Purpose

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 check frames.

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.

Modes

Each arm of UVES has two crossdispersers:

CD INS MODE 1 BLUE 2 BLUE 3 RED 4 RED

QC1 parameters

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 is calculated:

• average difference of Y positions (YDIFFAVG),
• rms of average Y difference (YDIFFRMS).

The trending plot shows YDIFFAVG versus time. Values less than a few tenth of a pixel indicate good positioning accuracy of the CDs.

Example for crossdisperser failure

An example for a failure of the red crossdispersers can be found in the JAN-MAR 2004 plot. Note that the positioning problems for CD#4 are indicated somewhat earlier than in the format check plot.