Space Telescope Science Institute   10.2.1 Sensitivity Units and Conversions  10.2.3 Point Spread Functions

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. The first figure shows S/N for point sources (GAIN=1). The second figure shows S/N for uniform extended sources of area 1 arcseconds2.

The different line styles in the S/N figures delineate regions where different sources of noise dominate. A particular source of noise (readnoise 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 arcseconds2 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.

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.
  2. Find the appropriate plot for the filter in question, and locate V+AB 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. 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.


 10.2.1 Sensitivity Units and Conversions  10.2.3 Point Spread Functions
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