IRAC Instrument Handbook - IRSA - California Institute of Technology

The noise pixels, Npix , are defined in Table 2.1.
2.5.2 Saturation
IRAC Instrument Handbook
The saturation limit for IRAC is calculated as follows. Using the same notation as earlier in this section,
F
Wf
− BT
w F ex
sat = (2.9)
STF
f cen f p
where W is the well depth (Table 2.2), fW =0.9 is the fraction of the well depth to which we can linearize
the intensities, and fcen is the fraction of the source flux falling onto the central pixel (Table 2.1). Table
2.11 shows the point source saturation limits of IRAC at each frame time. In an extremely bright area of
sky, such as an H II region, the saturation limit is lower. Note that the saturation value is conservatively
computed from the worst-case in which the PSF is directly centered on a pixel. To apply Table 2.11 for
extended sources,
Instrument Description 24 Sensitivity and Saturation
f
I sat = 28.6 × f cenF sat for compact (diameter < 30″) sources (2.10)
I sat = 28.6 × f cen f p
f ex
F sat for more extended sources, (2.11)
ext
where Isat is the total surface brightness (in MJy/sr) at which a pixel saturates; Ωpix is the solid angle, in
sr, subtended by the pixel; fp, fcen, and fex are as defined above, and Fsat is the saturating point source flux
density (in mJy) from Table 2.11, appropriate for the channel and integration time. For 8.0 µm
observations at low ecliptic latitude, an estimate of zodiacal light should be included in the surface
brightness.
Table 2.11: Maximum unsaturated point source (in mJ y), as a function of IRAC frame time.
Frame Time (sec) 3.6 um 4.5 µm 5.8 µm 8.0 µm
200 1.9 1.9 14 28
100 3.8 3.9 27 28
30 13 13 92 48
12 32 33 230 120