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Wild Card 006.1 "The Great Free Blanks Contest, PLUS!"

Posted by Rick Shaffer   08/10/2004 00:00:AM

Wild Card 006.1 "The Great Free Blanks Contest, PLUS!"
Last week I did a brief survey of unobstructed telescopes. This week, I’ll recap that, present a version of a relatively new unobstructed telescope, and give you the details of the contest I’m having to give away the glass necessary to make one. So, on with the show:

Here’s a restatement or recapitulation of what I’ve been presenting regarding telescopes that provide high contrast viewing.

CATEGORY01: You want a very high-contrast, color-free view of celestial objects, you want great compactness to allow for great portability, including the mount and tripod, you don’t want to assemble your telescope OTA when you get to your observing site, and you want the capability to use extremely low power to obtain a wide field. Price is a secondary consideration.

To my mind, the only telescope that satisfies these requirements is the premium apochromatic refractor. I doubt if I need to outline which manufacturers make these fine telescopes. The forums are ablaze with discussions of the relative merits of same. (And those made by several manufactures have many merits and only one demerit: price.)

CATEGORY02: You want a very high-contrast, color-free view, but you’re not too fussy about compactness or having to assemble your OTA from parts. You’d also accept a moderately wide field. You’d like the price to be less than that of the equivalent-aperture apo. There are several options:

An off-axis section of a larger paraboloid leaves little to be desired in image quality. Several manufacturers offer either the optics for such a telescope or the completed telescope. I haven’t used the current crop of telescopes, but assembled four of them in the 70s for others. Those were exquisite. Recent forum posts and reviews on other sites have indicated that the owners of the current crop of these instruments are extremely satisfied with them. (Note that one manufacturer offers a 9” f/10.5 off-axis paraboloid. How good a planetary telescope would you suppose THAT would make?!)

Another good choice is any one of the Schiefspiegler or Tri-Schiefspiegler telescopes. There was a 5” available from a manufacturer in Europe some years ago. I looked through one, and could find nothing to criticize. You might do a search to see who’s selling either telescopes or optics for them.

CATEGORY03: This is really CATEGORY02, except that you’ve completely removed any notion of compactness from the equation. There are several possibilities:

As I suggested a while back, an achromatic refractor telescope with an extremely long focal length is “sensibly apochromatic” if the size of the unfocused red and blue image of a star is no larger than the size of the Airy Disk when the telescope is focused for green light. Such a telescope would be very long. A 6” f/28 folded in half would be 7-feet long, but would perform extremely well over a rather modest field of view.

Folding a reflector that has a single long-focus spherical mirror in half is also an option to consider. A telescope with a 6” f/30 spherical mirror tilted just a bit and folded in half will have its eyepiece at the end of the tube near the ground. It’d be a big telescope, about 8-feet long, but it would also be the least-expensive high-quality, unobstructed telescope you could build.

Finally, you can build a telescope called a Stevick-Paul Telescope, which is a relatively new unobstructed reflector. It would find itself in either of the last two categories, depending on you definition of the word “compact”.

So, there’s the recap. I’d suggest, however, that a well-built Newtonian with a relatively long focal length should remain an option, IF you’re willing to sacrifice a large field of view. I’ve looked through several such telescopes, and their performance leaves little to be desired. I’d also suggest that careful design of a long-focus Cassegrain telescope can yield supurb planetary images. I’ll be discussing this option in an upcoming edition of “Wild Card”.


If you agree to build the telescope I’m about to describe, I’ll give you the blanks you need to do it. The catch? There are four: (1) You gotta pay the shipping, which won’t be very much, but will serve as some sort of indication that you’re serious. (2) You’ll agree to make the telescope within one-year of receiving the blanks. (Don’t worry. I won’t send the “blank-police” after you if you’re not finished in time!) (3) You’ll properly credit the design of this telescope to David Stevick. (What I’ve done is a mere variation of the design he put forward in the early 90s.) (4) That you’ll read the following carefully.


David Stevick is a very clever amateur optical designer who lives in Wheeling, WV. In the early 90s, he was perfecting an optical analysis program he wanted to use to evaluate unobstructed reflector telescopes. (It's now called "WinSpot", and you can just google on that to find out where to get it FREE!) To test the software, he was basically “fooling around” with several multiple element reflectors. One configuration featured a tilted paraboloidal primary, a convex spherical secondary, and concave spherical tertiary. He found, purely by accident, that spherical aberration, coma, and astigmatism all would vanish when the three mirrors were in a particular configuration:

ONE: Their radii were the same,
TWO: The secondary had the same focal point as the primary
THREE: The separation of the two “corrector” mirrors was equal to their radii of curvature.
FOUR: The tilts of mirrors were “just right”.

Stevick consulted Dick Buchroeder about what he’d found. Dick realized that David had discovered a variation of a telescope originally invented by the French optical designer Maurice Paul in the 1930s. (Paul was trying to find an all-reflective “add-on” system of correctors that could turn an existing large reflector into a wide-field telescope. Alas, it appears that such a telescope has never been built. The proposed “Large Synoptic Survey Telescope” is to have a catadioptric version of a Paul Telescope, but funding for that 7-m wide-field telescope appears not to be forthcoming.)

Buchroeder also remembered that Dr. James G. Baker of Harvard-SAO had rediscovered the Paul Telescope in the 1940s and had made a modest improvement to it by aspherizing the tertiary.

Stevick contacted Dr. Baker, who realized that David had really come up with an important new eccentric-pupil-telescope. Dr. Paul graciously suggested to David Stevick call what he had discovered the “Stevick-Paul Telescope”. I heartily agree with Dr. Baker’s suggestion, because, although it’s not new from some sort of an absolute perspective, it is an important advance in what we amateurs call “off-axis” telescopes.

David Stevick published the original article about the Stevick-Paul Telescope in “ATM Journal #3”. He also has a web page that shows how to lay out an SPT with the radii of the two correctors being 40% of the radius of curvature of the primary. This design is relatively conservative, and yields a fine telescope. The Missouri amateur A. L. Woods has made a 6” f/8 SPT about which I’ve heard nothing but glowing reports. It’s tube length is 80% that of an equivalent Newtonian. The included illustration will show the layout of a similar telescope SEE THE 40% VERSION BELOW

I noticed that an SPT could be made a little more compact than David Stevick’s original design by choosing the radii of the correctors to be exactly 1/3 of the radius of the primary. This allows the separation between the correctors to be 2/3 the focal-length of the primary, so and f/12 SPT could be only “f/8-long”.

As David Stevick has indicated to me in an email, the design option I chose leads to the focal plane being tilted with respect to the optical axis more than with the “40%-design”. That’s true. However, I was looking for a design that could allow the primary mirror to be left spherical. That requires that the primary be of a relatively long focal length, which minimizes the tilt angles so, the field won’t be tilted to much for the eyepiece to hand it.

I’ve come up with a design that features a 4.25” f/16 spherical mirror., so it has a focal length of 68”. But the tube length will be about 48”, so it’ll be about as compact as the typical 6” f/8 Newtonian. (The secondary and tertiary are also spherical, and can be used as the mirror/tool and tool/mirror by grinding one on the other. Once the tertiary is perfectly figured as a sphere, the secondary can be interference-tested against it.)

This telescope won’t have the large field of an apo. Nor will it even have the field of a general purpose Newtonian of that aperture. But, it will have an unobstructed field that has NO coma, NO astigmatism, and only about 1/25-wave of spherical aberration. If made correctly, this Stevick-Paul Telescope will have a perfectly corrected field of about 0.3-degrees, perfect for lunar, planetary, and double-star viewing. SEE THE "Contest" VERSION BELOW

The secondary and tertiary can be made with fairly small blanks, but standard 4.25” blanks will work just fine . You’d just mask off the unused portion of each finished optic. And, since you’d be using the center or the optics, you could reasonably ignore a turned edge, because it would be masked off.

I’ll send you three 4.25” Pyrex blanks if you’re the first person to send me a message at the AstroMart site. My userID is “rickshaf”. Your message must also convince me that you’ll comply with the requirements I outlined above about getting the telescope done and properly crediting David Stevick with the design.

HERE’S ANOTHER CATCH: I also have a 6” f/18 design. If you have your own 6” blank, or you want to buy one elsewhere, and you otherwise comply with the requirements above, I’ll send you two 4.25” blanks and the design specs for the larger scope. It would be about 75” long, and would leave little to be desired as a planetary scope.

One thing I haven’t mentioned earlier is that this design allows you to bring the light out the side of the tube, instead of out the top as the screenshots show. This would allow you to make the axis of the focuser be coincident with the altitude axis of an altazimuth mount. If you placed that axis about 42” above the ground, you’d have an eyepiece that’s at the height of most people sitting in a chair. And, the eyepiece wouldn’t change in height as the telescope changes in altitude.

I’m making an 11” f/11.4 variation on the Stevick-Paul Telescope with my friend Rob Schottland. It will break apart into two pieces like a piece of (large!) luggage. But I’d like the simpler version I’ve outlined above to be made as well. That’s why I’ve decided to hold this (“non-Mega-“) Contest. I suggest that, if you’re the winner, you’ll save a few bucks on glass, help me dispose of some of my “astro-debris”, and end up with a fine unobstructed telescope in the bargain!

NEXT WEEK: “About Extra-Ordinary Claims Made by Observers”

RICK SHAFFER is an astronomer, writer, teacher, and designer/builder of telescopes who lives in Sedona, Arizona, a place where the thoughts of at least some of its residents are unobstructed by logic….

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