IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
automatically converted into two/four repeats of 50 sec frames. The last column, Th, gives the extra time
spent taking the HDR frames (used in the observing time estimate equation below).
Table 2.6: Fowler numbers for IRAC frames
Frame Time (sec) Readout Mode Fowler Number Wait Ticks
200 Full 32 936
100 Full 16 468
50 Full (8 µm) 16 218
30 Full 16 118
12 Full 8 44
2 Full 4 2
1.2 HDR 1 4
0.6 HDR 1 1
0.4 Stellar 1 0
0.4 Subarray 8 24
0.1 Subarray 2 6
0.02 Subarray 1 0
Table 2.7: IRAC High-Dynamic-Range (HDR) framesets
Long Frame Time List of frames taken Th (sec)
200 0.6, 12, 200 15
100 0.6, 12, 100 15
30 1.2, 30 3
12 0.6, 12 2
Table 2.8, Table 2.9, Table 2.10 and Figure 2.8, Figure 2.9 and Figure 2.10 show the sensitivities for the
four IRAC channels for each of the three background models. The sensitivities in the tables are for point
sources extracted from single images (but perfectly flat-fielded). In the figures, the sensitivities are for
point sources extracted from coadded images (perfectly registered). We do not include “confusion noise”
(due to overlapping images of distant galaxies or other sources of background structure) in the sensitivity
estimates. The detectors are assumed to perform according to the IRAC detector measurements of read
noise, dark current, and quantum efficiency. The first 7 rows in each table show the sensitivity for fullarray
readouts, and the last three rows show the sensitivity for subarray readouts.
Table 2.8: IRAC point-source sensitivity, low background (1 σ, µJy).
Frame Time (sec) 3.6 µm 4.5 µm 5.8 µm 8 µm
200 0.40 0.84 5.5 6.9
100 0.60 1.2 8.0 9.8
30 1.4 2.4 16 18
12 3.3 4.8 27 29
Instrument Description 19 Sensitivity and Saturation