COAST Technical Details
The Telescopes
The COAST unit telescope design comprises a 50cm siderostat feeding a fixed horizontal 40cm f/5.5 Cassegrain telescope with a magnification of 16 times. The telescope frame is constructed from square steel tube and results in a very rigid structure.
The telescope is mounted kinematically on three feet, each consisting of a cone fitting over a hardened ball mounted upon the foundations. Due to possible differential expansion between the foundations and the telescope, the feet must have some compliance whilst still maintaining accurate location.
The siderostat mirror is mounted on two commercially available precision rotation tables in order to allow it to be pointed in any direction in the upper hemisphere. Carbon fibre tube is used to maintain the focus distance between the primary and secondary mirrors even on thermal expansion of the telescope frame.
A COAST unit telescope with its protective covers removed. The back of the siderostat mirror is visible in the foreground, with the concave primary mirror at the opposite end of the telescope.
The Beam Combining Building
The beam from each telescope passes through an aluminium tube, which serves to isolate it from the atmosphere, into the beam combining building. The beam combining building is a corrugated steel tunnel, 32m in length, 6m in width and a maximum height of 2.4m covered to a depth of 1m with approximately 300 tonnes of earth and sewn with grass. This arrangement provides a stable thermal environment internally. The annual variation in temperature is only 11K and diurnal variations are extremely small. Externally the building has a thermal signature almost identical to that of the surrounding ground and generates very little turbulence due to the passage of air.
Acquisition and Auto-guider System
Inside the beam combining building the outer annulus of each light beam is separated and used in the acquisition and autoguider system. Each telescope is pointed so that it acquires the field of interest, and then the beams are brought together sufficiently to give interference. The system is then switched to fast guiding mode and maintains the position of the object to within a fraction of an arcsecond.
A single CCD detector is used for both functions, light from each of the four beams being imaged onto different areas. Acquisition is achieved using a shutter exposure to integrate the image, followed by a slow read out of the chip, whereas in guiding mode data is read from the chip in such a way as to generate quadrature cells. A simple algorithm is then used to detect the offset from the centre of the field, and this is corrected using piezo-driven mirrors at the back of the telescopes. The guiding system operates at up to 100Hz.
The Correlators
Equalisation of light paths from a star through the telescope to the detectors is carried out by a movable trolley carrying a mirror running on precisely aligned rail tracks. The mirror is mounted to allow frictionless translation with respect to the trolley, and is driven by a loudspeaker voice coil. The position of the mirror is continuously monitored using a laser interferometer, and this information is used to correct the position via the voice coil.
The beams are then fed into one of the correlators (or beam combiners), either optical or infra-red, which lie at the heart of COAST. The total intensity of the emerging beams is measured using four Avalanche Photo Diodes (APDs), and this information is handled via a system of computers. The IR correlator is equivalent, except that the total intensity of each emerging beam is measured using a pixel of a NICMOS detector.
Both correlators are shown in the diagram below.
Optical layout at COAST.