Flux calibration issue for the XSHOOTER UVB data products
We recently found an issue with the flux calibration of the XSHOOTER data products, for the blue (UVB) arm. The calibration is done with master response curves, which however show significant deviations (above the ±10% limit quoted in the release description) from the proper nightly response curves. This is true since about April 2015. There are also two sudden changes introducing a spectral artefact at λ=365nm, approximately the Balmer discontinuity, around dates 2015-07-30 and 2016-02-10.
For science data products taken before 2015-04, all is fine. There is no issue for the other two arms (VIS and NIR).
We have stopped data processing at the end of October 2016 and are currently analysing the issue. We will accordingly update this information.
Update (May 2017): This issue has been solved, see dedicated archive news.