LX200R Mirror Lock Repair
It got to the point that no longer was I able to take long exposures on my Meade 10" LX200R. I was getting big image shifts in my integrations. Since both my main and guide image trains were solid and free of movement, I figured that the problem must lie with a shifting main mirror. Sure enough, when locked, I could not feel any resistance in the course focuser. Something that I know I could feel all along. Below is an outline of what I did to correct the problem. Please note; This is considered a temporary solution. It was done to test a theory and as you will see it seems to be working rather well. I will keep using this fix until it appears to fail. I guess you can consider it in beta.
The fist thing that needs to be done is to get the mirror assembly out. I want to thank David Nagel very much for outlining the procedure to do this. Here is his instructions on how this is done:
1) Loosen the Mirror Lock. Remove the Course Focus and Mirror
2) Carefully remove the screws holding the corrector lens in place. (Turn OTA
3) Note the orientation of the corrector lens. There should be a WHITE mark on
both the lens and OTA with a BLACK line on both. This is the alignment mark.
4) Grasp the central obstruction and gently pull out of the OTA. Note: There are
several cork spacers around the lens. These need to be in place when you but the
lens back in place)
5) There is a clip or screw on the end of the central tube. This keeps the
mirror assembly from falling off. Remove the clip or screw. The end of the tube
should be clear and unobstructed.
6) Grasp the outer tube and gently pull the tube and mirror out of the OTA.
7) The metal disk with teeth most of the way around it is the mirror lock.
Unless you need to tighten the lock, do not adjust this disk.
8) Wearing gloves reach in to the OTA and redistribute the grease that is most
likely accumulated at the bottom of the fixed tube.
9) If everything looks ok, reassemble your instrument. Don't forget the cork
spacers when replacing the corrector lens and align the alignment marks.
10) When you install the retaining ring do not tighten the hex screws to tight.
Once the mirror assembly is out, it will look like this:
A closer look more from the top reveals this:
Once the assembly is unscrewed and disassembled you have the three major components to the mirror lock assembly:
How it works is this; When you turn the mirror lock knob counter clockwise it turns the big aluminum gear clockwise. This in turn screws the hub inward. The tapered flange of the hub then works in concert with the opposing tapered area of the sleeve and the resulting wedge effect decreases the id of the sleeve. This in turn clamps down on the baffle tube thus stabilizing the whole mirror assembly.
What I found out in my case is that the hub, when screwed all the way in, was not compressing the sleeve enough to clamp down on the baffle tube. What I needed was a smaller id in the sleeve. Ideally, a new sleeve with a smaller id would be the proper solution. But since this was out of the question, I figured that perhaps building up the tapered flange on the hub could be a workable solution.
For this I used plastic electricians tape. I cleaned the flange area real good with soap and water to make sure all the grease was removed. Then I carefully put the tape all the way around the id of the flange being careful to keep the tape only on the flange area. What you then have is this:
The tape is .008 of an inch thick. That would give me .016 of an inch on the diameter. I figured that should be sufficient. Then I used a razor blade to trim off the excess tape:
I then reassembled everything back together. Put the hub/gear on but give it only a few turns. I had saved the grease for the sleeve and reused it. This is my one concern with using tape. The sleeve is greased so perhaps in time the grease will cause the tape to come off the hub. But I will see how long it takes if at all. After the mirror assembly is back in, I installed the focuser assembly. Then I turned the gear with my finger (through the mirror lock hole) clockwise until you can feel the resistance of the sleeve starting to clamp down on the baffle tube. I then installed the mirror lock knob.
They say the 'proof is in the pudding'. Below is my first light image after the repair. It is a minimally processed test image of M27. It consists of a stack of 6 images of Ha. Integrations were only 10 minutes long but this was sufficient to get an idea of whether or not the operation was successful. During the 1 hour imaging time, total shift was 4 pixels which is attributed to flexure. I was very pleased. Round stars can be a beautiful thing!