IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
Figure 2.10: IRAC point source sensitivity as a function of frame time, for high background. To convert to
MJy/sr, see equation 2.8.
In the sensitivity figures, the dashed line at 0.6 µJy is the confusion limit predicted by Franceschini et al.
(1991, [10]). This does not represent a hard sensitivity limit, but rather indicates where source confusion
affects reliability of source extractions for low background regions. Data from IOC/SV show noise
decreasing as N to 0.25 µJy (channels 1 and 2) or 0.6 µJy (channels 3 and 4). Moderately deep source
counts indicate that a source density equivalent to 36 beams/source is reached at 20.5 mag, or 1.8 and 1.1
µJy at 3.6 and 4.5 µm, respectively (Fazio et al. 2004, [9]). The confusion estimates by Franceschini et
al. and Fazio et al. are for low background, extragalactic observations only. For observations of higher
background or more “cluttered” regions (such as the Galactic Plane) the confusion noise will be much
more significant.
For diffuse emission, the surface brightness sensitivity per pixel (in MJy/sr) is
0.
03 f
f
ex
N
p
pix
× the point source sensitivity [in µJy]. (2.8)
Instrument Description 23 Sensitivity and Saturation