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I did think about the Hershelian but it needs to be quiet large and of long focal length not to need a corrector for off-axis observing.

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This seemed the best general thread. You get moaned at if you do a cross telescope type post in a specific thread.
Perhaps there needs to be a 'comparison' thread....Perhaps we should expect to pay more for some types of telescope to get the potential quality out of them?

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Out of morbid curiosity I hunted down the thread you're referring to...where it starts to say No one makes a "Premium" 6" Newt. Ah, yes. No one makes one, so you can't buy one, but since no one can buy them, it appears there's no market for one...which means no one will make one. Aside from making your own (which the refractor crowd almost never does) then there's no option to compete, nothing with fine tooled mechanicals to drool over, nothing with SOTA reflective optics in a minimal optical configuration that does that one planetary thing so well (and those who make their own planetary 'scopes know what I'm talking about). It's hard to say if this is a chicken-and-egg problem. The small slow newt market went away a long time ago. But if people are really interested in them it surely could come back...

For small mirrors (6-8") the production techniques differ, but that's not really a problem. What is a problem is that small mirrors never command much price in the market, and the time it takes to make them is similar to what it takes to make much larger optics, for which the price is higher. And you'd have to fight uphill all the way against the vast glut of off-shore entry-level optics, which realistically is the de facto standard of comparison at this time. Dismal prospect, that.

OTOH complete systems might find a small but ready market. Especially if they were, oh I don't know, Roddier tested to verify overall quality...

To answer a couple of the unexpressed issues and maybe a couple suggestions: You may (?) be looking for a large enough illuminated field for widefield Naglers in a longish FL 8". Your 2" secondary suggestion is overkill then, what you need is a minimal distance from the OTA center to the focal plane, not a big honking secondary for visual use. A sled focuser would accomplish all this and have the benefit of allowing various accessories to be plonked into the light path without calling for excessive focuser travel. An 8" f/8 can have no more than about a 1.5 degree true FOV (by my crude BOTE calculation) if it's feeding a 42mm diameter focal plane (the maximum entrance pupil for a 2" EP). For planetary alone this makes no difference, but the point is that full (100%) illumination of that 42mm focal plane, when it's located 5" from the optical axis, occurs with a minor axis secondary of 1.42", which leaves your 8" f/8 working at a secondary obstruction of less than 18%. This system will equal or beat any 6.5" unobstructed (be it APO dripping with exotic glasses, or unobstructed mirror design, or some combination thereof) in contrast transfer of planetary detail, though it will give up some ground on off-center abberations for highly corrected designs. OTOH it will have no spurious color and the center of the field will have zero aberrations.

Perhaps somebody could check the calculations (though they seem in the ballpark to me)?

OK I checked the calculations as something didn't sound right - brain malfunction! According to TeleVue the max field stop for a 2" is 46mm (not 42mm) and for a 1.25" it's 27mm. Reworking the above correctly I get a diagonal minor axis of 1.96" to FULLY illuminate a 2" EP, and a minor axis of 1.15" to do the same for a 1.25" EP. 1.96" (or 2") is thus correct for the stated application, if that's what wanted, and the obstruction would be 25% which is fairly high. For the 1.25" the obstruction would be (using an available 1.3" instead) 16%, quite reasonable. Running SEC I get the following illumination graphs. They don't agree with my calculation and since I've never actually used the program beyond looking at it I'll just have to leave that for others.

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Do you know of a supplier who can produce a 8" F8 mirror and 2" flat to a real 1/16th wave and get them dielectric coated? And how much would they cost.

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Then there is a Schupmann medial refractor which is made from two pieces of the inexpensive BK-7 crown glass, and is perfectly color corrected, much better then a APO. It also has a version with all spherical surfaces. Has very close to a perfect MTF curve for ultra high contrast. It can be diffraction limited for all colors over 1/2 degree field of view and of course is unobstructed.

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On my short-list, too. Can be made from refractive quartz if available (as it is now ).

Best,
Mark

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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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For planetary alone this makes no difference, but the point is that full (100%) illumination of that 42mm focal plane, when it's located 5" from the optical axis, occurs with a minor axis secondary of 1.42", which leaves your 8" f/8 working at a secondary obstruction of less than 18%. This system will equal or beat any 6.5" unobstructed (be it APO dripping with exotic glasses, or unobstructed mirror design, or some combination thereof) in contrast transfer of planetary detail, though it will give up some ground on off-center abberations for highly corrected designs. OTOH it will have no spurious color and the center of the field will have zero aberrations.

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Mark:

It would have been lovely to have your participation a week or so back on the "Aperture Rules" thread over in refractors. A bunch of APO fanciers claiming a 6-inch APO will outperform any reasonably sized reflector... In fact, touting their resolution advantage over 20-inch reflectors. I realize rational arguments make no difference under such circumstances. It still would have been fun.

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Fiske Miles
Nikon 8x42 LX / 12x50 SE Binos
Mini Borg 60ED, TV-101, AT80Ach, XT-8, C11/CI-700, 22-Inch Dob
Way too many Nagler eyepieces
http://www.fiskemiles.blogspot.com/
www.fiskemiles.com

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