Spectroscopic calculations compute the time or SNR at the specified observation wavelength.

In addition to the basic calculation results, the spectroscopic ETCs also present plots of the signal to noise as a function of wavelength, and the total counts from each component of the calculation.

Specifying the Wavelength

After selecting the central wavelength setting for the desired grating in the instrument configuration section, the user must also, in the next section, specify an observation wavelength that is within the wavelength range for that configuration. Information about the valid wavelength ranges is available on the forms. If the specified wavelength is not in the correct range, the calculation will not run and the user will get an error message.

In previous ETCs,

  • 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 was selected, the wavelength specified for the S/N calculation was checked and, if outside the valid range, was automatically reset to the midpoint of the valid range for that grating/CENWAVE combination. This automatic resetting is no longer performed.

Extended Sources

Increasing the slit width on a spatially extended source degrades the resolution. This degraded resolution is:



m_{\lambda} is the plate-scale in the 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 more correctly.

For slitless 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 accurate simulation could run the risk of being too optimistic.


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