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But the dielectric on the primary would not be wise, as the stress is quite high - to ensure the maintenance of the figure the primary would be much thicker than would be ideal. A high-quality enhanced coating won't stress the primary and will work quite well.

Best,
Mark

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Mark
That is the first explanation along such lines as to why dielectrics are not used on primaries...I have always wondered about this, and have never been entirely satisfied with the "if you make a mistake they are to hard to strip off" explanation, although this is probably true as well.
Also glad to hear you affirm the qualities of the on-axis performance of the paraboloid. I always wonder why certain implicit notions to the contrary don't go unchallenged.
Consider this one: It needs to be a Mak-newt to compete (on-axis) with an apo.
Why should this be so?
1) For a given focal ratio/field illumination the central obstruction is the same.
2) Yes, spherical surfaces are easier to make – but equally good aspheres are being made
by the likes of a couple of guys named Mark, and a few others.
3) Sure, there are no “visible” diffraction spikes – but this is a non-MTF-altering cosmetic issue.
4) And getting back to the spirit of this thread – two more optical surfaces, and a closed tube to boot.
Now none of this is to suggest that MNs are not outstanding planetary scopes – they are. But they should be no better on-axis than a simple newt of equal aperture/f-ratio, and equal wavefront error.
And then there are those Tak Mewlons – don’t get me started...
Joe

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Joseph Cannavo
16" F5 Zambuto, front collimating, scope nearly complete - WSP here we come!
16" F5 Royce, conical blank, front collimating (Mirror Sold)
10" F5 Dob
Mid 70's RV-6
4" Orion 100mm ED
Mr Keeyoots (My Cat)

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