Focus is maintained by two Wave Front Sensor probes (WFS) which are placed on point sources brighter than about 15th mag. These are generally centered up and their focus loops closed in the first few minutes of any given observation. For short observations there may not be enough time to acquire a WFS star and focus will be maintained by visual assessment of the FWHM of the guide star.
The WFS can achieve constant focus of less than 0.02 mm for a full trajectory unless the temperature is changing rapidly in which case the WFS tends to lag behind the changes in focus by up to 0.06 mm. When the Telescope Operator is required to keep focus manually (for short observations or if no WFS stars can be obtained) then the focus is generally kept to 0.05 mm.
Global radius of curvature of the primary mirror array is maintained by looking that the DMI during a trajectory when the telescope is in good focus. If this distance deviates by more than 200 um from the "ideal" distance then a GROC correction is applied at the end of the current trajectory. This GROC correction takes less than 4 minutes to apply and thus can be done while setting up on the next target.
Last updated: Sun, 06 Nov 2022 06:31:25 +0000 stevenj