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Design Plans

When I built my 8" (ALBIREO), I was pretty much following the plans out of Berry's Build Your Own Telescope. When I built the 12.5" (EQUINOX) I did draw up some plans (a few of which have survived), but not a complete blueprint of the design; a lot of it happeend organically in the shop as the scope was being constructed. For this scope, I thought it prudent to make the plans a bit more complete, since it is significantly larger than what I've built before. This is an archive of those plans.

Mirror Cell

To lay out my mirror cell, I used a bit of advice from Kriege & Berry, but calculated numbers using a finite element analysis program known as PLOP. PLOP takes the thickness of your mirror, the planned number of support points you are expecting to have, and then predicts the expected deformations in the surface of the mirror. It then automatically calcultes the precise size of the triangles and bars for your mirror cell needed to minimize that distortion. The rest of the cell was laid out using a lot of trigonometry and my calculator. I double checked all the calculations using a Mathematica notebook I wrote to make sure I didn't make any mistakes.

Early on I did a PLOP analysis and decided that the use of a 27 point cell did not gain me much; the first two frames show PLOP output for a 1.7" thick mirror using an 18 or an 27 point cell. I went with the 18 point (the same number that I used in EQUINOX). The last three plan pages below are ones I prepped for the machine shop that cut and welded my cell; the information is just spread out differently, hopefully making it easier to find/read/follow.

plop18 thumb plop27 thumb PDF thumb
triangles thumb bars thumb drill thumb
weld thumb top thumb layout thumb
drill1 thumb drill2 thumb weld thumb


I started with the Mirror Box once I knew how big the mirror cell was going to be. My principle requirement for the Mirror Box: it have an integral mirror cover, as I built on EQUINOX. I didn't want the mirror box to be enormously tall, so I settle on having the mirror cover fold in half then fold away.

mirrorboxSide thumb mirrorboxTop thumb rocker thumb
groundBoard thumb sideBearing thumb fanBoard thumb

On the Secondary Cage my requirements were: it have space for both a finder (a Stellarvue F80 RACI) and a TELRAD. This makes it a bit longer than traditional Dob cages, but I don't mind it so much --- the extra length makes it harder for stray light to get onto the secondary mirror.

cage 1 thumb cage 2 thumb cage 3 thumb

The Truss was the last part to be specified. I had tried to estimate what its overall length should be, but I was leery of wholesale cutting the poles down without first insuring that all my eyepieces came to focus (I had carefully chosen them from Televue's specifications so that they all fell, theoretically, within the focus travel of my focuser). In practice I built enough of the scope to do a rough assembly, then set it up with truss poles I knew were too long. I set the scope on the grass, pointing straight up (no altitude motion) and tried all my eyepieces. I cut 1/8 to 1/4 inch off the poles then tried all the eyepieces again, until I was happy with the length. The truss length here in these plans was arrived at by this process.

truss 1 thumb truss 2 thumb

The exact position of the side bearings was determined by doing a balance test --- I put all the optics in the telescope, together with the finders and eyepieces, then laid the truss across a round dowel. I moved the dowl back and forth until I found the balance point for the scope. This was the location where the virtual center of the side bearing was placed.

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Last Updated: 25 September 2012