The interaction between pointing and active optics on the VISTA telescope

The VISTA telescope has a field of view of 45 arc minutes radius and an f/1 primary mirror and so, in order to meet the image quality requirements at the edge of the field, the position of M2 has to be actively controlled in all 5 axes (focus, centring and tilt). Tilting M2 not only affects the image quality, it also shifts the image in the focal plane which introduces an interaction between the active optics and the telescope pointing. VISTA uses the VLT control system and the M2 hexapod does not allow movements of M2 and the telescope to be coordinated well enough for M2 to be tilted while a science exposure is in progress without introducing unacceptable image motion. Therefore, application of tilts requested by the active optics system have to be coordinated with the activity of the infra-red camera. This paper describes how the active optics system measures M2 tilt corrections and how the application of these tilts to the mirror and the compensating adjustments the telescope pointing are integrated with the operation of the telescope and camera in order to deliver the best possible image quality without reducing the survey efficiency.