Space Telescope Science Institute   9.3.1 Calculating Exposure Times for a Given Signal-to-Noise  9.4 Detector and Sky Backgrounds

9.3.2 Exposure Time Estimates for Red Targets in F850LP


At wavelengths greater than 7500 Ċ (HRC) and about 9000 Ċ (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 affects particularly the very broad z-band F850LP filter, for which the encircled energy depends on the underlying spectral energy distribution the most. In the currently available ETC, the treatment of such an effect has been ameliorated but not solved. 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 synphot package in IRAF/STSDAS for which the proper integration of encircled energy over wavelength has now been incorporated. To quantify this new synphot capability, we compare ETC results with synphot for a set of different spectral energy distributions and the observation mode WFC/F850LP. In Table 9.4, 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 arcseconds aperture for the ETC calculations and synphot.


Table 9.4: Encircled energy comparison for WFC/F850LP.
Spectral type
APT ETC
Synphot
O
0.76
0.74
M
0.71
0.7
L
0.69
0.68
T
0.61
0.6

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. To learn how to use the synphot tool, we refer to the instructions provided in the April 2003 STAN, http://www.stsci.edu/hst/acs/documents/newsletters/stan0302.html, and in ACS ISR 2003-08.


 9.3.1 Calculating Exposure Times for a Given Signal-to-Noise  9.4 Detector and Sky Backgrounds
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