ATM Folded 6" f/15 Refractor (IStar Lens)

Built by Tom Gideon of Langley, WA

After years of using various Schmidt-Cassegrain telescopes, I decided that I would build a refractor. I decided on a classic achromatic refractor and I wanted to experiment with building a folded refractor. My initial design was influenced by the classic look of the Unitron 131C. I ordered an Istar 6" f/15 lens, along with a used 4-inch 1/20-wave flat, and a commercial 70mm 1/10-wave elliptical secondary for the folding of the light path.

I started construction in early 2015. This design was built using a 10" diameter aluminum tube, with aluminum baffles, ½-inch aluminum rods as stringers, and had ¼-inch aluminum end plates. I found that putting all of this together was much like the proverbial 'ship in a bottle'. Alignment and collimating was very difficult and awkward. I also determined that the 26 pound weight was just too much for the mount I had planned to use, even though I had thought that the folded length of 34 inches would mean less weight as well.

Back to the drawing board!

I redesigned the tube, and chose the Istar 'Asteria' open tube design as a model. I used 1 x 1 x ⅛ inch aluminum and marine grade HDPE board (½ and ⅜ thick). I chose this material because it is easy to work with, machines easily, and of course, is impervious to moisture. I chose a shape that was aesthetically pleasing and a bit radical. The primary mirror holder was redesigned to use a spring and ball bearing mechanism with 100-TPI screws like a laser mirror holder. I also changed the secondary mirror holder to utilize 4 screw alignment.

Obviously, being able to adjust all the components easily made a big difference in alignment and collimation. I discovered that the smaller relay mirror was the critical piece in getting a proper diffraction pattern.

Current weight is 16 pounds with a classic Celestron turret eyepiece, and a laser pointer for the finder.

I completed the project by the middle of 2015, and took delivery of a EQ-AZ5 mount in August 2015. Unfortunately, the mount turned out to be too light for the moment arm of the tube, and the vendor (Skywatcher USA), traded it out for an EQ6 mount, at no charge.

I can report that the star images are text book sharp.

Original closed-tube design.	New open-tube redesign.	View of the secondary folding mirror, an elliptical flat.
Four-screw collimation adjustments on the secondary flat.	Laser pointer finder.	Completed open-tube telescope on mount.

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