A satellite view of the COAST telescope

Long-Baseline Optical/Infrared Interferometers in the world

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How the COAST telescope works


Differing from telescopes using radio interferometry, the mechanism of the COAST telescope places a huge emphasis on mechanical stability and precision. Here is an introduction to the mechanics of COAST and how the precision needed is achieved.


The telescopes

Four telescopes are focused on the object of interest and the image from each telescope passes through aluminium tubes into the beam combining building

Here's what one unit looks like:

The incoming light is reflected from the siderostat mirror onto the primary mirror, which focuses the light on a small mirror. The light beam goes through the hole on the primary mirror to an aluminium tube

The structure is mounted on three feet to provide the best stability.


A carbon fibre tube is used to keep the whole structure stable within nanometers!

In the beam combining building

The path lengths of the four beams of light are made equal using four movable trolleys. This is necessary so that the beams of light are in phase when they are brought together in the beam combiner, so that an interference pattern is produced. However, as each light beam comes from telescopes at different physical locations, the path lengths of the beams have to be adjusted to make sure they are in phase. As well as this, the path lengths have to be constantly adjusted owing to the rotation of the Earth moving the telescopes relative to the object of interest.

The positions of the trolleys are constantly monitored. By calculating the time it takes for a laser beam to travel the distance, the position of each trolley is judged with a precision within nanometers.



Here's what each trolley looks like.

Beam splitters

After being reflected by the trolleys, the light beams go through (half-silver coated) spotted mirrors, which serve as beam splitters. Part of each light beam is used in the guiding system which makes sure the telescopes are kept pointing at the object of interest.

Beam combiners

The other four beams of light are then combined in an optical beam combiner. The beams interfere with each other and this is detected by photo-diodes. When the beams interfere, a fringe pattern is produced. Data from the fringe pattern such as the phase of the light beams and the intensity of the fringes are measured by the photo-diodes. This data is then used to reconstruct the original image in a complicated series of processes. The result is an image with very high angular resolution, much higher than can be achieved by a conventional single telescope, for example the Hubble telescope.

The telescopes are arranged so that as the Earth rotates, an ellipse is traced out relative to the object of interest. Here is a diagram to help you see why.

The interference data is processed throughout the night and is reconstructed into an image. The final image has the angular resolution of a single telescope with the diameter of the traced ellipse.

This webpage was created by Oscar Chung and Sudhir Balaji as a work-experience project from 20/8/2012 to 24/8/2012

Creative Commons Licence Telescopes information by Oscar Chung and Sudhir Balaji is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported License.