This tutorial provides information for the cross dispersers Health
Check trending. It is based on Cross Disperser [CD]
alignment calibration data obtained with dedicated Health Check OBs executed
daily. These OBs obtain four sets of CD-Alignment data, one set for
each of the four CDs, each set comprising 2 RAW frames. The 2 RAW
frames of each set image the zeroth echelle order, with the CD being
moved away and then back into position between the two exposures. The
objective of the data is to measure the accuracy and precision of the
positioning of the CDs, in order to confirm that the CDs can be
repositioned in the morning after SCIENCE are acquired for acquisition
- average y difference of zero orders (QC1 database table uves_zflat, column ydiffavg, product keyword QC.YDIFFAVG)
Description: Average order position shift. We monitor the Cross Disperser
- rms of y difference (QC1 database table uves_zflat, column ydiffrms, product keyword QC.YDIFFRMS)
Description: RMS order position shift. We monitor the Cross Disperser
Cross-Disperser Alignment FLATs (ZERO ORDER FLATs)
The zero order flats are used to test the alignment and stability of
the positions of the four UVES crossdispersers (CD). They are measured
daily whenever UVES is operational and are a supplement to the format
The zero order flats are measured within a sequence. There are two exposures
for each CD: the CD is moved from two opposite sides to the nominal position.
Differences in Y (cross dispersion) position of the zero orders indicate
problems in the positioning of the CD.
Each arm of UVES has two crossdispersers:
The Y position of the zero order is measured at several X positions.
The average difference of the Y positions for each pair of zero flats
- average difference of Y positions (YDIFFAVG),
- rms of average Y difference (YDIFFRMS).