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VLT-MAN-ESO-14650-4942
P96
24.06.2015
26 of 161
ESO, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
2.2.2 ADCs problems and disabled ADCs observing mode in SLIT and IFU
During March to July 2012 the ADCs (atmospheric dispersion correctors) for the UVB and
VIS arms in X-shooter have been occasionally failing. Unfortunately the rate of such failures
has increased until being daily, leading sometimes to data taken in sub-optimal instrument
configuration, which needs to be taken into account when reducing and analyzing such
observations.
There is an ongoing investigation to find the cause for the ADCs' misbehavior, but it is
unlikely that the situation is back to normal for the next few months. Incorrect position of
ADCs might lead to slit losses worse than if they are not used. Consequently, the ADCs were
temporarily disabled (set at the non deviation position as in the IFU mode) on August 1st. A
major intervention to fix the problem is currently under investigation.
In the following pages, you will find useful information characterizing the observations without
working ADCs to compensate the atmospheric dispersion in UVB and VIS arms.
Measurements were performed in the various orders of the UVB/VIS arms, some
comparisons are performed and the average, the min/max values and the standard deviation
are provided. The slits used are 1.0”, 0.9”, 0.9” in the UVB, VIS, and NIR arms respectively.
In 2015 a study has been launched to repair the ADCs, if successful the repaired ADCs will
be made available possibly in 2016.