Exposure Time and S/N Calculation


	User's Guide Contents
	Return to ETC Welcome SYNPHOT SNR and Exposure Time SNR Time Optimal SNR Spectroscopy Target Acquisition Basic Use Page Heirarchy Reference Extraction Regions Spectral Distribution User Spectra Other Spectra Extinction Normalization Background CCD Parameters ID Number Calculations SYNPHOT Calls Call Descriptions Call Parameters NICMOS Spectroscopic Extraction Regions Tables Spectral Elements NICMOS Filters NICMOS Dispersers ACS Filters and Dispersers COS Dispersers WFC3 Filters and Dispersers STIS Filters and Dispersers NICMOS Tables COS Tables STIS Tables and Topical Help

Exposure Time and S/N Calculation

To specify the exposure parameters, you can give either the exposure time (in seconds), in which case the program will calculate the countrates due to the source, background, and the resulting S/N ratio. Or, you can give the required S/N ratio, in which case the program will calculate the required exposure time to achieve this S/N ratio and the corresponding integrated counts. Further details follow:

If you select the exposure time:

The program takes the filter transmission, throughputs of the optics and the efficiency of the detector, and calculates the expected countrate for the specified instrument configuration and finds the S/N ratio and the integrated counts from the source, the sky background, and the detector background for the specified exposure time.

If you select the S/N ratio:

The ETC takes the filter transmission, throughputs of the optics and the efficiency of the detector, and calculates the expected countrate for the specified instrument configuration and finds the required exposure time so that the observation will have the specified S/N. This exposure time is then used to calculate the integrated counts from the source and different backgrounds.

Specifying the Wavelength (spectroscopic exposures):

The S/N and associated counts values listed on the main output page are calculated at a specified wavelength, sometimes called the observation wavelength. The observation wavelength must be in the inclusive throughput range for the instrument, detector, spectral element(s) and disperser central wavelength (if applicable) choosen for this observation.