The VLT Adapter-Rotators
The ESO Very Large Telescope consists of an array of four
8-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 the
The adapter-rotators provide several key functions for the
operation of the VLT, including field
acquisition and guiding, wave front sensing
, and instrument rotation.
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 from
the telescope beam to pass through the center to the instrument.
The Nasmyth and Cassegrain adapter-rotators are not
identical in size but have similar functions.
Before 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.
As 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
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.
As the telescope field rotates during an observation, the scientific instrument
attached to the telescope as well as the Adapter itself must be rotated at the
same speed to allow compensation. The angular speed of this field rotation
varies according to the position of the observed object in the sky. During
observations, the Instrument Rotator (and the instrument attached to it), as
well as the Adapter, must rotate at the same speed to compensate for the
motion. During this mode of operation the Adapter and Rotator are effectively
(although not physically) locked together. Duriunit-tel/adapt-rot.html#irng the acquisition phase,
however, the Instrument Rotator, the Adapter and the Sensor-Arm can all be
moved independently of each another.