VLT-MAN-ESO-14650-4942
P96
24.06.2015
63 of 161
ESO, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
2.4.13 UVB/VIS ADCs problem
UVB/VIS ADCs intermittently show initialization problems, especially in cold conditions.
Since August 2010 a new operational procedure has been implemented to prevent starting
science observations with the UVB/VIS ADCs in a wrong position. In the evening at the time
of the instrument startup and during the morning calibrations the ADCs are closely monitored
to ensure that the systems are working as expected.
Unfortunately the ADCs failed more and more frequently between March and July 2012 and
it has been necessary to disable them since August 1
st
, 2012.
See section 2.2.2 for the relevant information about the efficiency of XSHOOTER with
disabled ADCs.
2.4.14 Drift of acquisition reference positions
It has been recently found (12/2013) a drift of the acquisition reference positions on
XSHOOTER.
All acquisitions are performed blindly (not possible to see the slits) and the reference
positions for the acquisitions were defined for all filters.
However, it appears that a function is drifting leading to a drift of those reference positions.
This is now monitored and rectified when the drift reaches a significant amount possibly
leading to flux losses for the narrower slits. Unfortunately, a bad software update may have
generated losses during the period 12/2013-06/2014 for the observations using narrow slits
and the U-band acquisition filter.
2.4.15 TCCD features
- The cooling system of the CCD produces small oscillations of the temperature of the CCD
around an average. Temperature variations affect the dark current level, so in case of short
exposure times, when the image sampling frequency happens to align with the frequency of
the temperature oscillations, this leads to "beats" and background level variations from one
image to the next one. These variations in background level disappear if a longer exposure
time is selected. In any case they do not affect the acquisition performance.
In addition since June 2011 the noise was improved and the quality of images allow under
good weather conditions to see objects as faint as magnitudes 25 in R, V bands in 3mn.
-Due to an upgrade of the VLT software driving all acquisition CCD in January 2014, the
snapshots saved during the acquisition process are of sub-optimal quality.
It is worth to note that the acquisition itself and the imaging mode are not affected at all by
this problem.
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