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Choosing an imaging scope - your views?


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I use my short focus frac as a telelens :)... not easy to focus, especially on a moving target, but there is no way I'll ever be able to afford a Canon 600mm lens... so double duty...

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Maybe I could add that. I do a very occasional bit of bird photography with the refractors - and not always the small ones. I probably terrified a hawfinch by training a 5 inch on the poor little fellow...

Olly

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All good stuff here Olly. Short focal length refractors are a great way to start if you are committed DSO imaing. A lot of people however want as scope that will do a bit of everything. In this situation I think a Newtonian makes a good case for itself if you can sort out the back focus issues.

A bit of a warning on relying too much on a fast focal ratio to allow you shorter overall exposures. An 200mm F6 scope will pull in x4 the light that a 100mm F6 scope will. However the greater magnfication means that the extra light has to be spread more thinly. If you were to image M51 with both scopes and present them at the same image scale the image from the large aperture scope would have less noise for a given exposure. This is the basis of the "focal ratio myth" argument.

Once you start to show this mathematically you will quickly turn off most beginners but it might be best to say that fast, short focal length refractors work very well when imaging extended objects or when using a small chipped camera. The fast ratio doesn't make up for lack of aperture if you have to start cropping.

A word on SCT focusers. You need to get used to their ways. If you take the star well out of focus then you can bring the the scope into focus always moving the focuser in the same direction. Gradually image shift reduces to virtually zero, this allows you to take a small sub frame. If you use a Bahtinov mask it is very easy to see when you have a precise focus. Unlike FWHM there is no need to go past the focus point. As soon as you reverse the focus movement image shift comes back with a vengeance. FWHM focusing is a nightmare with an SCT stock focuser.

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Thanks Martin. Very sound. This is what I concocted for the f ratio myth. Comments welcome;

However, beware of the 'f ratio myth.' The galaxy (say) in a fast scope of aperture 'x' is only brighter than the same galaxy in a slow scope of the same aperture because it is smaller. Crop and enlarge it to the scale it would appear in the slow scope of aperture 'x' and you have gained nothing.

Olly

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