Building a Dobsonian Telescope - By John Masterman (Mary's dad)

At OkieTex, I was mesmerized by the views through 18” Obsession Telescopes. Dave Kreige said he could build us a cheap 18” Dob with a chipped mirror. He was kind enough to refer me to Torus, which had an 18” mirror with 3/16” beveled off the edge. It was 82” focal length. When someone offers you a mirror with a beveled edge, it probably had an optical flaw of a turned down edge.

Mary helped with the design, gluing, sanding and painting. She really was too busy with school, and I wanted to try out that expensive mirror. She was very disappointed when she figured out I had conned her into a project which had nothing to do with spectroscopy.

I wanted to build a lightweight Dob, with no wheelbarrow handles. I really wanted it to be low, so a ladder would not be needed much.

I am not really a woodworker. I found a local cabinet shop with a $7000 CNC router table that could machine plywood. They could accept DXF Files.

I drew the parts in AutoCad. I then converted the outlines into polylines. Since I was using a 1/8” router bit, I offset the outlines outward by 1/16”. This cut out the parts accurately. This left a problem in that the radius kept the tongue and groove joints from fitting together properly. To fix this, I radiused the inside of the fingers with a carving knife and sandpaper to fit together properly. It does not have to be perfect, since the only the glue sees the inside of the joints. I drew spiral toolpaths to lighten the sides of the mirror box without cutting through the plywood.

Primary Mirror Cell

First, I downloaded PLOP and uses it to design the 18 point floatation cell. Its pretty much Kreige/Barry, but I wanted to make the mirror as low (far from the CG) as possible, to balance the secondary.

Detail of Collimation Bolt

To make the 3 primary mirror adjustment bolts, I used 3/8 fine thread bolts, using a drill and bench grinder to remove the boltheads and make a slightly convex surface for the bars to pivot on. I drilled the end of the bolts on the lathe and threaded them 6-32 to hold the triangle pivot bars. The bolts are threaded into 3/8” nuts welded into the tailgate. I cut short pieces of 1” by 1/8” bar to act as adjustment handles, with 3/8” holes drilled and tapped in the middle. 6/32 holes were drilled into the sides for a setscrew.

Pivot Bars

This picture shows two of the mirror support triangles, set up to pivot on the above adjustment bar. I beveled 6-32 brass nuts for a pivot bearing. I screwed the 6-32 screws into threaded holes in the 1” square tubing pivot bar, using a tiny wrench to hold the nuts. I used a second nut and loctite underneath to prevent the nut from unscrewing.

Tailgate

The above picture shows the tailgate. The layout is reversed from Kreige’s design, so the beveled part of the mirror box will clear better. I used 1.25” square tubing, with slots cut for the pivot bars. I primed the whole thing and brushed on black plasti-dip rubber, to be friendly to the mirror in case it ever touched the frame. . I made the holddown clips from plywood (in case the mirror ever bounces up.) I bought split bolts from Kreige/ Obsession to hold the sling.

This shows the tailgate mounted in the mirror box. The magnets are to counterbalance a certain eyepiece. The two small angles are to mount a fan.

Side View: Mirror box in rocker box

This shows a side view of the rocker box and mirror box. The rocker box was lightened by cutting out the sides, as was the mirror box.

In order to let the secondary store in the telescope (with the mirror cover on!) I had to use small gussets, rather than a big light baffle.

The round light baffle is 0.125” thin and held by 4 knobs and woodscrews.

Mary wanted to stain the telescope, but John didn’t want to. The secondary has Mary’s aesthetic. The secondary cage uses 2 layers of 5/8” plywood for the top end, since it is so thin where it stores into the mirror box. I used Kreige/ Obsession pole seats. The Rigel quickfinder fits (via 2 dowels) into 7/8” holes I drilled to lighten the top. I used a $25.00 used JMI focuser so I could afford a 31mm Nagler. It was a good tradeoff. With the Nagler 31, the telescope has a 1.2 degree field of view, and a 6.8 mm exit pupil. I can find things!

I used 2 layers of model airplane plywood and fibergless to make a thin stiff focuser mounting board.

The secondary is supported by foam in the mirror box, since the focuser sticks out too far. It does store in a small space, however.

Recommendations:

Astrosystems is great, they do what the say and ship it.

Discovery Telescopes (secondary mirror) was very good at promises and fake tracking numbers.

Buy Obsession / Kreige’s side bearings. The wood ones I made were a pain, since I could figure out how to accurately drill the holes for the truss poles. If you build the telescope in 3D modeling software, that will not be a problem.

The bottom truss blocks are from www.malcoprecision.com in New York. They work nicely.

Link to my DXF files of telescope drawing.

Warning, don’t use as plans, just as design ideas!