Mirror figuring question(s)?

I think I started figuring this 16" too soon. Should have worked on the long edge first but didn't recognize it because I only looked at the Ronchi screen inside of focus. The bend in the shadows just sort of blended in with the parabolic center. I kept wondering why the star test was so ugly. Anyway, I've begun using my 12" lap with straight center over center short strokes to try and blend it in with the middle. Also using tangential strokes at about the 70% zone to wear down the area where the focal length changes. Have reduced the width of the long edge so far from 3" to 2". It looks like this action is un-doing what correction I had achieved (note the straight shadows in the center of the mirror) up until this latest revelation. I'm not worried about that though. My question is... Does my approach make sense? Also, what effect on the optics does a long edge have? Before embarking on this latest plan the mirror seemed to work very well. It split the double-double in Lyra extremely well (refractor like), and it had absolutely no focus problems on either the moon, Jupiter or Saturn. Was able to view them all at around 440x (over 25x per inch). Of course the image is a little dim, but it resolves detail better than my 10". My only complaint has been the star test. Is this something that will show itself more once the mirror is coated? Has anybody run across this condition before? Any words of advice and experience to share? Any thoughts are welcome. Thanks and clear skies.

Bill

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Bill, This is exactly how my 10 inch looked and I had a hard time correcting it. Now that I've read Marks explaination I realise what I was doing wrong. How is your correction progressing? Any update?

Thanks Dave

Hi,

Well I was finally able to spend some time on this. It appears that the width of the long edge has been reduced to about 1", and a hill has been created in the center. I'm not worried about the hill. I can take that down easy. I am frustrated with how long it takes to reduce the edge. I have been using a small 6" & 8" laps with accented pressure very near the ridge between the two radii, and am now very near to working right near the edge. For a while I used my 12" lap with the center 8" of pitch removed in order to stop working the center. Anybody have an idea aboout what to do next?

Bill

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I guess I'm probably going to elected as the poster child for bad mirror making but here is the latest. Please give me your thoughts... good and/or bad.

I have been using a small 6" lap (which really is about 5") right at the edge with a short to medium length chordal stroke while walking around the barrel. Probably about 120 strokes per revolution. Have been around the barrel about 100 times since last update. Arms are getting stronger.

Looks like I have an problate spheroid developing and the long edge is beginning to blend in. TDE hasn't changed much. Surface is pretty smooth.

I was going to continue this approach and test again after ~30 minutes (~20-25 times around barrel). I don't have the resources available right now to make a bigger lap, but if that's what I need to do let me know. A 16" lap would be a mother to work with! If I buy some more pith what kind should I get?

Thanks again and clear skies,

Bill

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Well here is a photo of the latest progress. The Ronchigram outside of ROC (right side) looks a little chopped up because I have to piece together multiple images to make it whole. My thoughts on it are as follows:

1) Its evolved into an oblate spheroid (an ellipse rotated about its minor axis). Edges high relative to center. Undercorrected everywhere.
2) Surface roughness has smoothed out some since last photo, but still a little rough.
3) TDE down to about 1/4" but still no diffraction ring all around.

OPTION #1 -
I've read that this type of figure can proceed directly with parabolizing. Deepening the center will shift the average ROC in (make it shorter), which will bring the edge down (relatively speaking). There is a gamble in this that the amount of undercorrection at the edge is about equal to the the correction required once the center is deepened. If it is, then the TDE will remain and need to be masked. If it is not, then further work on the edge will be required. Work on the TDE can continue at that time.

OPTION #2 -
The alternative (which is what I've been doing) is to continue working the edge downward to match the center. The attempt being to return the surface as close to a sphereiod as possible. At that time the surface can be parabolized. This approach should also remove the TDE in the process of bringing down the edge.

I think that if the mirror didn't have a TDE I would definitely proceed with the first option. That would be the quickest way to bringing the mirror to completion. Because of the TDE I'm not totally sold on that approach yet. I am however, getting anxious to finish and the second option means a lot more work. What do you think?

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Howdy folks,

Check out the latest. Images are set up with inside ROC to left, outside ROC to right. 150 lpi Ronchi grating. Looks like I cleaned up the edge about as well as expected. Still a small TDE and no diffraction ring all around, but tremendous improvement. I see a figure beginning to appear. My interpretation is that the ~70% zone needs to be smoothed some. Looks like a pronounced break in slope as opposed to a smooth transition. Also, center ~40% needs to go deeper. What do you think?

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