IRAC Instrument Handbook - IRSA - California Institute of Technology

Performing Photometry on IRAC
Images
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IRAC Instrument Handbook
will need the IRAC spectral response curves, available in the IRAC web pages. Color corrections
are typically a few percent for stellar and blackbody sources, but can be more significant for
sources with ISM-like source functions (50% – 250% depending on spectrum and passband).
Measured flux density is the flux density at the effective wavelength of the array: 3.550, 4.493,
5.731 and 7.872 microns, for channels 1–4, respectively.
9. A pixel phase correction to the measured channel 1 flux densities should then be considered.
More information on the pixel phase correction can be found in Chapter 4 of this Handbook. This
effect is as large as 4% peak-to-peak at 3.6 microns and < 1% at 4.5 microns. To apply a
correction for mosaicked data is difficult as the pixel phase correction depends on the placement
of the source centroid on each CBCD. For well-sampled data the pixel phase should average out
for the mosaic. For precise photometry in low coverage data, the source centroids on the CBCDs
should be measured and the phase corrections averaged together and applied to the final source
photometry.
B. Point Source Photometry on Individual BCDs
Although most of the time it is a good idea to use the mosaic for performing photometry, performing
photometry on the (C)BCD stack is important for variability studies and can be useful for faint sources as
one can measure N out of M statistics (how many times you found the source). When performing source
profile fitting, performing photometry on the the (C)BCD stack is better as the phase information of the
PRF is preserved.
1. Examine your data (CBCDs) and identify artifacts that could affect your photometry and that
need to be corrected.
2. First perform artifact mitigation on the pipeline-produced CBCDs. While the pipeline-reduced
CBCD files are mostly artifact-free, some residual artifacts remain. The pipeline and contributed
software have difficultly recognizing very saturated pixels that produce artifacts. As a result they
will not usually correct artifacts from very saturated point sources and extended saturated regions.
Data at 5.8 and 8.0 microns exhibiting the bandwidth effect should be masked. If performing
aperture photometry on the CBCDs, a particular CBCD should not be used for a source when
there are masked (bad) data in the source aperture.
3. Make a mosaic of artifact-corrected images, for example with the MOPEX package. This needs to
be done to create the proper rmask files to be applied to the CBCDs when performing the
photometry on them, and also to get a nice comparison of CBCD-revealed and mosaic-revealed
image features. When creating the mosaic, the overlap correction option should be used in
MOPEX, most importantly in channels 3 and 4, to match the backgrounds. Inspect the mosaic to
confirm that outlier rejection is acceptable, if not, then remosaic with more appropriate
parameters. Comparing mosaics of adjacent channels on a per-pixel basis will readily identify if
outliers remain in a mosaic. The mosaic coverage maps should be inspected to verify that the
outlier rejection has not preferentially removed data from actual sources. If the coverage map
systematically shows lower weights on actual sources, then the rejection is too aggressive and
should be redone. One result of making the mosaic is the production of rmask files which identify
bad pixels in the CBCDs. One should apply the rmasks when performing the photometry in the
next step so that bad pixels are not included within the apertures.