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The telescope typically points with an accuracy of 1.5 - 2.5 arcsec rms at Cassegrain and prime focus, and 2.5 - 3.5 arcsec rms at Nasmyth, depending on when the pointing model was last refined.
At intervals, ideally every few weeks, and at least once after each instrument change, a short pointing test (CALIBRATE) is carried out. During the CALIBRATE, the telescope is pointed at 7 stars near the meridian, and the 3 coefficients of the telescope's 21-coefficient pointing model which are most likely to change are re-measured: the azimuth and elevation zero points ('index errors' IA, IE) and the angle between instrument rotator axis and azimuth axis ('collimation error' HC). The test also calculates an rms pointing accuracy (albeit one obtained over a restricted range of azimuth).
The figure below shows these four numbers (rms, IA, IE, HC) plotted against date, for the WHT Cassegrain focus:
The top panel of the plot ('Rms') may include a few outliers contributed by tests carried out in poor conditions (bad weather or technical problems). At any given time, the median of the plotted values gives a reasonable indication of the pointing rms to be expected on-sky, near the meridian. Measurements made in 2015 indicate that away from the meridian, the rms is typically higher by a factor of 1.5 - 2.
Click on the links below for similar plots for other focal stations:
Full pointing tests, involving observations of ~ 100 stars, are carried out whenever the rms starts to drift upwards from the typical values quoted above.
The history of the derived Cassegrain-focus pointing coefficients during 1990 - 2010 is shown below. Most of the coefficients change little with time, despite the many changes in loading of the telescope structure over the last two decades.
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