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The prime focus cage and an observer, 1976

Tags: David Malin, AAT, Anglo-Australian Telescope, prime focus, astrophotography, photography

When the Anglo-Australian Telescope was used to take photographs it was the equivalent of using huge camera that was fixed to the ground but could point anywhere in the southern hemisphere sky. The 'camera' is able to track objects with great precision, effectively compensating for the Earth's rotation. The AAT's 4m diameter primary mirror can be thought of as the camera's lens, made large to capture as much light as possible. Considered as a camera, it has a focal length of 12.7 m and a operates at f/3.3, with a field of view 1 degree square, twice the apparent diameter of the full Moon. Of course, a telescope is more than a camera, and collects light for a wide variety of instruments.

In the configuration shown here the observer used to ride in the prime focus 'cage' at the top end of the telescope where light from the primary mirror is brought to a focus. With his back to the sky, he spent many hours taking photographs on specially sensitised glass plates each of which was 10 inches square and required exposure times of 30-90 minutes to reveal the faintest objects. As the telescope moved across the sky the small seat could be rotated around the plate holder to provide the observer with some degree of comfort.

Photography is no longer used in astronomical photography, either amateur or professional, and neither the prime focus cage nor an observer sitting under the stars are necessary. Imaging is now done using solid state electronic detectors (CCDs), and the observer works in a comfortable control room.

The plates that taken with this camera were mostly intended for scientific purposes, but many of them have been used to make the 3-colour images seen elsewhere on these pages. Some care has been taken to ensure that the colour in the images derived from these plates is realistic, and more information on astronomical photography is available here. The picture above picture was made in 1976.

Credit: David Malin

© Australian Astronomical Observatory