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Reflectors & 35mm Prime Focus AP

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#1 ???


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Posted 31 July 2020 - 04:48 PM

In the textbook Astrophotography for the Amateur, author Michael Covington states that Newtonians (Reflectors) normally only have 1-2cm of back focus, which would be insufficient for 35mm film (or full-frame digital) astrophotography. The work-around that the author suggests for this problem is modifying the telescope to bring the primary mirror forwards about 5cm. From what I've read online, it seems this involves either substituting the screws holding the mirror for longer ones, and/or cutting the tube itself. 


I'm interested in Reflectors because of their relatively fast focal ratios and modest pricing. They also seem to be one of the more often recommended types of telescopes for imaging, which makes it all the more surprising to me that they are generally unusable for prime astrophotography until they have been modified. My copy of this book is the revised edition published in 1991. Has anything changed in the design of reflectors so that they are better suited to full-frame imaging without modification? Are there any Reflectors available that are preconfigured for full-frame prime focus astrophotography? 


#2 jessebear


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Posted 31 July 2020 - 04:56 PM

There are many astrograph reflectors designed for photography currently on the market. There is also the option to install a low profile focuser in a reflector, which may get you the focus you need without any substantial modifications (though this will depend on your equipment). Another option is to use a Barlow lens, which will often move the focal plane back far enough to allow use without modification (though this option is far from ideal).
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#3 Todd N

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Posted 31 July 2020 - 05:29 PM

As the previous poster stated, many newtonians are astrophotography ready so no modification is needed. This was more so a common problem from yesteryear. I have a 6" f/6 TPO and their entire line will accommodate a D/SLR with coma corrector but not off axis guider. If the scope is f/4 to f/6 be sure to buy a coma corrector and a laser collimator. You'll have to collimate everytime you set up with newts but the laser makes it a cinch.


Scopes like TPO and others usually have a 2" crayford focuser that will cover 35mm full-frame. Faster scopes like the smaller 6" f/4 varieties will not be as sharp and uniform across the full-frame even with a CC especially if there is light pollution, illumination is not uniform- vignetting. A longer focal length  F/6 scope is way better in this regard.


There is also some new super fast newt astrographs I can't recall at the moment. 6" & 8" f/2.8 with built in correctors good for 44mm image circle but again I would probably not use this scope where there is significant light pollution due to uneven illumination.

Edited by Todd N, 31 July 2020 - 05:37 PM.

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