ACS Insturment Handbook for Cycle 24

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10.2 Using the Information in this Chapter

10.2.1 Sensitivity Units and Conversions

10.2.2 Signal-to-Noise

10.2.3 Point Spread Functions

10.2.1 Sensitivity Units and Conversions

This chapter contains plots of throughputs for each imaging mode, in Section 10.3.1. 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 handbook, the overall detector efficiency curve and most filter throughputs have been adjusted based on in-flight data. The total system unitless quantum efficiency, i.e. throughput, at any wavelength is defined as the probability that a monochromatic photon incident on the primary number produces a detected photo-electron. For the CCD, “counts” is the number of electrons detected. For the MAMA, “counts” is the number of valid photo-electron 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 reflections and transmissions 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:

> calcband acs, wfc1, f475w sdssg_thpt.

The resulting throughput table is stored in the file sdssg_thpt.

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.

10.2.2 Signal-to-Noise

For each imaging mode, plots are provided to estimate the signal-to-noise ratio (S/N) for a representative source, see Section 10.3.1. 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 is 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 is used. For the CCD the read noise is 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 50 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.

Follow these steps to use the signal-to-noise plots:

1.	Determine the AB magnitude of your source at the wavelength of interest. There are several ways to do this.

-	Examine Table 10.1, 10.2, or 10.3 and find ABν for the desired spectral type and filter. Sum the V magnitude of the target and ABν derived from the table.

-	Alternatively, compute ABMAG (=V+ABν) from the source flux, using the relation , or .

2.	Find the appropriate plot for the filter in question, and locate V+ABν on the horizontal axis. Then read off the signal-to-noise ratio for the desired exposure time, or vice-versa.

The “x” characters at the top of each plot indicate the onset of saturation, in the case of the CCD. For the MAMA detector, 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.

10.2.3 Point Spread Functions

All information about the PSF are based on the modeled encircled energy data presented in ACS Point Spread Functions in Section 5.6

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Synphot will soon be replaced by the pysynphot package, a significantly improved re-implementation of Synphot written in Python. Please visit the pysynphot webpage at http://stsdas.stsci.edu/pysynphot.