This chapter contains plots of throughputs for each imaging mode. Section 9.2 explains how to use these throughputs to calculate expected count rates from your source.
The first figure for each imaging mode gives the integrated system throughput. This is the combination of the efficiencies of the detector and of the optical elements in the light path. The throughputs in this handbook are based in part on ground test data, although, at the time of writing the overall detector efficiency curve and most filter throughputs have been adjusted based on in-flight data. The throughput is defined as the number of detected counts/second/cm
2 of telescope area relative to the incident flux in photons/cm
2/second. For the CCD, “counts” is the number of electrons detected. For the MAMA, “counts” is the number of valid events processed by the detector electronics after passing through the various pulse-shape and anti-coincidence filters. In both cases the detected counts obey Poisson statistics. The throughput includes all obscuration effects in the optical train (e.g., due to the
HST secondary).
To recalculate the throughput with the most recent CCD QE tables in synphot1, you can create total-system-throughput tables (instrument plus OTA) using the
synphot calcband task.
calcband takes any valid obsmode command string as input and produces an
STSDAS table with two columns of data called “wavelength” and “throughput” as its output. For example, to evaluate the throughput for the F475W filter and the WFC detector, chip 1, you would use the command
The ramp filters are not included in this chapter because the passband will change depending on the chosen central wavelength. The width of the passband and available range of central wavelengths for each ramp segment are listed in
Table 5.2. Additionally, the passband for a ramp segment can be obtained with
synphot1 using the following command
calcband acs,wfc1,fr388n#3880 sdssg_thpt where the #3880 is the desired central wavelength in Angstroms.
For each imaging mode, plots are provided to estimate the signal-to-noise ratio (S/N) for a representative source. The first figure shows S/N for point sources (
GAIN=1). The second figure shows S/N for uniform extended sources of area 1 arcsecond
2.
The different line styles in the S/N figures delineate regions where different sources of noise dominate. A particular source of noise (read noise for example) is presumed to dominate if it contributes more than half the total noise in the observations.
The point- and extended-source S/N figures are shown for average and low sky levels. For point sources, an aperture size of 5 x 5 pixels has been used for the WFC, 9 x 9 pixels for HRC, and 15 x 15 pixels for the SBC S/N evaluation. For extended sources, a 1 arcsecond
2 aperture was used. For the CCD the read noise has been computed assuming a number of readouts
NREAD= integer (
t / 1000 seconds), where
t is the exposure time, with a minimum
NREAD=2. That is, each exposure has a minimum
CR-SPLIT=2. Different line styles in the figures are used to indicate which source of noise dominates.
To the left of the vertical line in the SBC S/N plots, the count rate from the source exceeds the 150 counts/second/pixel local count rate limit. This is computed from the model PSF, which gives 14% to 22% of the flux in the central pixel.
In situations requiring more detailed calculations (non-stellar spectra, extended sources, other sky background levels, unknown target V magnitude, etc.), the ACS
ETC should be used.
The “x” characters at the top of each plot indicate the onset of saturation, in the case of the CCD. The “x” shows where the total number of counts exceeds the 16 bit buffer size of 65,535.
Note that the plots show the S/N as a function of source magnitude for exposure times as short as 0.1 seconds, although the minimum exposure time for the WFC CCD channel is 0.5 seconds.
, or 
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