 and NICMOS instruments on HST (Lauer 1999 [18]; Anderson
& King 2000, [2], see also Mighell 2005, [19]). These techniques involve building a ''point response
function'' (PRF; Anderson & King use the alternative terminology ''effective PSF''), and users interested in
the detailed theory of the PRF should refer to these papers. In summary, the PRF is a table (not an image,
though for convenience it is stored as a 2D FITS image file) which combines the information on the PSF,
the detector sampling and the intrapixel sensitivity variation. By sampling this table at regular intervals
corresponding to single detector pixel increments, an estimate of the detector point source response can be
obtained for a source at any given pixel phase.
PRFs for IRAC have been created by William Hoffmann of the University of Arizona, a member of the
IRAC instrument team. The starting point for these PRFs was the Code V optical models for
Spitzer/IRAC, made at the Goddard Space Flight Center. These were constructed on a 5x5 grid covering
each of the IRAC arrays. Observations of a calibration star made during the in-orbit checkout at each of
these 25 positions per array were then deconvolved by their respective optical models. The results were
averaged into a single convolution kernel per array which represents additional PRF scatter from
unmodeled optical effects and spacecraft jitter. A paper on “simfit” that gives more details is included in
the IRAC section of the documentation website. The intrapixel sensitivity function was estimated using a
polynomial fit as a function of pixel phase. The PRFs were then transposed, and flipped in x and y to align
them with the BCD coordinate system.