Telescope Reviews: Re: The 'Best' telescope and eyepieces

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.