In most cases, you will find it convenient to use the online WFC3 Exposure Time Calculator (ETC)
to make estimates of the required exposure times for your project. The ETC is available from the WFC3 Web site at:
The ETC calculates counts (e−
) and count rates (e−
/s) for given source and background parameters and assumed characteristics of the detectors. Once these are entered, the ETC then outputs signal-to-noise (S/N) ratios achieved for a given exposure time, or the exposure times required to achieve a given S/N ratio. The ETC supports both direct-imaging and spectroscopic (grism) observations. The aXeSIM tool is also available for simulation of grism observations (see Section 8.4
). A variety of extraction apertures may be assumed in the ETC, both circular and square, allowing the user to select either a radius in arcseconds or a size in pixels. It is also possible to input a calibrated spectral-energy distribution (SED) of your source directly into the ETC. The ETC also outputs peak per-pixel count rates and total count rates, to aid in feasibility assessment. Warnings will appear if the source will saturate the detector, which would not only compromise CCD and IR observations, but might even affect subsequent exposures with the IR channel (see Appendix D:Bright-Object Constraints and Image Persistence
). The ETC has online help for its execution and interpretation of results.
It is also possible to use synphot
to calculate count rates and the wavelength distribution of detected counts.