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Curved-Vane Secondary Mount
Gary Seronik shows an innovative and simple curved-vane secondary mirror holder on his website. I had encountered these often in the past, but they always seemed daunting to contemplate. Two things, however, gave me pause in considering his design.
The first issue was that my secondary is only 1.3 in., and I was considering an even smaller one. The design of Gary's spider hub, with its crossing bolt-holes, left a very thin segment beneath the head of the pivot bolt. If the whole assembly is to lie within the shadow of the secondary, then there is simply not much space left after recessing this hole. Constructing this of wood, even hardwood, left me very nervous about its strength in this area. He shows a simplified version, used on his StarBlast Travelscope, which would eliminate this issue; however, I felt that it would be very difficult for me to adjust properly to the center of the optical path.
The other issue was that which I just mentioned, bending the vane to properly align it in the optical path. This is what a friend calls, "main strength and awkwardness," but on a smaller scale. Do-able, but difficult. I wanted something equally simple, but hopefully resolving these difficulties.
I eventually came up with the variation shown above and in the photos (1, 2 & 3) below. The mounting brackets are made of 1⁄8" aluminum stock instead of wood. This allows me to tap into the metal and reduce the profile, while enhancing the strength and reliability of the unit. The secondary holder could have been made of wood, as is Gary's, and it undoubtedly would have been easier to do so. But since I was making the spider hub of aluminum, I just went ahead and used it for both. I feared that this thick aluminum might crack upon bending, but it worked nicely. I used a 10-32 brass bolt and tapped all holes to match. The knurled brass nuts are also 10-32, available at Lowe's hardware, and probably many other places as well. One could also use simple wing-nuts, but these only seem to come in 10-24, and I wanted the finer thread, both to give me a finer adjustment for the mirror, and also to provide more threads in the 1⁄8" sheet. Similarly, the pivot bolt is 10-32 also.
The vane is constructed as Gary's, from an 18" stainless rule. My telescope is a 6", with a 6-7⁄8" I.D. tube; this determines the length of the metal for the vane. My design being somewhat different than Gary's required a longer rule. I knew there would be some "springback" from bending the rule, but it was more than I expected. I first tried bending it around cardboard tubes, but it quickly became apparent that to obtain the approximately 3" radius curve I needed would require a tube significantly less than the 4" diameter I was using. Also, it was going to be extremely difficult to obtain a uniform curve, bending it by hand, without multiple "kinks" along the curve.
To solve this, I constructed a bending jig (photo 4), much like a wire bending jig, only larger, using a scrap board and oak doweling I had. Aligning the pegs along an arc of the required radius allowed me to obtain a uniform curve by forcing the rule between them. I did this mostly by trial-and-error, at first over-estimating the radius. I think the final radius was just over 1" to produce my 3" radius curve, after springback. I made this a little less than the tube curvature, so that it would press snugly against the inside of the tube when attached at the two ends. If I had taken pictures during the process, it would be much clearer.
The mounting holes in the vane are elongated to allow for some latitude in drilling the tube, and slight adjustment if necessary (photos 5 & 6). Note that the center section of the vane attaches to the inside of the tube, and the two ends bend upward to join at the center. The pivot bolt attaches the holder through elongated holes in the two ends of the vane, which then also allows for adjusting the "meeting point" of the two ends. By allowing the two ends to slide against one another, the length of the arc may be increased or decreased. Thus the holder may be moved transverse to the tube axis both horizontally and vertically, as well as rotated. You will need to draw this out full-scale before forming, to determine the actual dimensions for your tube. The vane is simply an arc, passing just shy of the tube center, and where it intersects the tube determines the length of its center section.
I painted the vane matte black, and darkened the remaining parts with Birchwood Casey® Aluminum™ Black. While made for aluminum, it will also darken the brass. Use it sparingly on the brass, it makes a thick black oxide which then mostly wipes off. The surface then is not black, but it does not shine. I attached the mirror with 3M™ clear VHB™ tape 4910. Because of the way the holder is formed, it was difficult to figure out how to get a uniform spacing using silicone. The tape is 1 mm thick, which works out well to provide a flexible bond. If you use the VHB™, be certain to follow the directions to achieve satisfactory results.
Finally, it quickly became apparent to me that, although the spider was easily adjusted, it was not so easy to get it centered in the optical path. I cut a cardboard sector, using a hole punch at the very center, and taped it to the inside of the tube (photo 7). To center the holder, simply loosen the pivot bolt and adjust the vane to put the center bolt exactly into the center hole of the sector, then tighten the pivot bolt (photo 8). From there, it is a simple matter to align the mirror. (photos 9 & 10)
1) Secondary mount in final fitting before blackening and adding mirror.
2) Hub is turned sideways to show complete assembly.
3) View from below showing vane centering adjustment.
4) Metal vane forming jig — insert one end and push through.
5) Finished parts before assembly.
6) Finished parts before assembly, showing reverse side of vane.
7) Cardboard sector alignment tool, taped into place.
8) Inside view of alignment sector, with center bolt in adjustment.
9) Completed & adjusted mount with mirror attached. Safety string attached to mirror with silicone.
10) Finished assembly, looking down tube at primary. Vane is painted, all other parts darkened with Aluminum Black.
Photos Copyright © 2019 Karl Swartz
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