My first Magazine Sky & Telescope - 04.2019

ASTRONOMER’S WORKBENCH by Jerry Oltion Frank’s Four-Mirror Binoscopes the ﬁ rst mention I can ﬁ nd of it is in Albert G. Ingalls’s 1932 book Amateur Telescope Making — but for some reason the idea has never really caught on. It should have, though, because the design is simplicity itself: Simply lay two Newtonian scopes Frank shows off “Popeye” side by side and at the Oregon Star Party offset them both telescope walkabout. horizontally and vertically so the light path of one crosses through the IT’S COMMON KNOWLEDGE in the light path of the other. When the scopes ATM world that a reﬂ ecting binocular are oriented properly, their light cones telescope requires six mirrors. You need shine directly into an observer’s eyes the primaries to gather light and bring without any further ado. it to a focus, you need secondaries to It’s that “oriented properly” part that bounce that light off to the side, and probably scares off a lot of people, but you need tertiaries to make the two compared to collimating six mirrors on light cones parallel again and aim them a more traditional binocular scope, it’s into your eyes. Like most common knowledge, that’s not even in the same league. There are a couple of ways to do it: not necessarily so. There’s a way to You can strap two separate OTAs side by make a binocular scope with just four side, or you can build both OTAs into a mirrors, and Oregon ATM Frank Szcz- single framework. Frank has gone with epanski has become a master at it. the second method. The idea has been around a while — Secondary mirrors Eyepieces 72 A PR I L 2 019 • SK Y & TELESCOPE Primary mirrors tq The light cone from the far OTA crosses through the near one, eliminat- ing the need for tertiary mirrors. One of the ﬁ rst things you notice when you look down the front of one of Frank’s binoscopes is that the secondary mirrors are different sizes. The one in the OTA that’s farther from the eyepiece must be larger than the nearer one, since it’s intercepting the light cone several inches more distant from the eyepiece. That creates a greater obstruc- tion, but the effect is barely noticeable to the eye. The longer distance between eyepiece and secondary on the more distant OTA means that the primary must be mounted farther forward. Frank puts each primary on its own bulkhead. His scopes are designed to come apart for transport, so there are four bulkheads, all cut and drilled as a single sandwich so everything is perfectly aligned. By doing that and being very careful about centering each optical element, Frank doesn’t need any fancy mounting for the primaries. They have the traditional tilt capability for collimation; no side- to-side adjustments are necessary. To ﬁ ne-adjust image merging, he tilts just one of the primaries with extended col- limation screws that he can reach from the eyepiece. People’s eyes vary in separation. To adjust the interpupillary distance, Frank mounts the secondary mirror of the nearer OTA on a sliding plate that also contains the focuser. The entire works slides up and down the tube, mov- ing the left eyepiece closer to or away from the right one. Once the correct spacing is achieved, the left eyepiece is refocused. Fr ank’s latest binoscope, a 10-inch f/5, uses 2″ reverse-Crayford focusers, but other designs simply use snug tubes, and you slide the eyepieces REGG Tertiaries? Who needs tertiaries?

My first Magazine Sky & Telescope - 04.2019 | Page 74