Quote:
Originally Posted by ngcles
Hi Nick,
I concede an unobstructed telescope is best of all for this application but given nearly all telescopes sold or made (commercial or home-made bespoke telescopes) today do have a central obstruction (apart from refractors) , can you explain why a Newtonian is not a "tool of choice" for close double stars ?
Best,
L.
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Putting refractors and exotic reflecting designs like Off Axis Newtonians, Schiefs and the like, which are all unobstructed, but have other drawbacks aside, a standard Newtonian has to be the next best option over any of the compound telescope designs.
The laws of physics determine that this is so. It has less air to glass surfaces, less reflecting surfaces and a smaller central obstruction. All of these things introduce diffraction and light loss and the larger central obstruction also has a signifcant effect on the MTF curves.
If I had to pick something other than a refractor, to run a close 2nd to a Newtonian, it would be a Maksutov Newtonian. These basically have the same parameters as a Newtonian with the only additional being the front corrector plate. It still has a small central obstruction and no diagonal, unlike the other compound designs. I have previously done some observing in the US with an Intes MN-61 which was a 6"/F6 Mak Newt, that was also sold by Orion as the Argonaut on the Vixen GP-DX mount. This was just about as good as any 6" refractor.
Cheers
John B