Comparison of STIS and SNAP spectrograph throughputs

Abstract

This is a comparison of the measured throughput of STIS on HST versus what we might expect from the spectrograph on SNAP. The principle reference is Woodgate et al. (1998) PASP, 110, 1183. Additional material was taken from the STIS Handbook, available on-line at www.stsci.edu.The goal is to demonstrate that there are sound reasons to expect better performance for a SNAP spectrograph (even one with a grating) than would be expected by scaling from HST+STIS.In Table 1 I attempt to reverse-engineer the STIS system throughput at 0.77 µm by using published or assumed efficiencies for constituent STIS optical components. In particular, the CCD efficiency is given Table 10 of Woodgate et al and the Al efficiency is the textbook value for fresh Al. The grating efficiency, and the dewar window throughput are assumed quantities. The behavior with wavelength of several STIS components is shown in Figure 1. The HST OTA throughput is not included in Table 1. The reference wavelength of 0.77 µm was chosen since Woodgate et al quote their efficiency at this wavelength. The measured throughput for the G750L grating at 0.77 µm given in Table 1 of Woodgate et al 1998 is 10.0%, while the G750M gratings has a measured throughput of 14.8%. (The lower efficiency of G750L compared to G750M is due to a Lyot stop) The value of 14.8% is in good agreement with the ab initio estimate of 18% in Table 1. Indeed, the STIS efficiency in the G750M grating at 0.77 µm given by the HST calibration pipeline tables (i.e. in SYNPHOT) is 17.6% -slightly higher than the value published in Woodgate et al. and in excellent agreement with the ab initio estimate. This suggests that the assumed efficiencies of the spectrograph components are plausible.In Table 2 I show how a similar spectrograph on SNAP would fare relative to STIS at 0.77 µm. Here the OTA efficiencies for both telescopes are included. Basically, we should expect SNAP+spectrograph to be roughly 4× more efficient than HST+STIS.

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