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/instruments/wfpc2/Wfpc2_etc/wfpc2-etc.html.
 

To use this program, the user fills out an HTML form giving the target information (magnitude, color, and reddening), camera configuration (PC or WFC, desired gain setting, and filter), and either the exposure time or the desired signal-to-noise ratio. There are separate HTML forms for point sources, extended sources, point sources with a diffuse stellar background, and extended sources on a diffuse stellar background. 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. 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 latest version (V4.0) includes calculations of exposure times and/or signal-to-noise ratios for point sources (plus background) using either the traditional "optimal PSF weighting" method or simple aperture photometry in a fixed aperture radius specified by the user. The latter option is more appropriate when comparing with the ACS ETC, which assumes the use of aperture photometry as a default.

In addition, the ETC allows for a flexible specification of the sky background. There are now three options. The first option uses a rough estimate of "average" or "high" or "low" sky background conditions. The second option estimates the sky background based on the position of the target and (optionally) an estimate for the heliocentric longitude of the target (sun angle). The last option allows the user to explicitly provide a value for the sky background, in magnitudes per square arcsecond. Finally, the program also returns advice on CR-SPLITing, use of CLOCKS=YES, and warnings about saturation, if appropriate. 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.