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 systemic1 spectroscopic point-source sensitivity,
, has units of counts pixλ−1
per incident erg cm-2
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. STMAGλ
is the color-dependent correction from V magnitude to STMAG at wavelength λ.
Values of STMAGλ
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 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.
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 STMAGλ
values of Table 13.2
are based on observed spectra of each object type.