Mirror mounting preferences/opinions

[joe1950]

 

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

 

 

 

 

[rl]

The mirror you describe is both small enough and thick enough to be very tolerant about its mounting. Just make sure it's not pinched and all will be well. Air circulation for cooling might be a more important issue. Any standard 3-point suspension should be just fine.

If you want to see the bending effects then the software program Plop might be of interest; it allows you to enter the mirror data and mounting method and will predict how much surface distortion will happen. And it's free. The latest version GuiPlop will cope with mirror tilt as well. Remember to double up on the answers to get the focal plane errors which is what you're really interested in.

RL

[joe1950]

Thank you, RL. Very helpful and much appreciated!

[joe1950]

I went with the silicon RTV mounting using 3 spots of silicon at the PLOP points and 3 spots around the circumference. I read the recommended thickness of the silicon should be between 3 and 5 mm so I used spacers to keep that thickness. Each spot is about 16 to 20 mm in diameter. Those are the recommended measures.

It's been done and recommended by the Stellafane ATM site and often done in mounting the secondary mirror.

One thing I learned that was confusing in some of the posted information. It was recommended to bond the RTV to bare wood. That is incorrect according to other sources and in a previous use I found that silicon RTV does not bond well to bare wood. The wood should be painted, have a coating of shellac or sprayed with clear gloss and the RTV will bond nicely to it.

Also, it is always recommended to use a brand new tube of the silicon each time a bond is made. One tube for the primary and secondary is okay, but if the silicon RTV sits for any length of time, some of it may partially cure and not be as effective.

Thanks again for the information RL and hope anyone who read the post found some interesting information also.

joe