IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
Figure 4.7. Extended source flux correction factors; solid lines represent exponential function fits to the data.
Also indicated are correction factors derived from zodiacal light tests, and Galactic HII region tests (e.g.
Martin Cohen's GLIMPS E vs. MSX, private communication).
Figure 4.8. Extended source flux correction factors for galaxies (solid lines) versus the PSF aperture
correction factors (dotted lines). The main difference between the two is the truly diffuse scattering internal
to the array.
Aperture photometry should also include background subtraction; we recommend that you use an annulus
that is located just outside the boundary of your galaxy. Circular or elliptical apertures may be used.
The procedure for correcting extended source photometry is to apply the correction factor to the
integrated flux measured from the IRAC image (subject to the standard or point source calibration). The
correction factor is a function of the circular aperture radius or the effective circular aperture radius (if
using ellipses). These corrections should be good to 10%. For convenience, we have converted the
empirical curves into a functional form:
Calibration 58 Extended Source Photometry