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Is afocal Marsenne configuration possible with two concave parabolas?

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#1 hamishbarker

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Posted 17 October 2019 - 09:46 PM

Having read about a large amateur marsenne telescope on the Televue website (parabolic primary, and convex parabolic secondary, feeding a refractor via the altitude axis), I was wondering if any optical designers could comment about the possibility of using a concave parabolic secondary instead, but otherwise similar arrangement. Obviously the secondary would be further from the primary by 2x secondary focal lengths.  

 

Or does the afocal marsenne configuration only work for convex secondary? What is the main aberration?



#2 JamesMStephens

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Posted 17 October 2019 - 10:43 PM

Yes, the focal point of the concave paraboloidal (not parabolic!) secondary would coincide with the focal point of the primary.  The separation between the two mirrors would be the sum of the two focal lengths.  I don't know how the off axis aberrations would compare with those of a Mersenne.  I suspect you'd have a small field of view, not because of aberrations but just geometry.

 

Jim


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#3 luxo II

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Posted 18 October 2019 - 05:12 AM

If you put a diagonal in there near the focus of the primary then yes it’s possible and I’ve seen this referred to as a “hammerhead” configuration.

Edited by luxo II, 18 October 2019 - 05:12 AM.


#4 Vla

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Posted 18 October 2019 - 07:31 AM

The only aberration for the two mirrors is field curvature, stronger than in the Cass configuration, since the two mirrors add up. But since the effective aperture stop is at the secondary, it is also farther away from the imaging objective, which means more vignetting and more aberrations on that end.



#5 hamishbarker

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Posted 20 October 2019 - 10:38 PM

If you put a diagonal in there near the focus of the primary then yes it’s possible and I’ve seen this referred to as a “hammerhead” configuration.

I'm not sure that I understand. Do you mean a tiny diagonal, sending the expanding beam out to the side, to the second paraboloid, then the afocal beam coming back across to the imaging objective (refractor or whatever) on the other side?

 

What advantage over a normal newtonian configuration? Smaller diagonal? But it doesn't get the advantage of nice low eyepiece (unless a second diagonal sent the afocal beam down towards ground for the refractor etc.) which seems to be the attractive feature of the mersenne.




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