IRAC Instrument Handbook - IRSA - California Institute of Technology

f ex (throughput correction for background) 1 1 0.72 0.88
Table 2.5: Background brightness in IRAC wavebands.
3.6 µm 4.5 µm 5.8 µm 8 µm
“low” background model
Iν fS (MJy/sr) 0.093 0.32 1.7 6.6
Fν BG (µJy) 3.2 11 57 220
B (elec/sec) 2.5 9.9 18 184
“medium” background model
Iν fS (MJy/sr) 0.15 0.44 2.3 9.3
Fν BG (µJy) 5.1 15 79 320
B (elec/sec) 4.1 14 25 260
“high” background model
Iν fS (MJy/sr) 0.52 1.0 5.6 22
Fν BG (µJy) 18 35 190 750
B (elec/sec) 14 32 60 620
IRAC Instrument Handbook
The quantity fF is the flat field pixe l-to-pixel variance, which depends on the observing strategy. In what
follows, we will set fF =0, which would apply strictly in the case of stable detectors with perfect flat field
measurements, and should apply practically for highly-dithered observations. An observation with no
dithering will be limited by the correlated noise. The accuracy of a flat field derived from a single
observing campaign was measured to be 2.4, 1.2, 1.0, and 0.3% in channels 1, 2, 3, and 4, respectively, by
comparing flats in several campaigns. Using combined flats (“super sky flat”) from the first two years, the
estimated fF is 0.14%, 0.09%, 0.07%, and 0.01% in channels 1–4, respectively. Using these values for fF
in equation 6.1, single frames are dominated by background and read noise. When combining multiple
frames to generate a mosaic, the background and read noises will average down (as square root of the
number of frames), while the flat-field noise will only average down for dithered observations. For N
undithered observations on the “medium” background, flat-field noise dominates when the total exposure
time, ex T N × , exceeds approximately 420 sec (using individual campaign flats) or 2.5 hrs (using the
super sky flat). For dithered observations, the flat-field noise will also average down, and will only be
important for the very deep observations of high background fields.
For the frame times used in IRAC operations in flight, Table 2.6 gives the readout mode and Fowler
number. For full array readout mode, only the 2, 12, 30, and 100 sec frame times can be chosen in the
IRAC AOT; the 0.6 and 1.2 sec frame times come as part of the “high dynamic range” (HDR) sequences.
The 0.4 sec full frame time is only available for channels 1 and 2 in Stellar Mode. The frame sets that are
taken for each pointing in HDR mode are shown in Table 2.7. Long frame times at 8 µm are backgroundlimited.
Therefore there is a maximum frame time of 50 sec at 8 µm, and the 100/200 sec frames were
Instrument Description 18 Sensitivity and Saturation