Just for the experience I made the Foucalult tester as described by Richard Berry and tested the mirror. I tested using 5 zones. The Radius of curvature is 114 inches. The mirror is overcorrected. It is within 1/4 wave tolerance as a 6 inch mirror but the outer zones are off quite a bit.
Here are my measurements:
1/4 1/4 Zone Wave wave Measured Zone Radius Dmin Dmax Delta R 1 1.2 -.04 .04 -.01 2 2.2 .0 .04 .005 3 2.8 .01 .05 .03 4 3.6 .035 .06 .08 outside tolerance 5 3.7 .05 .07 .15 outside tolerance
Zone radius is the radius of the center of the zone. Dmin Dmax is the delta Radius of curvature tolerance for 1/4 wave.
So now what are my options? 1. I could mask the mirror down to a 6" but I already have a 5" Celestron Comet Catcher. I don't need a big scope with a small aperture.
2. Can I correct the mirror without the original tool? I have never made a mirror but have read a lot about it. I'm handy with my hands and do a lot of wood and metal working. I have a home shop where I could do it. How do I make a tool to do this? Is it worth it?
3. Maybe the mirror is not all that bad so I should just have it recoated and use it the way it. I used it as is last summer. I live in a suburb of St. Paul, Mn. and do my viewing from my back yard. The seeing is not very good. I have never seen more than the two brightest star of the little dipper with my naked eyes. So I think the magnitude limit is about 3.5. I am a rank beginner so there are a lot of objects that I can capture that are interesting to me. My goal is to log as many of the Messier (sp?) objects as I can from my back yard. I have about 10 so far.
4. Warp the mirror to the proper correction by pushing on the center of the back when it is mounted. I tried it on the bench with the tester and I got the mirror into 1/4 tolerance. I don't know what other distortion I caused. I saw and article in S&T about doing that to a spherical mirror by epoxying a bolt to the center of the mirror and supporting it all around the edge. Each time the scope was setup for use the mirror was adjusted using a Ronchi star test. The bolt pulled the mirror into a parabolic shape. I would need to do the opposite. That is push it into shape. I wonder if pushing it against only 3 mirror clips would cause other distortion. Does anyone have an opinion? Perhaps I could devise some sort of ring that would rest against the front perimeter of the mirror.
Thanks for any advice.
Dale Eason eason@stpaul.ncr.com