7.1.6 Spectroscopic Sensitivity
Tables 7-5 and 7-6 provide samples of the MDCF and Minimum Detectable Line Flux (MDLF) calculated at three different wavelengths across each grism bandpass for each of the available spectroscopic modes – long slit and cross-dispersed. The data are provided for point sources only. The MDCF and MDLF estimates are for the raw integration time of 900 seconds and do not include observing overheads, but do account for a a two-position chop (perpendicular to the slit).
Figures 7-5 and 7-6 present the continuum point source sensitivities for the FORCAST grisms. The plots are the MDCF in Jy needed for a S/N of 4 in 900 seconds at a water vapor overburden of 7 μm, an altitude of 41K feet, and a zenith angle of 60°. The rapid variations with λ are due to discrete atmospheric absorption features (as computed by ATRAN).
To determine the required integration time necessary to achieve a desired S/N ratio for a given source flux, GIs should use the FORCAST on-line grism exposure time calculator. The on-line calculator also allows for calculation of the limiting flux for a given integration time and required S/N. Since FORCAST observations are background limited, the values given in Tables 7-5 and 7-6 and Figures 7-5 and 7-6 can be used to make an estimate of the required integration time using the following relation,
where [S/N]req is the desired signal-to-noise ratio, Fsrc is the continuum flux of the target, texp is the exposure time on source (without taking into consideration observational overheads), and the MDCF is taken from the tables for the point-source sensitivities or estimated from the figures. For emission lines, simply use the line flux for Fsrc and use the MDLF value instead of the MDCF. However, these tables may not contain the most recent or best determined sensitivity values and therefore the on-line calculator results should be used in the actual proposal.
Table 7-5: FORCAST Long Slit Point Source Sensitivities
| Grism | λ (μm) | R = (λ/Δλ) | MDCF (mJy) | MDLF (W m-2) | R= (λ/Δλ) | MDCF (mJy) | MDLF (W m-2) |
|---|---|---|---|---|---|---|---|
| 4.7'' Slit | 2.4'' Slit | ||||||
| FOR_G063 | 5.1 | 120 | 79 | 2.3E-16 | 180 | 98 | 2.9E-16 |
| FOR_G063 | 6.4 | 120 | 219 | 5.2E-16 | 180 | 268 | 6.3E-16 |
| FOR_G063 | 7.7 | 120 | 496 | 5.2E-16 | 180 | 724 | 6.3E-16 |
| FOR_G111 | 8.6 | 130 | 419 | 4.9E-16 | 300 | 532 | 6.2E-16 |
| FOR_G111 | 11.0 | 130 | 449 | 4.1E-16 | 300 | 575 | 5.2E-16 |
| FOR_G111 | 13.2 | 130 | 593 | 4.5E-16 | 300 | 764 | 5.8E-16 |
| FOR_G227 | 17.8 | 110 | 715 | 8.6E-16 | 140 | 936 | 1.1E-15 |
| FOR_G227 | 22.8 | 110 | 834 | 7.9E-16 | 140 | 989 | 9.3E-16 |
| FOR_G227 | 27.2 | 110 | 1979 | 1.6E-15 | 140 | 2586 | 2.0E-15 |
| FOR_G329 | 28.9 | 160 | 1365 | 6.5E-16 | 220a | 1899 | 9.0E-16 |
| FOR_G329 | 34.1 | 160 | 1408 | 5.6E-16 | 220a | 1994 | 8.0E-16 |
| FOR_G329 | 37.0 | 160 | 1763 | 5.6E-16 | 220a | 2439 | 8.0E-16 |
a The 2.4'' long slit mode for G329 will not be available during Cycle 5.
Table 7-6: FORCAST Cross-Dispersed Point Source Sensitivitiesa
| Grism | λ (μm) | R = (λ/Δλ) | MDCF (mJy) | MDLF (W m-2) |
|---|---|---|---|---|
| 2.4'' x 11.2'' Slit | ||||
| FOR_XG063 | 5.1 | 1200 | 238 | 1.2E-16 |
| FOR_XG063 | 6.4 | 1200 | 703 | 2.8E-16 |
| FOR_XG063 | 7.7 | 1200 | 918 | 3.0E-16 |
a XD modes are not available during Cycle 5
Figure 5-5. Cycle 6 grism continuum point source sensitivities for both wide and narrow long slits overlaid on an atmospheric transmission model (light blue). Values are for S/N = 4 in 900 s under nominal conditions.
Figure 7-6: FORCAST grism sensitivities for a continuum point source using the grisms 1 and 2 listed above in Table 7-3. Otherwise as above.
