What is the stability of the spectrograph throughout a night?
As the night goes on, the spectra move with respect to the CCD (or the CCD moves with respect to the spectra, or both). When looking at the corrections provided by Libre-Esprit (position of telluric lines), the corrections are increasing towards negative values (for an initial value around 0.0). At the end of the night, the correction ends up around -0.3 km/s with respect to the beginning of the night. This is only a small fraction of a pixel, about 0.1 pixel (one CCD pixel = 2.6 km/s), but it is a clear and systematic effect. Note that the latest version of Libre-Esprit incorporates analysis of telluric lines to provide a better wavelength calibration. Therefore, it should not be necessary to take calibration exposures during the night to track that effect.
