XSHOOTER - List of Known Problems

All the points mentioned below are better described in the user manual.

  • Recently an oscillation of the dichroic absorption band in the UVB and VIS arms was reported. The last UVB arm order and the first 4 orders of the VIS arm suffer of this oscillation. However, the NIR arm remains not affected. This oscillation corresponds to a shift of the dichroic features on the order of 1 to 2 nm. As a consequence, dichroic features variation of up to 30% between 2 flat fields was observed. The reasons for such variations are not clear yet but this problem is under investigation.
  • The technical CCD (Acquisition and Guiding camera) is currently experimenting some stability problems. As a consequence its efficiency is lower than measured during the commissioning. It means that for objects fainter than the 21st magnitude, a blind offset from a brighter object has to be used.
  • The technical CCD had some communication problems, they seem to be fixed by a software upgrade. This is under monitoring.
  • The UVB ADCs are failing from time to time leading to observations withbad angles of the ADCs. They are now monitored every day several timesthe day. And if they fail, the proper action is taken to avoidobservational problems.
  • The AFC were failing from time to time in the UVB arm, it was related to the warmup of the lamp used. This problem is fixed.
  • It was reported that the absorption band at ~570nm in the UVB arm was oscillating with the time. This problem is hardly reproducible, however it is under investigation, it may be related to the AFC of the UVB arm. Since no oscillation was observed in the VIS the dichroic is probably not at the origin of that problem.
  • In case of the observation of a very bright object, some very faint ghosts appear.
  • In case of CCDs saturated, strong remnants can stay for a couple of hours. It is particulary true when arc calibrations are taken, specially with the 2x2 binning mode.
  • There is a vignetting in the 11th order (K-band) of the NIR arm, affecting the top of the order. This vignetting introduce a 10% decrease of the flux. This is not corrected by the FF due to different illumination in the daily calibrations.