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Old 04-08-2019, 06:47 PM
Wavytone (Nick)
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Les the two I’ve looked through were both Mewlon 250 with thin vanes. As with Newtonians they are not the tool of choice for close double stars.
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Old 04-08-2019, 07:34 PM
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ngcles
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Hi Nick,

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Originally Posted by Wavytone View Post
Les the two I’ve looked through were both Mewlon 250 with thin vanes. As with Newtonians they are not the tool of choice for close double stars.
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 ?

I repeat my earlier specific question regarding the Mewlon's failure to split a pair with 0.6" separation, were you viewing with the Mewlon 210 or the 250? Can you supply the id of the pair you were observing?

Please note my earlier posted observation (above) of splitting Gamma Sextantis (0.54 arc seconds at the time) with 25cm Newtonian at x580:"Well split with a dark hairs breadth between. Clearly split 90 - 95% of the time. In PA 70, diffraction rings surrounding both components virtually stationary. Another faint star 1' away magnitude11 in PA 340. "


Best,

L.
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  #23  
Old 06-08-2019, 12:48 PM
ausastronomer (John Bambury)
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Quote:
Originally Posted by ngcles View Post
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.
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
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Old 06-08-2019, 08:38 PM
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Hi Bammo & All,

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Originally Posted by ausastronomer View Post
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
Amen!
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