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

FORCAST Filter Sensitivities
Filter Channel Single Chan. MDCF (mJy) Dual Chan. MDCF (mJy)
FOR_F054a SWC 65 895
FOR_F056b SWC 74.5 285.2
FOR_F064 SWC 82.1 92.1
FOR_F066 SWC 92.4 120.6
FOR_F077 SWC 85.5 104.1
FOR_F086a LWC 422 -
FOR_F111a SWC 143 155
FOR_F113a LWC 320 -
FOR_F118a LWC 194 -
FOR_F197 SWC 116.2 123.4
FOR_F242 LWC 154.3 -
FOR_F253 SWC 236.2 250.9
FOR_F315 LWC 197.9 269.1
FOR_F336 LWC 414.1 586.9
FOR_F348 LWC 283.6 399.9
FOR_F371 LWC 383 579.3

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.


FORCAST sensitivity plot

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.