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Mirror Cell for 16" Conical
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Author:  llanitedave [ Wed May 14, 2008 10:14 pm ]
Post subject:  Mirror Cell for 16" Conical

Hi, Folks, thought I'd post about the progress of the mirror cell I'm making for my 16" Royce conical primary.

Because the mirror is perforated, and supported by a central post, I figured this would be a great opportunity to try something different: not a true dobsonian mount, but a "split ring altazimuth". The advantage of this is that it can pass right through the zenith without a need to rotate the mount all the way around. It might be a tracking advantage at some point.

This allows a mirror cell with symmetry, instead of the dobsonian sling-supported mirror. In addition, I wanted to be able to collimate from the front, while keeping the center of gravity (and the elevation of the eyepiece) as low as possible.

What came out of this was a triangular cell with support arms outside of the edge of the mirror, sweeping upwards to approximately the plane of the mirror front at the suspension points. It has a wooden core, covered and sealed with multiple layers of fiberglass cloth and epoxy.

The beginnings were humble enough:

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Just a single slab of 23/32" plywood.

Author:  llanitedave [ Wed May 14, 2008 10:15 pm ]
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From there, I cut a series of triangles:

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Author:  llanitedave [ Wed May 14, 2008 10:17 pm ]
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I sanded each one into shape as carefully as I could (although I overdid it just a little), and began stacking them.
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Author:  llanitedave [ Wed May 14, 2008 10:19 pm ]
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Finally, after using a bit of epoxy and glass bubble filler to smooth out some of the worst of the rough spots,, I had the wood core ready for fiberglassing.
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Author:  llanitedave [ Wed May 14, 2008 10:21 pm ]
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I cut individual strips of fiberglass cloth, and put them into place before applying liquid epoxy to them.

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Author:  llanitedave [ Wed May 14, 2008 10:23 pm ]
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The fiberglass coating stiffened the structure up significantly, but not enough. I decided to add braces.

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I used 1/8" oak plywood for those -- primarily because I had some on hand.

Author:  llanitedave [ Wed May 14, 2008 10:25 pm ]
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A few more strategically placed layers of fiberglass, and the core structure is complete. I HOPE it's stiff enough!

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The bracing did make a big difference.

Author:  llanitedave [ Wed May 14, 2008 10:27 pm ]
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For the center post, I'm taking a bit of a risk. I'm using PVC pipe, a smaller diameter nested inside a larger diameter, sealed with glass-fiber-reinforced epoxy. The material itself is soft, but I believe the nested tubes will make it plenty stiff and strong.

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Author:  llanitedave [ Wed May 14, 2008 10:30 pm ]
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I put together a "lathe", of sorts -- a rotary tool with a spherical grinding bit clamped to the table. I used that to cut O-ring grooves in the PVC.

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(The tool was my "bonus" for 15 years at my current job. Better than a poke in the eye, I guess!)

Author:  llanitedave [ Wed May 14, 2008 10:32 pm ]
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When the pvc was properly grooved and tested, I epoxied it to the center of the mirror cell core.

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Author:  llanitedave [ Wed May 14, 2008 10:34 pm ]
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Finally, I did a fit test with the mirror, just to make sure the wasn't any serious mismatch in the design and construction. Looks good so far!

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Author:  llanitedave [ Wed May 14, 2008 10:35 pm ]
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Like so many of my other projects, I'm learning as I go, and my ambition is kind of out-reaching my skill level. It looks ugly, but I think it's going to work.

Author:  Erik [ Fri May 16, 2008 8:30 am ]
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Looks great, Dave! Very unique design! :)

Author:  Tom [ Mon May 26, 2008 4:11 pm ]
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So paint it black, it will look awesome!

Nice job, Dave.

Author:  llanitedave [ Mon May 26, 2008 10:14 pm ]
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It's painted black now, Tom, and all the holes are drilled and the o-rings are installed. I'll be taking an "after" photo in the next day or so. Right now I'm building a cover for the mirror, so that I can store the mirror and the cell together.

I'm still not completely convinced that the assembly is stiff enough, but I'm going to try it as is. I have some ideas for further stiffening modifications should they be required.

Author:  rboe [ Sun Jun 01, 2008 8:25 pm ]
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Not sure if the little blue pill will help - but I'm willing see you give it try. :razz:

Author:  Chopin [ Tue Sep 30, 2008 6:55 pm ]
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Dave, how's the project coming. You must be getting close a complete scope, no?

Author:  Erik [ Tue Sep 30, 2008 8:04 pm ]
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Yes, please, Dave- an update!!!

Author:  llanitedave [ Wed Oct 01, 2008 6:20 am ]
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I'll throw in pictures when I have time.

So far, I've discovered that the cell was not stiff enough after all. I'm using styrofoam to thicken it in the right places and will be fiberglassing that probably next week.

I've finished the UTA, the secondary holder, the altitude bearings, and one of two sets of cross-members. The foam/fiberglass combination is very strong and stiff, and I'm happy with that part of it. It's extremely time-consuming to lay up, though, and the finished pieces are more prone to chipping and cracking on impact than dense wood would be. I'm almost done with the upper and lower truss brackets; they're plywood but I'm using a layer of fiberglass over them to prevent splitting. I'm also making knobbed bolts out of casting resin to hold the assembly together.

I need to finish the lower cross members, stiffen the mirror cell, construct the mount, assemble, test and collimate the whole thing. I've got three weeks.

It's going to be close!

Author:  Erik [ Wed Oct 01, 2008 8:58 am ]
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Thanks for the update, Dave. Man, 3 weeks- you'd better get busy! Looking forward to seeing pics..

Author:  snorkler [ Wed Oct 01, 2008 10:26 am ]
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Very cool project. I'm not sure wood and fiberglass is enough to support 50 lbs. (or whatever) of mirror. You may end up taking your model to a welder ;)

Author:  llanitedave [ Wed Oct 01, 2008 11:48 am ]
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Darrell, the stiffness of fiberglass part depends almost exclusively on the cross section of the piece, not the material the core is made of. The wood and fiberglass was 3/4" thick on the base of the cell, which might be enough to support the 26 lb mirror if it didn't have the cantilever of the three support arms projecting beyond the mirror edge.

What I need to do is build up the cross section, which I'm doing with pieces of styrofoam. Once I've got the form reshaped, a few more layers of fiberglass over that should stiffen it up nicely.

I've already subjected the OTA rings, cross members and altitude bearings to a lot of force, and they haven't budged.

Author:  llanitedave [ Wed Oct 01, 2008 12:13 pm ]
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OK, here are some progress photos, although strictly speaking they're not about the mirror cell.

First is the secondary holder -- the assembly is PVC and wood, and will support a wire spider.

Parts:
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Assembled:
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Next is the UTA after the first round of painting. I'm not happy with the boundary between the black and red, and Elaine has promised to try to work her magic on it:

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Here is one of the altitude bearings while the epoxy was curing on its final fiberglass layer. Notice that it's symmetrical rather than having the usual dobsonian rotational offset. There are wooden blocks embedded at strategic points that provide anchors for the truss brackets:
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And here are the wooden brackets for the upper truss assemblies. The lower brackets have only a single slot. These have since been fiberglassed and painted.
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So, I've made progress, but I still have a ways to go.

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