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/cm2
of telescope area relative to the incident flux in photons/cm2
/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
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 synphot1 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 arcsecond2
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 arcsecond2
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 Exposure-Time Calculator
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.