System Throughput and SNR / Exposure Time Estimation

6.2 On-Line Exposure Time Calculator


We note that most of the calculations below are incorporated in the on-line WFPC2 Exposure Time Calculator (ETC)
program, which is available on the WFPC2 WWW pages at

http://www.stsci.edu/ftp/instrument_news/WFPC2/wfpc2_top.html

on the software tools page.

To use this program, one merely fills out an "HTML" form giving the target information (e.g. magnitude and color), camera configuration (PC or WFC, desired gain setting, and filter), and either the exposure time or the desired signal-to-noise ratio. As of this writing there are separate HTML forms for point sources, extended sources, point source with background light, and extended targets with background light. After filling out the form the user then clicks on "calculate" and the program returns the resulting signal-to-noise ratio if the exposure time was specified, or vice versa. Examples of completed HTML forms and results are shown in "Sample SNR Calculations" on page 123. Note that clicking on any colored text on the HTML form will give a description of that item.

The ETC program handles sources with stellar spectra, power law sources, and emission line sources; point sources and extended sources; and sources superposed on a diffuse stellar background. The program also returns advice on CR-SPLITing, use of CLOCKS=YES, and warnings about saturation, if appropriate. The program currently (Version 2.0) assumes stellar data will be analyzed by PSF fitting. Results are typically accurate to a few percent.

While observers should familiarize themselves with the material below, most will find the ETC program faster and easier to use for actual calculations. The ETC program will also be updated to reflect any changes in instrument performance, so observers can be assured of up-to-the-minute information.