i) Arc and Flat-Field Calibrations
In principle we expect the system to be stable over sustained
periods, and indeed test data obtained during the commissioning runs
show this to be generally the case. However, very occasionally
relatively large variations have been seen that defies explanation. While
this problem remains under-investigation, `over-calibration' would seem a
wise procedure to adhere to (especially as the overheads in obtaining
calibration data are small).
ii) Swing Plate Mechanism
The SP mechanism is stable at the level of our uncertainties when
determining centroids (i.e., less than 0.2 arcsec / 40 microns).