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Advanced Camera for Surveys Instrument Handbook for Cycle 27 > Chapter 9: Exposure-Time Calculations > 9.3 Computing Exposure Times

 9.3 Computing Exposure Times
To derive the exposure time to achieve a given signal-to-noise ratio, or to derive the signal to noise ratio in a given exposure time, there are four principal ingredients:
 • Expected counts C from your source over some area.
 •
 • Sky background (Bsky) in counts/pixel/second.
 • The detector background (Bdet) or dark count in units of counts/second/pixel and the read noise (R) in counts of the CCD.
 • Section 9.4 provides the information for determining the sky-plus-detector background.
The signal-to-noise ratio, is given by:
Where:
 • C = the signal from the astronomical source in counts/second, or electrons/second from the CCD. The actual output signal from a CCD is C/G where G is the gain. You must remember to multiply by G to compute photon events in the raw CCD images.
 • G = the gain is always 1 for the SBC, and 0.5, 1, 1.4, or 2 for the WFC after SM4, depending on GAIN. For archival purposes, gains prior to SM4 for WFC and HRC were ~1, 2, 4, or 8.
 • Npix = the total number of detector pixels integrated over to achieve C.
 • Bsky = the sky background in counts/second/pixel.
 • Bdet = the detector dark current in counts/second/pixel.
 • R = the read noise in electrons; it is equal to zero electrons for SBC observations. See Table 4.1 for WFC after SM4. For archival purposes, see Table 4.2 and Table 4.3 for WFC and HRC prior to SM4.