Space Telescope Science Institute   10.1 Introduction  10.2.2 Signal-To-Noise

10.2 Using the Information in this Chapter


10.2.1 Sensitivity Units and Conversions


This chapter contains plots of throughputs for each imaging mode. "Determining Count Rates from Sensitivities" in Section 9.2 explains how to use these throughputs to calculate expected count rates from your source.

The first figure for each imaging mode gives the integrated system throughput. This is the combination of the efficiencies of the detector and of the optical elements in the light path. The throughputs in this handbook are based in part on ground test data. Although, at the time of writing the overall detector efficiency curve and most filter throughputs have been adjusted based on in-flight data. The throughput is defined as the number of detected counts/second/cm2 of telescope area relative to the incident flux in photons/cm2/second. For the CCD, "counts" is the number of electrons detected. For the MAMA, "counts" is the number of valid events processed by the detector electronics after passing through the various pulse-shape and anti-coincidence filters. In both cases the detected counts obey Poisson statistics. The throughput includes all obscuration effects in the optical train (e.g., due to the HST secondary).

To recalculate the throughput with the most recent CCD QE tables in synphot, you can create total-system-throughput tables (instrument plus OTA) using the synphot calcband task. calcband takes any valid obsmode command string as input and produces an STSDAS table with two columns of data called "wavelength" and "throughput" as its output. For example, to evaluate the throughput for the F475W filter and the WFC detector, chip 1, you would use the command calcband acs,wfc1,f475w sdssg_thpt. The resulting throughput table is stored in sdssg_thpt.


 10.1 Introduction  10.2.2 Signal-To-Noise
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