My first Magazine Sky & Telescope - 01.2019

ASTRONOMER’S WORKBENCH by Jerry Oltion A Folded Serrurier Refractor Here’s a practical solution to long-focal-length refractors. REFRACTING TELESCOPES HAVE faced a fundamental design challenge since the beginning: The longer the focal ratio the less chromatic aberra- tion, but the more unwieldy the tele- scope becomes. Early refractors reached a ridiculous extreme with scopes like Johannes Hevelius’s 46-meter (150- foot) monster of the 1640s. Triplet lens designs that correct for color fringing have largely solved that problem, but triplets are expensive. Doublets do okay and are relatively inexpensive, but they still function bet- ter at longer focal ratios. French amateur Bertrand Remy has come up with an excellent solution that neatly bypasses the length compromise. Inspired by articles in the December 1984 and March 2001 issues of Sky & Telescope, he designed a combination refractor/reﬂector that folds a long-focal- q The objective lens, tertiary mirror, and focuser all nestle into the front box. 72 JA N UA RY 2 019 • SK Y & TELESCOPE ratio (f/15) light cone into an easily manageable package. He started with a two- element, air-spaced 150-mm (6-inch) f/15 Clavé achromat. A little less than halfway down its light cone he positioned a 4-inch Edmund Optics 1 / 20 -wave silica mirror, which bounces the light back toward the objective, but off to the side where a 66-mm (2.6 inch) 1 / 30 -wave Antares diagonal mirror reﬂects the light through a Moonlite focuser to the eyepiece. To support the secondary, he chose an eight-truss Serrurier design that holds the secondary perpendicular to the light path even if the trusses sag a bit under its weight. This little bit of magic is achieved by using eight trusses instead of six. There’s a triangle on each side, making it very stiff in all directions, but even if the trusses do ﬂex a little, their length won’t change. The entire structure acts as a paral- lelogram, and we all remember from basic geometry (right?) that when you ﬂex a parallelogram, the opposite sides remain parallel to one another. In Bertrand’s case, the objective lens, the tertiary mirror, and their mount- ing box make up one end of the paral- lelogram, while the secondary mirror makes up the other. If the secondary end of the telescope sags a bit, it’s still held parallel to the objective. And since the secondary is ﬂat, ﬂexure-induced collimation change is not an issue. The result is a dramatic reduction in the scope’s overall length while actu- ally reducing the worry of tube ﬂexure affecting the collimation. The ﬁnished scope folds a 2,250-mm (7.4-ft) focal Bertrand Remy and his folded Serrurier refractor length into an impressively short 1,090- mm (3.6-ft) package. This design has several additional advantages: It’s light. The whole works weighs only 8 kilos (18 lbs) including the dovetail plate. The scope is compact, so it’s easy to transport and easy to place on its Los- mandy G-11 mount. The damping time is only half a second. This extremely low ﬁgure is undoubtedly due to the Serrurier trusses. The image is sharp, even at magni- ﬁcations above 300×. Bertrand reports that color fringing is minimal, even on the bright Moon. It’s easy to collimate. Bertrand says, “Thanks to the large coma-free ﬁeld of the objec- tive, collimation is very forgiv- ing. I only had to do it once to get crisp images, simply viewing u The secondary is actually the i rst mirror in the light path but the second optical element.

My first Magazine Sky & Telescope - 01.2019 | Page 74