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WFPC2 Exposure Time FAQs

NOTE: Due to limited resources, these pages may not have been regularly updated. It is possible that the information provided below and/or in the links given may be outdated or inaccurate. If you come across conflicting information or are confused by the answers given, please contact the STScI helpdesk at: help@stsci.edu>.

Q: How do I calculate the exposure time for my target if the target is a point source?

A: Exposure times can be calculated by using the WFPC2 Exposure Time Calculator. Exposure times can also be calculated by using Equation 6.1 in V 4.0 of the WFPC2 Instrument Handbook, or by using the SYNPHOT package in STSDAS. Please see Exposure Time Estimation for Point Sources for a description and examples of exposure time estimation for point sources.

Q: How do I calculate the exposure time for my target if the target is an extended source?

A: Exposure times can be calculated by using the WFPC2 Exposure Time Calculator. Another method for calculating exposure times for extended targets is given in Exposure Time Estimation for Extended Sources. In this document, we show how to derive count rates using both analytic formulae and SYNPHOT for extended targets with stellar, non-thermal, and emission line spectra. These count rates are then used to derive signal-to-noise ratios and exposure times.

Chapter 6 of the WFPC2 Instrument Handbook contains further information on exposure times and their estimation.