Spectroscopy
Extended Sources
Increasing the slit width on a spatially extended source degrades the resolution. This degraded resolution is:D = (slit-width) / (mλ) * (δ-λ per pixel)
where mλ is again the plate-scale in dispersion direction. In other words, the actual resolution is degraded by a factor R, the slit width in pixels/2. In the current ETC implementation, this degradation in resolution is simulated by a convolution of the input spectrum over the slit width. Thus cases that were handled only in an approximate way by older ETC versions (e.g. input spectra with narrow lines, geocoronal lines in the STIS/FUV) are now handled correctly more correctly.
With slittless spectroscopy, the sky background is assumed to uniformly fill the detector. In truth, the Geocoronal lines don't uniformly fill the detector, but attempting a more acurate simulation could run the risk of being too optimistic.
Specifying the Wavelength
It is important for the user to specify a wavelength that is in the range for the instrument settings that they have choosen. If the wavelength is not in the correct range, the calculation will not run and the user will get an error message giving the correct range.
In previous ETC's the central wavelength setting was selected based upon the observation wavelength setting specified by the user. In this version, the user must select the central wavelength setting that they want to use. This allows the user to choose, in some cases, between two central wavelength settings that both cover the desired observation wavelength. When a new grating or central wavelength value is selected in the ETC web interface, the wavelength specified for the S/N calculation is checked, and if it is outside the range allowed for that grating/CENWAVE combination, it is reset to the mid-point of the valid range. Users should exercise caution, as the wavelength value specified for a given calculation may have been reset by a previous selection.