joe1950 Posted August 16, 2016 Share Posted August 16, 2016 I would like to solicit opinions on mounting a mirror in a recently built DOB. The finished DOB, 150mm, f/6. Mirror c. 1976, Superior Optics, Columbus Ohio. Secondary 33mm minor axis, 22% CO, 3 vane (0.4mm thickness) spider. low profile helical focuser. The mirror is a vintage (1976) 150mm, f/6, Pyrex, and has a full 25mm, 1/6 ratio thickness, as most did back then. The coating is still good and the mirror performs well. I have it mounted in the scope, but may change the mounting arrangement, depending on feedback. 1. Presently, the mirror is in an aluminum mirror cell made way back in the 1960s by Edmund Scientific, NJ. It had the three adjustment knurled nuts and is spring tensioned. It also has 15mm felt pads the mirror sits on about 0.5r. And it has 3 metal clips holding the front of the mirror, about 14mm wide with about 5mm over the mirror and adhesive rubber pads just barely touching the mirror. This is the aluminum cell. The nylon screw heads that the mirror sits on were replaced with felt pads. 2. The alternative is using a wood mirror mount which I've already made, working the same as the one above, except using the silicon RTV mounting method (described on many web sites). It would use 3, roughly 18mm to 25mm blobs of RTV under the mirror at about 0.5r, and having the prescribed thickness determined by spacers placed under the mirror as it cures. Also there would be 3 small blobs around the edge of the mirror at 120 deg. each giving it some side support. There would not be any mirror clips. Also the wood surface would be prepared to better adhere to the RTV as bare wood is not the best as I now understand. I somewhat favor 2 for simplicity and because there are no mirror clips to fiddle with, it would probably hold collimation better, and without clips, the overall diffraction would be somewhat less, but I'm not sure if the difference is significant. Basically, the question is, does it matter which method I use? Is #2, if done properly, safe from introducing pinching or astigmatism especially considering the small size and full thickness ratio of the mirror? If anyone has used either or both feeling there is a benefit of one over the other, I would certainly appreciate hearing from you. Thanks very much!! joe Link to comment Share on other sites More sharing options...
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