VLT XShooter


This file summarizes several instrument specific settings that are related to the VLT/XShooter spectrograph.


As it is common for ESO to obtain calibrations with different slit widths and binning, this can lead to various challenges for PypeIt.

As regards wavelengths, the varying binning and slit widths lead to differing FWHM of the arc lines. And because the RMS threshold for a good solution is scaled to FWHM, the default is to measure the FWHM from the lines themselves.

If too many orders are being rejected, you may wish to adjust things in one or more ways.


For the UVB or the VIS, you may turn off measuring the FWHM (in units of binned pixdels) from the arc lines by adding this to your PypeIt Reduction File:

        fwhm_fromlines = False

This will set the FWHM to the default value for UVB/VIS which may yield a better set of discovered arc lines.


Another option is to increase the RMS threshold for a good solution. This may be done in the PypeIt Reduction File as well:

        rms_thresh_frac_fwhm = 0.2

Note that rms_thresh_frac_fwhm is expressed as a fraction of the FWHM. See WavelengthSolutionPar Keywords for further details.