 Space Telescope Science Institute ACS Instrument Handbook for Cycle 27 help@stsci.edu     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.
 • Nread = the number of CCD readouts.
 • t = the integration time in seconds.
This equation assumes the optimistic (and often realistic) condition that the background zero point level under the object is sufficiently well known (and subtracted) to not significantly contribute; in crowded fields this may not be true.
Observers using the CCD normally take sufficiently long integrations that the CCD read noise is not important. This condition is met when: For the CCD in the regime where read noise is not important and for all SBC observations, the integration time to reach a signal-to-noise ratio , is given by: If your source count rate is much brighter than the sky plus detector backgrounds, then this expression reduces further to: i.e., the usual result for Poisson statistics of .
More generally, the required integration time to reach a signal-to-noise ratio is given by: At wavelengths greater than 7500 Å (HRC) and about 9000 Å (WFC) ACS CCD observations are affected by a red halo due to light scattered off the CCD substrate. An increasing fraction of the light as a function of wavelength is scattered from the center of the PSF into the wings. This problem particularly affects the very broad z-band F850LP filter, for which the encircled energy mostly depends on the underlying spectral energy distribution. The encircled energy fraction is calculated at the effective wavelength which takes into account the source spectral distribution. This fraction is then multiplied by the source counts. (The effective wavelength is the weighted average of the system throughput AND source flux distribution integrated over wavelength). However, this does not account for the variation in enclosed energy with wavelength.
To quantify this new pysynphot capability, we compare the ETC results with pysynphot for a set of different spectral energy distributions and the observation mode WFC/F850LP. In Table 9.3, the spectral type is listed in the first column. The fraction of light with respect to the total integrated to infinity is listed in the other two columns, for the ETC and pysynphot calculations respectively. These values are derived for a 0.2 arcsecond radius aperture for the ETC calculations and pysynphot.
 Spectral type ETC pysynphot O 0.76 0.74 M 0.74 0.72 L 0.70 0.69 T 0.65 0.64

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