by Scott Milligan, Springfield Telescope Makers
Page 1 of 7
The year 1984 marked my second attendance at Stellafane. As I strolled around the old camping field, I happened across two gentlemen who were busily engaged in setting up a rather strange looking wooden-tubed telescope. The were preparing to view Venus during the daylight. While waiting for my first-ever glimpse of Venus during the day, I engaged these two enthusiasts in conversation about their unusual telescope. "It's a single glass refractive apochromat," one said, "and it uses only spherical surfaces." If I would stick around, they assured me, I could see for myself that residual color aberration was essentially zero!
Those of you familiar with recent Stellafane history will recognize the telescope I have described as the six-inch Schupmann built by Mike Mattei of the Amateur Telescope Makers of Boston. The instrument had won first prize for optical excellence at the 1983 Stellafane convention. The second gentleman was Jim Daley, whose self-published booklet "Amateur Construction of Schupmann Medial Telescopes" had inspired and guided Mike's efforts. Working in collaboration with Optical Engineer Bert Willard, Jim developed and prototyped the design that would eventually inspire the efforts of the Springfield Telescope Makers to make the world's largest operational Schupmann Telescope.
The seed that would eventually become McGregor observatory was planted in June of 1987. That month, I decided it was time to strip and recoat the ten inch mirror that had served as my gateway to the stars for the past two years. After receiving some advice that suggested the use of a rather strong aqueous solution of sodium hydroxide to accomplish this task, I set up such a solution, and immersed "my precious." Within 5-10 minutes, most of the old coating was gone; a few stubborn spots, however, remained untouched. I then made a decision which, in hindsight, proved to be both pivotal and stupid. I decided to leave the mirror immersed overnight in the stripping solution. Arriving the next morning, I found the mirror to be completely free from aluminum deposits; unfortunately the polished optical surface had also been quite thoroughly etched, and was now totally useless as an objective.
Working at a determined pace to refigure the mirror in time for Stellafane, my thoughts drifted towards dreams of owning a large refractor. At that moment, the primary virtue of such an instrument was seen to be the absence of finicky mirror coatings; issues like central obstruction, tube currents, and coma free fields of view were minor considerations when compared against the possibility that I would never have to strip and recoat anymore #$%&$#@ mirrors! A problem with large refractors, however, was the presence of secondary spectrum. Although I knew observers who claimed not to be bothered by residual color, my own experience was that I found the presence of the bright violet halo to be an objectionable distraction. I considered the various options, including the use of special dispersion glass, and the various all reflective TCT (Tilted Component Telescope) designs. Use of special glass was attractive from a theoretical standpoint only, since the exorbitant cost coupled with the substantial risk of breakage in relatively inexperienced hands made the construction of a large instrument as much a test of financial courage as anything else. All reflective TCT's, while in vogue among advanced ATM's, had a reputation for being somewhat difficult to collimate, had an awkward appearance, (yes, appearance does count for something!), and most importantly, did not solve the freedom from mirrors problem. Only the Schupmann, with its unobstructed, primarily refractive, and yet color free optical system offered the possibility of satisfying most of my wish list. A Further advantage of the Schupmann concept were the incredibly detailed yet clearly explained instructions for building the design contained in Jim Daley's booklet. Given the depth and clarity of the information presented, I felt confident that I could successfully complete the construction of a large Schupmann Telescope.
Figure 1 shows an optical layout of the Daley-Willard all-spherical "Super-Schupmann." Light arriving from the heavens first encounters the main objective lens, labeled "L1" in the figure. The light then is focused to an intermediate image at the field mirror, labeled M1. The intermediate image is aberrated, containing both under-corrected spherical and chromatic aberrations.
The field mirror M1 has a weak concave curve, and reflects the incident radiation out to the corrector lens, L2. The corrector lens is a negative meniscus refractive component, which has a reflective coating applied to the rear (convex) surface. The negative element is made from the same (Bk7) glass as the main objective, preferably from the same melt, and the power and aperture of the negative element are chosen to correct the chromatic and spherical aberration introduced at L1.
The reflective coating creates a concave mirror, which serves to return the light back through the negative meniscus and on its way to a small fold mirror, M2, where the corrected image is directed towards an accessible focal plane. The field mirror M1, besides folding the optical path, also by virtue of its weak positive power serves to reimage the aperture stop at L1 onto the corrector L2. This allows for complete correction for lateral color, and improves the already superb correction of the axial chromatic aberrations by equalizing the ray height at which the various wavelengths arrive at the L2 corrector.
A "Lyot stop" is machined into the corrector lens cell; it is this aperture which determines the actual aperture of the telescope. Together with an adjustable field stop at M1, the Lyot stop creates the potential for a fully baffled optical train. The only possible unwanted rays are "straight shots" emanating directly from L1 to M2; these are blocked by baffles inside the tube.
To allow access to the final corrected image, this image must be laterally separated from the uncorrected intermediate image at M1. This is accomplished by introducing a slight tilt in the plane of Figure 1 at the L2 corrector. Introducing this tilt has the unfortunate consequence of introducing astigmatism into the final image. This astigmatism may be removed by several means; by far the simplest, and therefore most likely to succeed technique is to introduce an offsetting tilt at L1. This tilt must be orthogonal to the tilt introduced at L2 if it is to correct the astigmatism introduced by the tilt in L2.
A friendly word of advice for anyone wishing to set-up a ray trace of the design: whatever tilt you select for the field mirror, you will probably find, as I did, that it is necessary to add a minor "tweak" to the angle that defines the location of the L2 corrector after reflection at the field mirror. I found that optimum performance was achieved by tweaking both the x and y tilt angles here. Without the tweak, some lateral color remains on-axis. Remember also that the tilt of the objective lens must be introduced in a plane that is orthogonal to the L2 tilt, or else the objective tilt will add its own astigmatism to that already introduced by the tilt of L2.
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