IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
photometry). And in the presence of extended emission, a small off-source annulus is normally used. The
calibration aperture does not capture all of the light from the calibration sources, so the extended emission
appears too bright in the data products we delivered. See the more detailed discussion under 4.11.
Similarly, observers will often use smaller apertures and will want to correct their photometry to match
the absolute calibration.
Users should note that the spatial extent of the PSF in channels 3 and 4 is much larger than the subarray
area. In other words, a large amount of the total power in the PSF is scattered onto arcminute size scales.
As a result, special care needs to be taken when measuring fluxes in these channels, since accurate
measurement of the “background" is difficult. Proper application of aperture corrections is very
important.
For photometry using different aperture sizes, the aperture correction can be estimated with Table 4.7. All
distances in this table are in native pixels (~ 1.2”). Note that the post-BCD mosaics currently available
from the Spitzer data archives use pixels that correspond to exactly 0.6” x 0.6”. The aperture corrections
as written will INCREASE the flux measured by the listed method, i.e., your measured brightness should
be MULTIPLIED by the aperture corrections in the table. The third decimal place in these numbers is
included only to illustrate the trends; the accuracy of these corrections is presently ~ 1% – 2%. The
aperture corrections in Table 4.7 are averages of the values derived from PSFs measured using stars at 23
different positions on the array. Standard deviations (including measurement errors and true variations
across the array) are less than 0.5% for all entries except the smallest aperture, in which they are still less
than 1%. The extended source (infinite) corrections in Table 4.7 come from Reach et al. (2005, [23]). The
measured flux densities can then be converted to magnitudes, if desired, using the zero-points in Table
4.1.
Table 4.7: IRAC aperture corrections.
.
Radius on source
(native ~1.2” pixels)
Background annulus
(native ~1.2” pixels)
Aperture correction
3.6 µm 4.5 µm 5.8 µm 8.0 µm
infinite N/A 0.994 0.937 0.772 0.737
10 12-20 1.000 1.000 1.000 1.000
5 12-20 1.049 1.050 1.058 1.068
5 5-10 1.061 1.064 1.067 1.089
3 12-20 1.112 1.113 1.125 1.218
3 3-7 1.124 1.127 1.143 1.234
2 12-20 1.205 1.221 1.363 1.571
2 2-6 1.213 1.234 1.379 1.584
Calibration 54 Point Source Photometry