If you've ever tried to take an astro-image with a fixed camera, you'll have seen how the starlight is smeared across your camera's sensor as the Earth rotates, even during relatively short expsoures of a few seconds.
Here's an illustration, using a single 1-minute image of the constellation Lyra:
Now the "barn-door" (aka "Scotch mount") is is a very simple home-made camera platform for astro-photography. It consists of a large wooden hinge, which is slowly opened by a motorised screw at a rate of one degree every four minutes. When fixed on a tripod with its hinge-pin pointed at the celestial pole, a camera mounted on it will track the Earth's rotation for long-exposure astro-photography.
Using the same example of Lyra , this picture was formed by adding 14x 1-minute expsoures (taken before the sky was completely dark). There was no significant trailing of the stars in each individual picture, although tracking errors (due to imperfect polar alignment and inaccuracies in the screw-drive) were apparent when comparing images across the whole sequence. But these errors were easily removed by re-alignment of the images in the IRIS program.
This prototype unit is now available for loan to BAS members - see our our Loans Book: http://www.boltonastro.org.uk/Forums/vi ... f=17&t=161
For those interested in imaging and image processing with DSLR, CCD, webcam or film.
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