Friday, December 14, 2007

Big Bertha's Rebuild: Mirror Cell

Big Bertha's Rebuild: Mirror Cell

National Metal Fabricators promised the two aluminum rings from which I am planning to make the lightweight mirror cell for December 13. Sure enough, that's when they arrived. I was just slightly nervous about whether they would be stiff enough for the job, but the math said that 1/8" thick 6061 aluminum would be up to the task. Having put out about $135 for them, I was still concerned that I might end up with two of the more unusual paperweights.

This is the 17.5" diameter ring that the mirror will rest on. (Actually, on some felt that will be on top of this.)

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This is the 20.4" diameter ring that the smaller ring will connect with springs and bolts. And yes, the protractor is because I was measuring 120 degree angles to get the flanges as close to even as possible.

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They are certainly stiff enough for the task. I don't think I would have wanted to go any thinner, but I don't think that they in any way marginal for the intended purpose. Best of all, these two rings weigh about four pounds! Even with the springs, bolts, flanges, and side supports for the mirror, it is going to be under six pounds. Since the mirror end of the scope is the heaviest part--and therefore the one most prone to deform the tubes--the more weight that comes off here the better.

Here you can see the flanges that will bolt the mirror cell into the tube, in roughly their final position.

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I made the flanges by taking a 3" x 2" rectangular aluminum tube (1/8" wall), cutting sections, then cutting these L-brackets out of the sections. There are two 1/4"-20 threaded holes in each of the horizontal sections that will be used to hold the base plate to the flanges. There is a 2.25" long, 1/4" wide slot in the vertical sections that will be used to hang the mirror cell to the tube wall. The reason for the slot is to let me move the mirror between the visual and photographic positions. (The focal point needs to move about 2.5" farther up for the camera; this lets me optimize the secondary mirror for visual use, and yet still be able to do prime focus astrophotography.)

The L-brackets were an interesting experience. I had originally planned to rough cut them with the chop saw, then machine them more precisely to length and width with the vertical mill. But it turned out that the vertical mill doesn't have quite enough motion in one direction to do what I wanted. To my pleasure, I was able to get the length and width of these L-brackets with .010" using the bandsaw. Then I used the vertical mill to very precisely (within .005") position the slot. I used a 1/4" end mill to create the slot. The more precisely you position the mirror in the cell, and the cell in the tube, the less play you need in the collimation screws to get everything exactly aligned. I've seen some telescopes where sloppiness in placing the mirror cell in the tube meant a lot more movement was required in the collimation screws.

The horizontal part of the L-bracket will be attached to the base plate of the cell with two 1/4"-20 hex head bolts. The L-bracket is tapped; the base plate will be through hole; and there will be nuts on the bottom of the base plate. I may use a lock washer on the bottom to make sure that once I have these screwed down, they don't move. Six 1/4" bolts should be more than sufficient to hold an assembly that weighs total (with the mirror) about 32 pounds.

The slots will take 1/4"-20 bolts again, probably hex head. I will use a wing nut and a lock washer on the outside of the tube to hold the mirror cell in position to the tube. This way I only have to loosen the lock washers and push or pull the mirror cell to reposition it at either the visual or photographic position. Of course, that still means recollimating the mirror, but that's not all that hard to do. I may look for the higher grade of bolts on this, since that's a total of three bolts holding about 32 pounds. That still seems more than sufficient.

The finish isn't much, but I am going to either have it black anodized or (if that turns out to be too expensive) flat black paint it.

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