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
| 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.
Figure 7-4: FORCAST imaging sensitivities for a continuum point source at the effective wavelengths of a selection of the FORCAST filters listed above. The values reported are for a S/N of 4 in 900 seconds at an altitude of 41,000 feet and a water vapor overburden of 7 μm. The red squares indicate the MDCF values in Single Channel mode while the blue circles are for Dual Channel mode using the dichroic. The horizontal bars indicate the effective bandpass at each wavelength. At the shorter wavelengths the bandpass is sometimes narrower than the symbol size. Note that the higher MDCF values at 8.6, 11.3, and 11.8 µm are due to lower sensitivity at short wavelengths in the LWC.