IRAC Instrument Handbook - IRSA - California Institute of Technology

4.7.1 Core PRFs
IRAC Instrument Handbook
The FITS files of the core PRFs are linked off the IRAC web pages. These core PRFs can be used for
PRF-fitting photometry and source extraction in (C)BCDs for all but the brightest sources. We still
recommend performing aperture photometry instead of PRF fitting in all instances except in crowded
fields and regions with a strongly varying background, because aperture photometry is much simpler,
straightforward and faster to do. In addition, especially in channels 1 and 2, the PSF is undersampled by
the native IRAC pixel size, causing futher uncertainty to PRF fitting. PRF fitting does not work in image
mosaics where the information from the PSFs has been ``scrambled’’ together. Aperture photometry is
the correct way to perform point source flux density measurements in image mosaics.
The PRFs are provided in two different samplings, 1/5th and 1/100th native pixels. The 1/100th native
pixel sampling PRFs have been created by interpolating the 1/5th sampled PRFs onto a finer grid. These
PRFs are designed to work with the photometry extraction software APEX. The 1/5th pixel sampling
versions are the originally derived versions and are appropriate for use with custom PRF-fitting software,
but not APEX. For both versions of sampling, the PRFs are provided for 25 positions in a 5x5 grid upon
the array for each channel. The PRFs are normalized such that the flux is unity within 12 arcsecond (10
pixel) radius around each point source with the zero pixel phase instance (centered on a pixel).
Calibration 48 IRAC Point Spread and Point
Response Functions