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Scope+dslr question


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This may be a stupid question but, how does imaging with a dslr work?

(with reference to magnification.)

When viewing through a 'scope, eyepieces give magnification. When imaging DSOs, are images usually cropped to create a "zoomed" image?

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The telescope becomes the camera lens. If you are using a 80mm f/5 scope, you effectively are imaging with a 400mm f/5 lens. To get more magnification, you require to make your focal length longer (e.g use a barlow lens) which will also increase the f-stop you are using. Images can be cropped to show just the object you are imaging, but personally I think they can look better when given some surrounding context, but that is personal preference only.

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But the other thing is to maintain a fast focal ratio to concentrate enough light onto your DSLR for DS imaging. If you use a barlow you will slow down the f ratio maybe to the extent that DS imaging would be out of the question.

Rather than extending the focal length of their telescopes with Barlows, DS imagers almost invariably do the reverse, using focal reducers to speed up the system (or just to have a second choice on field of veiw, wider with the reducer.)

Planetary imagers do use Barlows and Powermates because 1) they want to get a good image scale on a small object and 2) they have enough light to play with on bright planets not to need a fast focal ratio in order to get an image.


Edited by ollypenrice
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