For the FUV gratings, the wavelength ranges sampled at each FP-POS position are plotted separately. For the NUV gratings, the total wavelength range sampled by all
FP-POS positions is plotted for each central-wavelength setting.
This section presents sensitivities and effective areas as a function of wavelength for each grating. The target is assumed to be a point source centered in the PSA. For both the FUV and NUV detectors, the total systemic
1 spectroscopic point-source sensitivity, 
, has units of counts pix
λ−1 sec
−1 per incident erg cm
-2 sec
-1 Å
-1, where
The count rate per pixel is simply the product of the target flux and the point-source sensitivity at a given wavelength. To estimate the signal-to-noise ratio (S/N) achieved at a given count rate and exposure time, follow the directions in
Section 7.3 or use the S/N plots in this chapter.
For each grating, a plot is provided to help you estimate the S/N that can be achieved from a point source observed at a fiducial wavelength near the peak of the effective-area curve. The fiducial wavelength is indicated in the ordinate label of each plot. To estimate the S/N at other wavelengths, scale your source flux or magnitude by the relative sensitivities at the wavelength of interest and at the fiducial. The plots show S/N as a function of
Fλ and of STMAG for a range of exposure times. STMAG
λ is the color-dependent correction from V magnitude to STMAG at wavelength λ
. Values of STMAG
λ for various stellar and extragalactic sources are presented in
Table 13.1 and
Table 13.2, respectively. In producing these plots, we assumed an average sky background (as described in
Chapter 7) and the dark current appropriate for each detector. These plots should be used only for rough estimates of exposure times. When constructing your proposal, use the
COS ETC to estimate S/N values.
For example, consider a V = 15 star of spectral type B0V, for which we want to derive the S/N achieved in a 100 sec exposure using the NUV grating G230L. The S/N calculations for G230L are presented in
Figure 13.27, where we learn that the fiducial wavelength for this grating is 3000 Å. Assuming an effective temperature of 30,000 K, we obtain STMAG
λ ~ –2.1 at 3000 Å from
Table 13.1, making STMAG
= 12.9. Returning to
Figure 13.27, we find this value on the horizontal axis. For an exposure time of 100 seconds, the S/N ~ 9.5.
The STMAGλ values of
Table 13.1 are derived from the stellar models of Castelli and Kurucz (2003, 2004), assuming solar metallicity ([Fe/H] = 0.0) and a surface gravity of log(
g) = 4.5. The STMAG
λ values of
Table 13.2 are based on observed spectra of each object type.
, where N is the number of sub-exposures to be averaged.
For example, consider a V = 15 star of spectral type B0V, for which we want to derive the S/N achieved in a 100 sec exposure using the NUV grating G230L. The S/N calculations for G230L are presented in Figure 13.27, where we learn that the fiducial wavelength for this grating is 3000 Å. Assuming an effective temperature of 30,000 K, we obtain STMAGλ ~ –2.1 at 3000 Å from Table 13.1, making STMAG = 12.9. Returning to Figure 13.27, we find this value on the horizontal axis. For an exposure time of 100 seconds, the S/N ~ 9.5.