by Maryann Arrien, August, 1995
On July 15, 1995 the Springfield Telescope Makers of Springfield, Vermont announced the completion of a 13-inch f/10 Schupmann medial telescope. It is a refracting telescope which uses spherical optical components to produce a high contrast image free of primary and secondary chromatic aberration. The "Super Schupmann" design was first discovered in 1969 by Cliff Ashcraft. By using a precise ratio of diameters of the objective lens to corrector lens, specifically the ratio 1.83, a diffraction limited, coma and color free image can be achieved without resorting to aspheric components. It is for this reason, Jim Daley explains in his publication Amateur Construction Of Medial Telescopes, that this type of design should be considered by advanced amateur telescope makers. According to the calculations of Daley's collaborator Berton Willard, an all-spherical 13-inch telescope with a 7-inch corrector could be successfully fabricated by members of the Springfield Telescope Makers to yield the largest operating Schupmann in the world.
The Schupmann type refractor has been a long neglected design, partially because large reflecting telescopes with parabolic mirrors took the fore. Only those interested in planetary observations considered building the more difficult refractors. Because they have no central obstruction in the form of a secondary, they do better to reveal low contrast detail on the surface of the planets. Due to the influence of Rev. Edwin Olson in recent years (1964-1971) there was a resurgence of interest in the Schupmann refractor embodied by "The Schupmann Club". In the early 1980's a 6-inch Schupmann medial telescope made by Mike Mattei of the Boston ATM's took first place in optical excellence at the annual Stellafane Convention in Springfield, Vermont.
A few years later Scott Milligan of the Springfield Telescope Makers, after having lost a mirror while stripping a failed coating, read Jim Daley's book on amateur Schupmann medials. He had come to dream of building a refractor, and was struck by the elegant simplicity of the all-spherical, diffraction limited design. With the help of his friend, optician Philip Rounseville, he worked on weekends and in his spare time for a period of 8 years to complete the optics for the 13-inch Schupmann. With the donated facilities of OSTI (Optical Systems and Technology) and the encouragement of their employer Mr. Harry Vandermeer , Scott and Philip were able to obtain the BK-7 glass blanks from Schott Glass as a donation to start the project. The ATM's of Boston also contributed $3,000 for the tube assembly.
Both the objective and the corrector are preferably manufactured from the same "pour" of BK-7 glass. Because of the "in glass" reflection, the so called Mangin corrector must have a surface accuracy of 1/20 wave. Chief optician Philip Rounseville nonetheless fabricated the seven inch corrector to within 1/30 wave of smoothness. Working in his basement in Hubbardston, MA, chief optical designer Scott Milligan completed the testing of the telescope on an improvised vibration isolation table made of wood and inner tubes. It was balanced atop steel drums weighted with sand. Once the optical adjustments were made in both monochromatic and white light, the optics were temporarily removed for the telescope's ride to Vermont in the back of a pickup truck.
The light from the 13-inch objective lens first forms an image on a tilted component field mirror. It is then relayed to the rear-surface silvered negative corrector lens. It is here that axial color is removed. After leaving this tilted "Mangin" mirror light is re-imaged and relayed to the eyepiece by a simple fold mirror which effectively sends the image outside the tube for viewing through a conventional eyepiece. The 7-inch negative refractive corrector simultaneously cancels out the positive color and the spherical aberration of the 13-inch primary lens. The result is a bright, high contrast image which suffers from no central obstructions. The tilted component refractor / reflector design has the advantages of an unobstructed refractor, without resorting to the use of $30,000 of FK glass that would be required for making an apochromatic triplet. The cost of the BK-7 glass used in the Schupmann is more of the order of $1,000 which makes the design more attractive in terms of cost effective aperture.
Unlike a catadioptric such as a Schmidt Cassegrain, the Schupmann uses refractive components for primary imaging, and reflective mirrors as relay elements. The Schupmann has a narrow field design which is primarily intended for visual observation at high magnification or with CCD imaging where correction is even better at the red end of the spectrum. The field mirror can be adjusted to remove lateral color effect introduced by the atmosphere dispersion near the horizon. Conversely, the field mirror can be used to add lateral color to observe the spectrum of a star.
This 13-inch f/10 Schupmann medial is housed at the newly built McGregor Observatory located on the grounds of the Stellafane convention in Springfield, Vermont. The observatory was dedicated to Springfield Telescope Maker Douglas McGregor (1951-1988). McGregor was the Master of Ceremonies at the Stellafane conventions for many years before his untimely death in a car accident. His fellow club members constructed the massive observatory in his name to honor him for his love for telescope making and observational astronomy. At the dedication, observatory designer and builder John Martin commented on how dearly Doug loved Stellafane. He was a "true observer" who "really loved the sky".This past July, the Schupmann telescope was in use during the convention to observe the giant planets Jupiter and Saturn. McGregor's longtime friend George Scotten remarked "Doug would be...proud to bursting about this telescope." Seventy years after the founding of the amateur telescope making movement in the United States, the amateur spirit survives.
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