IRAC Instrument Handbook - IRSA - California Institute of Technology

5.8 µm 0.66−0.73
8.0 µm 0.74
IRAC Instrument Handbook
Surface Brightness = measured surface brightness x correction_factor, where the correction factors
represent the infinite aperture value. Note that for IRAC channel 3 the recommended correction is
somewhere between 0.66 and 0.73, depending on the downward curvature of the aperture corrections
(which is highly uncertain). These aperture corrections should be good to 10%.
Examples of LSB objects: large, late-type galaxies (e.g., NGC 300); Magellanic-type galaxies (e.g., NGC
6822); diffuse dwarf galaxies (e.g., M81 DwA); HII regions that are larger than ~100 arcseconds and not
very centrally condensed.
4.11.4 Caveats & Cautionary Notes
At small radii, r < 7–8", the extended source aperture corrections should not be used. Instead, we
recommend using the point source aperture corrections for small radii.
It remains uncertain how much the spectral shape of the extended object determines the flux corrections;
the aperture corrections presented here were derived using relatively "old" spheroidal galaxies. To first
order, the extended source aperture corrections apply to most types of galaxies.
Likewise with the spectral color caveat, it remains uncertain how much the spatial distribution of the light
determines the flux corrections; these corrections were derived using relatively high surface brightness
spheroidal galaxies; it is unknown whether these corrections apply to lower surface brightness galaxies
(e.g., late-type spirals; irregulars; Magellanic-types).
4.11.5 Faint Surface Brightness Behavior
Note that the discussion in this section applies only to warm IRAC data. For more detailed information,
please see Krick et al. (2011, [16]).
4.11.5.1 Binning
Binning data by essentially making larger “pixels” should reduce the noise in the image linearly with
binning length. Figure 4.9 and Figure 4.10 show a plot of noise versus binning length for a set of deep
mapping data in the Virgo cluster (PID 60173). These data have been carefully corrected for the first
frame effect using the data themselves. The measured noise does not achieve the expected linear relation
with binning length.
Calibration 60 Extended Source Photometry