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	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

Calculating SNR

The SNR is calculated based on the user input exposure time, instrument configuration and simulated stellar spectrum. Total photon counts from the various light sources are then computed by multiplying the SYNPHOT derived count rate (in photons/sec) by the user input exposure time. Each photon count term is depicted below with a _csubscript. All terms are calculated for a particular extraction region (aka photometric extraction aperture). Some terms, shown in parenthesis, are optional in that they apply only to certain exposures. Thermal background, for example, only applies when an infrared detector is used.

(i)

where:

S_c = Source counts

N = Noise

where:

BG_c = Background counts

RN= Read Noise

where:

DC_c = Dark Current counts

Sky_c = Sky counts = Earth Shine + Zodiacal Light + (Geocornal Emission Lines - optional)

(Thermal_c) = Thermal Background counts (Optional)

(CS_c) = Coronagraphic Source counts (Optional)

So if (i) is fully expanded with each of its individual terms substituted it becomes:

(ii)