We use the same basic calculations presented above to estimate the sensitivity and S/N for PUI observations. Table 2.4 summarizes the instrument sensitivity in the PUI mode. The flux density is the peak-up target flux density at 16 microns (blue) or 22 microns (red). The underlying procedure used to calculate the PUI sensitivities is presented for informative purposes here:

1. The background photocurrent from sky and telescope is given by Table 2.4, where, in the case of PUI, .

2. For faint sources, the total noise in electrons/pixel is the sum of the photocurrent shot noise, read noise, and dark current shot noise. It is given by:

Equation 2.13

where is the dark current contribution, RN is the read noise in electrons, N_{reads} is the number of nondestructive samples up the ramp, and t_{int}is the ramp time in seconds.

3. The relationship between photocurrent per pixel and the incident flux density, S_{mJy}, is given by Equation 2.4, except that the slit losses, , are replaced by the fraction of encircled energy in the assumed beam size, which, for PUI, is larger than the 4 pixels assumed in Equation 2.4. For the nominal S/N calculations, we assumed a beam size equal to that of a circular aperture containing half the light of a point source.

4. The S/N for a point source is then the ratio of S_{mJy} to N.

Table 2.4 PUI 1σ sensitivity (microJy) for 6/14/30 second ramp times