IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
Figure 2.8: IRAC point source sensitivity as a function of frame time, for low background. To convert to
MJ y/sr, see equation 2.8.
Figure 2.8, Figure 2.9 and Figure 2.10 show the point source sensitivity as a function of integration time
for each background model. The “time” axes in the plots represent the frame time for the images, which
does not include time for moving the telescope. The IRAC full-array frame times are 0.6, 2, 12, 30, and
100 seconds (200 seconds was also available in the early mission). Other times plotted below are assumed
to use multiple exposures of those fixed times.
For bright sources, shot noise due to counting statistics in electrons from the source itself becomes the
dominant source of noise. We can estimate the total noise by adding the shot noise in quadrature, so that
σ tot = σ ( 1+
( F / Fb
))
(2.5)
Instrument Description 21 Sensitivity and Saturation