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The last three gratings are used with detector D2, which admits some second-order light, hence the comments. For example, Lyman-a light (1216 Å) can appear at 2432 Å in second order. Except for this possibility of geocoronal contamination, many cool stars have very little short-wavelength flux, so that the best resolution can be achieved without undue extraneous light by observing in first order near the high-wavelength limit.
Note that the G270M grating has an order-sorting filter which eliminates light below about 1650 Å so that no cross-order contamination occurs below 3300 Å.
By providing a sharper image of a point source, COSTAR restores the resolving power achieved with the LSA to within about 20% of that possible with the SSA. There is no effective change for the SSA, however.
Table 8.1:
The "bandpass" column provides the number of Ångstroms per exposure one can expect, the range being from the blue end of the spectrum to the red.
Useful Wavelength Ranges for First-Order Gratings. Resolving Power
The following figures illustrate the resolving power as measured for each of GHRS' gratings. In this case the resolving power was computed as R = l/Dl, where 
is the measured full-width-at-half-maximum (FWHM) of lines from an exposure of a spectrum calibration lamp. Tests have shown that the measured FWHM does not change significantly with wavelength (for the first-order gratings) or with ml, the product of the wavelength and order number (for the echelle gratings). The nominal design specification for the GHRS was R = 20,000 for the first-order gratings, but in fact one can exceed a resolving power of 25,000 at virtually all wavelengths. Similarly, the low-dispersion grating G140L has R in excess of 2,000 over most of its useful wavelength range. The true resolving powers for the echelle gratings are closer to 80,000 than the nominal 100,000.Figure 8.1: Spectrum resolving power as a function of wavelength for the GHRS medium-resolution (holographic) gratings. From left to right the curves are for G140M, G160M, G200M, and G270M, respectively.
Figure 8.2: Resolving power for grating G140L.
Sensitivity Functions for the First-Order Gratings
Below are given sensitivities for the first-order gratings measured after the installation of the COSTAR mirrors, using the Large Science Aperture and in units of 1011 (counts sec-1 diode-1) per incident (erg cm-2 sec-1 Å-1). SSA sensitivity is about 50 to 70% of these values, with the larger value applying at longer wavelengths.Table 8.2: Sensitivities for First-Order Gratings When Used with the LSA.
