IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
Figure 4.5. The in-flight IRAC point response functions (PRFs) at 3.6, 4.5, 5.8 and 8 microns. The PRFs were
reconstructed onto a grid of 0.3” pixels, ¼ the size of the IRAC pixel, using the drizzle algorithm. We display
the PRF with both a s quare root and logarithmic scaling, to emphasize the structure in the core and wings of
the PRF, respectively. We also show the PRF as it appears at the IRAC pixel scale of 1.2”. The reconstructed
images clearly show the first and second Airy rings, with the first Airy ring blending with the core in the 3.6
and 4.5 µm data.
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 (Section 4.6). To deal with these
problems, we have developed Point Response Functions (PRFs) for IRAC. A 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 centered at any given subpixel positition.
Calibration 47 IRAC Point Spread and Point
Response Functions