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ACS Insturment Handbook for Cycle 24 |
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• Expected counts C from your source over some area.
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• 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.
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• Nread = the number of CCD readouts.
• t = the integration time in seconds.As a consequence, in order to obtain correct estimated count rates for red targets, observers are advised to use the synphot package in IRAF/STSDAS (or Pyraf), or the pysynphot package in Pyraf.To quantify this new synphot capability, we compare the ETC results with synphot 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 synphot calculations respectively. These values are derived for a 0.2 arcsecond radius aperture for the ETC calculations and synphot.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 synphot software package can be used. Further information about filter F850LP can be found in Sirianni, M. et al. (2005) and ACS ISR 2012-01.