The VLT Adapter-Rotators
IntroductionThe ESO Very Large Telescope consists of an array of four8-meter Unit Telescopes, each of which has two Nasmyth foci and a Cassegrain focus. Each focus is equipped with an adapter-rotator,which forms the mechanical interface between the telescope and thescientific instruments.
The position control functions are contained within a mechanical assembly called the adapter, and the instrument interface assembly is called the instrument rotator. Together they form an integral unit which is referred to as the adapter-rotator. The adapter-rotators are basically annular in shape to allow the light fromthe telescope beam to pass through the center to the instrument.
Field Acquisition and GuidingBefore starting an astronomical observation, the telescope control system will point the telescope towards the astronomical object of interest.The Adapter acquisition/guide CCD sensor, located in the Adapter sensor-arm, can be used to view the central part of the telescope field to provide visual identification of the object to be observed. This function is referred to as acquisition.
During the course of the observation the position of the object in the telescope focal plane must be maintained with a high degree of accuracy.To achieve this a reference star close to the object being observed to provide a reference position. The Adapter acquisition/guide sensor continually measures the position of the reference star and any position error detected is passed to the Telescope Control System (TCS) for correction. This function is referred to as guiding.
Wave front SensingAs the primary mirror of the telescope is relatively thin, the optical quality can be degraded by the changing gravitational flexure during observations as well as by other slow effects such as thermal gradients.In order to allow the correction of these optical errors, a second CCD sensor in the sensor arm measures the shape of the wave front reaching the focal plane. This wave front sensor detects the light of the same reference star used for guiding through an optical system which splits the pupil image of the telescope into an array of sub-images which are imaged separately onto the wave front sensor in a grid pattern of light spots.
The position of the light spots indicates the shape of the wave front. The light coming from a fibre-optic reference source within the adapter, imaged through the same optics, is used to give the reference positions for the light spots so that systematic optical errors in the measuring system itself can be compensated. The difference vectors between the two grids are analysed in the Adapter-Rotator control electronics to determine the errors in the shape and position of the telescope mirrors. This information is then passed to the TCS for correction in a similar way as for guiding errors.