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
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.