Saturday, December 29, 2007

The Diagonal Holder

The Diagonal Holder

I've decided to give up on fiberglass, and have National Metal Fabricators make the two tube sections I need. Just to be sure that the mirror cell tube can handle it, I'm having them make it out of .125" aluminum sheet, and the focuser/diagonal section out of .080" aluminum sheet. The focuser and diagonal components only weigh about eight pounds, so most of the increased weight on the mirror cell section is gained by going thinner at the other end. (Unfortunately, most of the weight that causes deflection problems is at the mirror cell end--if only there was a way to go lighter on that section.)

Here you can see the diagonal holder, almost complete. I haven't attached the spider legs yet, which will be .050" carbon steel, or the three clips that will hold the diagonal mirror to the 45 degree face. Why carbon steel for the legs? Because the thinner the legs, the less they interfere with the optical path. If thickness didn't matter, I would use aluminum instead of steel; aluminum is slightly stiffer for the same weight. But when it has to be really thin--.050" steel will be stiffer than .100" aluminum.

Click to enlarge

Click to enlarge

The main body is constructed of a 3" diameter piece of Delrin, hollowed out to reduce weight. I used Delrin also because it is, relative to aluminum, very light, and yet still adequately strong and stiff for this application.

To make sure that I can reach in and easily turn the collimation wing nuts, the cylinder to which the legs will attach has to be small, so I used aluminum, because it is very stiff for its weight (unlike Delrin). There's a 0.25" hole bored through the center of that piece of aluminum, and a 1/4"-20 hex head bolt allows me to move the entire diagonal assembly up and down.

There are two 8-32 x 0.5" socket head screws in the side of that piece of aluminum to let me lock the screw in position. I had thought of using thumb screws for that, but once I get the diagonal positioned relative to the main mirror, I can't imagine having to move it again.

The hex head bolts that go into the 45 degree cutoff section are going into threaded holes. There are also nuts on the top of the 45 degree cutoff section to prevent any motion of those hex head bolts.

The plate that the hex head bolts go through (with wing nuts on the top) is also Delrin. This is attached to the aluminum cylinder with the 1/4"-20 bolt which is screwed into a threaded hold in the Delrin plate. There is a nut and a lock washer on the bottom of the plate to prevent motion, and a nut on the top of the plate to prevent rotation there as well. (Maybe I am being overly cautious, but the prospect of the diagonal assembly dropping down on to the main mirror makes me nervous.)

Would I have ever attempting anything this ambitious without a lathe and a drill press? No way!

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