IRAC Instrument Handbook - IRSA - California Institute of Technology

Performing Photometry on IRAC
Images
149
IRAC Instrument Handbook
Appendix B. Performing Photometry on IRAC Images
This is a quick guide for performing point source photometry on IRAC images.
A. Point Source Photometry on a Mosaic
1. If you are only interested in photometry down to about 10% accuracy and have bright point
sources, you can usually perform photometry on the pipeline mosaic. Set the aperture size to 10
pixels and the sky annulus to between 12 and 20 pixels. The IRAC calibration is based on an
aperture of this size, so for this aperture no aperture correction is necessary. For fainter stars, it is
better to use a smaller aperture and then apply an aperture correction. Remember that the units of
the images are in MJy/sr, so you need to convert your measured values into flux density units in
micro-Jy, by accounting for the pixel size in steradians. Conversion into magnitudes is
magnitudes = −2.5*log10(f/f(0)), where f is your measured flux density and f(0) is the zero
magnitude flux density. If using software such as "phot" or "qphot" in IRAF/DAOPHOT which
requires a magnitude zeropoint, the "zmag" keyword in photpars should be set to 18.80 (ch1),
18.32 (ch2), 17.83 (ch3) and 17.20 (ch4) if using a mosaic pixel scale of 0.6 arcsec/pixel. Other
zmag values will be needed for other pixel sizes. Note that if you require photometry to a higher
accuracy than 10% – 20%, you should follow the steps listed below.
2. Examine your data (CBCDs) and identify artifacts that could affect your photometry and that
need to be corrected.
3. First perform artifact mitigation on the pipeline-produced CBCDs. While the pipeline-reduced
CBCD files are mostly artifact-free, some residual artifacts remain.For example, the pipeline and
contributed software have difficulty recognizing very saturated pixels that produce artifacts. As a
result, they will not usually correct artifacts from very saturated point sources or from extended
saturated regions. Data at 5.8 and 8.0 microns exhibiting the bandwidth effect should be masked
before performing photometry.
4. Make a mosaic of artifact-corrected images, for example with the MOPEX package. 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 MOPEX 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.
5. If you are interested in blue point sources (sources with spectral energy distributions, SEDs, that
decline toward the longer wavelength IRAC passbands) you should create an array locationdependent
photometric correction image mosaic. If you are interested in only red sources (with
SEDs that rise toward the longer wavelength IRAC passbands), you do not need to apply the
photometric correction images and make a mosaic out of them. We recommend making a
correction mosaic, instead of multiplying the correction images with the CBCDs and then
mosaicking these CBCDs together, since you may need to iterate this a few times and/or you may
have both red and blue sources in the field, and thus the correction only applies to a subset of
sources. This location-dependent effect is as large as 10%. It is the dominant source of