Spitzer Documentation & Tools
S18.18 IRAC Flux Conversion Issue

THE S18.18 FLUX CONVERSION UPDATES TO VERSION S18.25 WERE COMPLETED IN MID-AUGUST 2012. THIS MEMO IS LEFT HERE FOR INFORMATIONAL PURPOSES ONLY.

1 Nov 2011

In the process of performing a detailed cross comparison of the calibration of Spitzer instruments and comparing the IRAC calibration to the HST calibration database, an error was identified in the S18.18 IRAC processing. Only the S18.18 IRAC processing is affected; previous versions have a correct photometric calibration as given by Reach et al. (2005). In updating the flux conversions (FLUXCONV), erroneous color corrections were used for the calibration stars. The color corrections used were incorrect for the 5.8 and 8.0 micron bands. As a result, the corresponding flux conversions are incorrect and the scaling of the data to flux density for IRAC channels 3 and 4 is incorrect as well. All IRAC image data with processing version S18.18 and any derived flux density measurements should be multiplied by 1.000, 1.000, 0.968 and 0.973 for the 3.6, 4.5, 5.8 and 8.0 micron channels, respectively.

We are in the process of updating the data in the Spitzer Heritage Archive. Processing version S18.25 will contain the correct FLUXCONV values. Note that the final cryogenic processing includes updated array location dependent photometric correction maps and intra-pixel gain correction functions that will soon be available on the Spitzer website.