7.1.4 Imaging Sensitivities

The FORCAST imaging sensitivities for a continuum point source for each filter are presented in Table 7-3 and Figure 7-4. The Minimum Detectable Continuum Flux (MDCF; 80% enclosed energy) in mJy needed to obtain a S/N = 4 in 900 seconds of on-source integration time is plotted versus wavelength. The MDCF scales roughly as (S/N) / √(t) where t = net integration time. The horizontal bars indicate the effective bandpass at each wavelength. At the shorter wavelengths the bandpass is sometimes narrower than the symbol size.

Atmospheric transmission will affect sensitivity, depending on water vapor overburden. The sensitivity is also affected by telescope emissivity, estimated to be 15% for Figure 7-4.

Observations with FORCAST will be performed using standard IR chop-nod techniques. The GIs can choose chop/nod amplitudes small enough to leave the source on the array in each position or large enough that the source is positioned off the chip for one of the chop positions. For background-limited observations, as will be the case with FORCAST on SOFIA, chopping and nodding off-chip in nod-match-chop (NMC; see Section 7.2.1) will generally result in the same signal to noise (S/N) as chopping and nodding on-chip in nod-perp-chop (NPC; see Section 7.2.1). Calculations of S/N for various chop-nod scenarios are provided here.

Table 7-3: FORCAST Filter Sensitivities

^a MDCF values shown are those measured from Cycle 2 and Cycle 3 data.

^b No observations were available to test the theoretical MDCF values.

Figure 5-4. Cycle 6 continuum point source sensitivities for single and dual channel modes. Values are for S/N = 4 in 900 s under nominal conditions. Investigators are encouraged to use the SOFIA Integration Time Calculator (SITE) for their calculations.