Please refer to Appendix B for a detailed description of how to achieve the highest possible accuracy when performing point source photometry. Appendix C summarizes the proper use of PRF fitting to obtain high accuracy point source photometry in a crowded field or in a field with highly varying background. Photometry using IRAC data is no different from that with any other high-quality astronomical data. Both aperture photometry and PRF-fitting work successfully. Aperture photometry is most commonly used, so we will discuss it briefly.

Aperture photometry measurements should be corrected by multiplying by the aperture correction, the array location-dependent correction, and the pixel phase correction. It is customary to perform the latter two corrections first. IDL functions to perform the correction for the cryogenic and warm mission data can be found at

on IRSA’s Spitzer/IRAC documentation website to apply the latter two (position dependent) corrections. You will need to measure the centroid location in addition to the aperture flux. The corrections in the IDL functions have been derived assuming first moment centroids. The link to the IDL function contains a link to a code to derive first moment centroids as well.

The improvement in data quality after applying these corrections can be significant. We show in Figure 8.1 the cumulative distribution of the flux percent difference from the mean (absolute value), for the original (BCD) and corrected data for calibration star NPM1+67.0536 (approximately 3000 data points). The corrected data have a factor of two smaller dispersion than the original data. The 68% confidence interval (1σ for a Gaussian distribution) for the original data is about 2.5% in channel 1. In comparison, the array location-dependent photometric and intrapixel sensitivity corrected data have a 68% confidence interval of about 1.4%. The corrections available earlier (now obsolete), including the radial pixel phase correction, were only about half as good at reducing the scatter in the data, with a 68% confidence interval of about 1.9%.

Figure 8.1: Correcting photometric measurements of calibration star NPM1+67.0536 using the pixel phase and array location-dependent corrections. Black curves show the cumulative distribution of aperture photometry measurements throughout the cryogenic mission, for which a value is within a given per cent of the mean. Blue curves show the distribution after correction using an earlier (now obsolete) set of location-dependent functions. Red curves are the result of the improved (now final) correction. A gray horizontal line at 68% (1σ for a Gaussian distribution) is labeled with the corresponding spread in values for each of the three distributions.