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The telescope typically points with an accuracy of ~ 1.5 - 2 arcsec rms at Cassegrain and prime focus, and ~ 3 arcsec rms at Nasmyth, depending on when the 21-coefficient pointing model for each focal station was last refined. Below are described the tests carried out to update these coefficients on (1) short timescales and (2) longer timescales.
(1) Quick pointing tests (7-star CALIBRATEs)
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:
(2) Full pointing tests
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