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ACS Insturment Handbook for Cycle 24 |
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This chapter contains plots of throughputs for each imaging mode, in Section 10.3.1. Section 9.2 explains how to use these throughputs to calculate expected count rates from your source.To recalculate the throughput with the most recent CCD QE tables in synphot1, 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:The ramp filters are not included in this chapter because the passband will change depending on the chosen central wavelength. The width of the passband and available range of central wavelengths for each ramp segment are listed in Table 5.2. Additionally, the passband for a ramp segment can be obtained with synphot1 using the following command: >calcband acs, wfc1, fr388n#3880 sdssg_thpt where the #3880 is the desired central wavelength in Angstroms.10.2.2 Signal-to-NoiseFor each imaging mode, plots are provided to estimate the signal-to-noise ratio (S/N) for a representative source, see Section 10.3.1. The first figure shows S/N for point sources (GAIN=1). The second figure shows S/N for uniform extended sources of area 1 arcsecond2.The point- and extended-source S/N figures are shown for average and low sky levels. For point sources, an aperture size of 5 x 5 pixels is used for the WFC, 9 x 9 pixels for HRC, and 15 x 15 pixels for the SBC S/N evaluation. For extended sources, a 1 arcsecond2 aperture is used. For the CCD the read noise is computed assuming a number of readouts NREAD= integer (t / 1000 seconds), where t is the exposure time, with a minimum NREAD=2. That is, each exposure has a minimum CR-SPLIT=2. Different line styles in the figures are used to indicate which source of noise dominates.In situations requiring more detailed calculations (non-stellar spectra, extended sources, other sky background levels, unknown target V magnitude, etc.), the ACS ETC should be used.
- Examine Table 10.1, 10.2, or 10.3 and find ABν for the desired spectral type and filter. Sum the V magnitude of the target and ABν derived from the table.
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2. Find the appropriate plot for the filter in question, and locate V+ABν on the horizontal axis. Then read off the signal-to-noise ratio for the desired exposure time, or vice-versa.10.2.3 Point Spread FunctionsAll information about the PSF are based on the modeled encircled energy data presented in ACS Point Spread Functions in Section 5.6Synphot will soon be replaced by the pysynphot package, a significantly improved re-implementation of Synphot written in Python. Please visit the pysynphot webpage at http://stsdas.stsci.edu/pysynphot.