Calculating SNR

The SNR is calculated based on the user input exposure time, instrument configuration, and simulated target spectrum. For spectroscopic modes, these values are calculated at a particular wavelength specified by the user in Section 2 of the ETC forms.

Please make sure the wavelength that appears in this section is covered by the selected mode.

Total photon counts from the various light sources are computed by multiplying the pysynphot-derived count rate (in photons/sec) by the user input exposure time. Each photon count term is depicted below with a _c subscript. All terms are calculated for a particular extraction region (aka photometric extraction aperture). Some terms, shown in parentheses, are optional in that they apply only to certain exposures. Thermal background, for example, only applies when an infrared detector is used. We start with equation (i),

where:

S_c = Source counts

N = Noise

where:

B G_c = Background counts

RN = Read Noise

where:

DC_c = Dark Current counts

Sky_c = Sky counts = Earth Shine + Zodiacal Light + (Geocoronal Emission Lines)

(Thermal_c) = Thermal Background counts (optional)

(CS_c) = Coronagraphic Source counts (optional)

(Geocoronal Emission Lines) = (optional parameter)

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