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Hi Rowena.
For the few deep sky I've taken I've used a 6.3 focal reducer. It has a considerable effect on exposure time. This is a quote from Astro Photography by HJP arnold.
:quote:
To give an example: an exposure of f/8 at 1/125 of a second is equivalent to one of f/11 (one stop less light) at 1/60 of a second (one shutter speed more time), And also to one of f/5.6 (one stop mor light) at 1/250 of a second (one shutter speed less time). :end quote:
So everytime you double your f/ratio (5.6 to 8) you double the exposure time., or halve your ratio (8 to 5.6) you halve your exposure time. From f/10 (if tha's what your telescope operates at?) to f/6.3 is a strange ratio as it doesn't conform to the standard camera f/ratios the results are very similar (though it is a common astronomical/telescope ration). That is you will end up roughly halving your exposure time. Hence the benefit of a 6.3 reducer.
However if you're using an already fast scope (f/6 or 5 or faster) the focal reducer may have strange effect on your images, depending on your type of scope.
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