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

• Expected counts C from your source over some area.

•

• 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.

• 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.

•

• Bsky = the sky background in counts/second/pixel.

•

•

• Nread = the number of CCD readouts.

• t = the integration time in seconds.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.As a consequence, in order to obtain correct estimated count rates for red targets, observers are advised to use the pysynphot package available in AstroConda.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.Table 9.3: Encircled energy comparison for WFC/F850LP.

The ETC results are off by 3% (O star), 2% (M star), 2% (L star), and 1% (T star). If this small effect is relevant to particular observations, then the pysynphot software package can be used. Further information about filter F850LP can be found in Sirianni, M. et al. 2005, PASP, 117, 1049 and Bohlin 2016, AJ, 152, 60.

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