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 below 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,

equation

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 90 79 2.3E-16 180 98 2.9E-16
FOR_G063 6.4 90 219 5.2E-16 180 268 6.3E-16
FOR_G063 7.7 90 496 5.2E-16 180 724 6.3E-16
FOR_G111 8.6 150 419 4.9E-16 300 532 6.2E-16
FOR_G111 11.0 150 449 4.1E-16 300 575 5.2E-16
FOR_G111 13.2 150 593 4.5E-16 300 764 5.8E-16
FOR_G227 17.8 70 715 8.6E-16 140 936 1.1E-15
FOR_G227 22.8 70 834 7.9E-16 140 989 9.3E-16
FOR_G227 27.2 70 1979 1.6E-15 140 2586 2.0E-15
FOR_G329 28.9 110 1365 6.5E-16 220 1899 9.0E-16
FOR_G329 34.1 110 1408 5.6E-16 220 1994 8.0E-16
FOR_G329 37.0 110 1763 5.6E-16 220 2439 8.0E-16

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 4

 

FORCAST grism sensitivities

Figure 7-5: FORCAST grism sensitivities for a continuum point source in each of the grisms listed above in Table 7-3 overlaid on an ATRAN model in light blue. The values reported are for a S/N of 4 in 900 seconds at a water vapor overburdens of 7 μm at an altitude of 41K feet and a 60° zenith angle. Sensitivities are plotted for each of the three available slits.

 

FORCAST grism sensitivities

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.