From a sequence of ThAr spectra taken during a continuous 28 hours run Dr. Eric Stempels (University of St. Andrews, UK) measured the Echellogram shift due to atmospheric air-pressure variations. The ThAr spectrum shifts by approximately 120 m/s per millibar (hPa). Note that the diurnal atmospheric pressure 'tide' amounts to about 1 hPa peak-to-peak amplitude. When using simultaneous-ThAr mode this air-pressure effect is largely calibrated out.
Note that normally the drifts due to temperature changes should be (much) lower than those due to pressure effects, as the temperature is kept constant (within 0.1 degree) in the FIES room and spectrograph.
Using the latest version of FIEStool and simultaneous ThAr mode, an intra-night velocity zero-point accuracy of 5m/s can be achieved for well illuminated spectra.
The next step in looking at the stability of the spectrograph is to derive and specify the long-term wavelength zero-point accuracy for observations with and without simultaneous ThAr mode.
For the different calibration exposures there are script which first take a (windowed) test exposure to define the required exposure time. To monitor the health of the calibration lamps the idea is to record the results from these scripts in a data base.
Thomas Augusteijn 2009-05-14