The Jones medial refractor: this optical design builds on the strengths of several previous ones.
Telescope making is a hobby with many facets. Some people enjoy assembling scopes with commercial parts, whereas others want to make everything from scratch. And there are also the adventurous ones such as Ed Jones of Ohio who enjoy creating new designs.
I featured Ed's Chiefspiegler reflector in the November 2008 issue, page 87. His newest eort is a high-performance design that he calls the Jones Medial Refractor. It builds on some of his previous scopes, including the Chiefspiegler, and it also shares DNA with exotic designs such as the Schupmann and Honders/ Busack Medial refractors. "When developing this design, " Ed says, "I started with the Honders/Busack Medial, but I tilted the lenses to avoid an obstruction, and I used Chiefspiegler-type correctors. "
Ed's scope has a single-element, 7-inch objective (made from BK7 or K5 optical glass). The next element in the optical path is a 6.66-inch diameter Mangin corrector -- a lens whose back surface is aluminized (or chemically silvered in Ed's case) to reflect light back through the lens a second time. As such, it functions as both a lens and a mirror. The Mangin corrects for the chromatic aberration of the single-element objective.
In Ed's design, the Mangin is tilted to divert the light cone toward the side of the tube, thus avoiding the need for an obstructing secondary mirror. But, as is typical in the optical-design game, the solution to one problem often creates a new one. In this case, the new problem is a tilted field. "To correct for this, I changed the folding element from a 2.19-inch flat mirror to one with a very weak convex curve,"Ed says. "The result is a telescope that is highly corrected, like the Honders, but unobstructed."Light from the convex mirror is fed through a 2-inch-diameter plano-convex field lens that corrects for lateral color error and astigmatism. This is an o-the-shelf component (part number 011-4660) from OptoSigma (optosigma.com)
Ed's scope has five optical surfaces (excluding the field lens), each of which must be generated, polished, and tested. That sounds like a lot of work, but all the surfaces are spherical, which makes the task considerably easier. The extra eort does, however, pay o. The resulting telescope is compact, features a convenient eyepiece position, and most importantly is a superb performer. "The scope works very well at high magnification with no hint of color error," Ed reports. "It puts a lot of dark sky between all four of the components in Lyra's famed Double Double. I also enjoy using a low-power eyepiece and seeing perfect star points all the way out to the edge of the field."
Ed's 7-inch is just one possible variant of the design; his recipe can be modified to produce other examples that retain its excellent performance. For instance, Ed says that with some changes to the focal ratio and the spacing of elements, the design can be scaled up to a 40-inch scope and still be diffraction limited across the entire visible spectrum!
Readers interested in exploring the Jones Medial Refractor further can obtain detailed information by e-mailing Ed at firstname.lastname@example.org.
Gary Seronik Telescope Workshop
Contributing editor Gary Seronik is an experienced telescope maker and observer. He can be contacted through his website, www. garyseronik.com.
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|Publication:||Sky & Telescope|
|Date:||Mar 1, 2014|
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