Quote:
Originally Posted by silv
the f/5 is good for astrophotography.
But whether the tube was "designed" for AP - which criteria would tell me that?
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OK, to explain a little further. And the astrophotographers can correct any mistakes I make.
When you fit a camera into a focusser, the image has to be formed some distance above the focusser to be correctly on the film (in the good old days) on at the sensor (CCD). When you put an eyepiece in, the image needs to be formed much lower, inside the focusser body. So the first difference is to have the tube and focusser mechanically designed to place the image where it is needed. Using a visual designed scope for photography is difficult - it usually needs mechanical adjustments to get the camera closer to the primary mirror. There are various ways: get the primary mirror higher in the tube on long mounting screws; saw some of the length off the tube and rebuild it; change the focusser to a lower profile focusser.. Using an imaging tube for visual is fairly easy - insert an extension tube in the focusser to lift the eyepiece above the focusser to where the image is being formed.
There might be some other differences in the size of the secondary mirror, but the above mechanical length is the main difference.
The typical visual 8" scope is an f6. An 8" f5 or f4 scope always suggests it might be designed as an imaging scope. Not always the case, but worth checking.
So you really have to see where that image is being formed with respect to your focusser so you can then decide how to proceed.
Cheers
Eric