IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
Figure 4.4: Dependence of point source photometry on the distance of the centroid of a point source from the
nearest pixel center in channel 1. The ratio on the vertical axis is the measured flux density to the mean value
for the star, and the quantity on the horizontal axis is the fractional distance of the centroi d from the nearest
pixel center.
4.7 IRAC Point Spread and Point Response Functions
Figure 4.5 shows the IRAC point response functions (PRF) reconstructed from images of a bright star
obtained during IOC/SV. (Here we use the language common in the optics field; the point spread function
[PSF] is before sampling by the detector array, and the point response function [PRF] is after sampling by
the detector array. The PRFs, which are undersampled at the IRAC pixel scale, were generated by
combining 108 individual IRAC images in each band. By offsetting each image by a fraction of a pixel
width, fully sampled PRFs can be extracted from the data. The resulting PRFs are the optical point spread
function projected onto the focal plane by the IRAC and telescope optics, convolved with the response
function of a single detector pixel. The images were combined using a drizzle algorithm (Fruchter &
Hook 2002, [11]) to minimize smoothing of the PRF during the reconstruction process. The resulting
pixel scale was ¼ the width of an IRAC pixel. Images of a bright star at the native IRAC pixel scale are
also displayed for comparison.
FITS images of the IRAC PRFs are available in the IRAC section of the SSC website. The
appropriateness of a given PRF is dependent on the observation sampling and the photometric reduction
package used.
Calibration 46 IRAC Point Spread and Point
Response Functions