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The prime focus cage and an older observer (1991)

Tags: AAT, Anglo-Australian Observatory, David Malin, astrophotography, photography

When the telescope is used to take photographs it acts as huge camera. The camera 'lens' is the AAT's 4m primary mirror which operates at f/3.3. It has a focal length of 12.7m and a field of view 1 degree square. In this configuration the observer rides 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, the observer can spend many hours taking photographs on specially sensitised glass plates each of which is 10 inches square (that's how Kodak delivered them) and which may require exposure times of 60–90 minutes to reveal the faintest objects. As the telescope moves across the sky the small seat can be rotated around the camera pedestal to provide the observer with some degree of comfort. Since this photograph was taken the prime focus 'cage' and its optics have been extensively modified for a number of  innovative instruments that were never imagined by its original designers. 

 The plates 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. This picture was taken in 1991.

Credit: David Malin

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