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[ATM] Mirror figuring problem
I am figuring my first mirror and have been trying to work out a problem for a while now. I am sending out this email as a distress signal since I can not seem to solve my problem. My problem is that my mirror has a depressed zone at around the 75% radius (and a TUE). Here is the necessary foucault information: 8in mirror, full thickness. ROC: 90.125in. I am using a slitless moving source tester (Stellafane tester) with the Couder mask suggested in Texereau Fig 2-32 p. 77 (4 zones). Here are my most recent zone readings (average of 4, all readings are within 0.006 of each other).
Zone 1: 0
Zone 2: 0.025
Zone 3: 0.054
Zone 4: 0.049
Effective mask Radius: 0.854, 2.028, 2.96, and 3.654 (From Figure XP)
I have been working for a while now using the normal stroke to try and get back to a sphere and remove the depressed zone. Unfortunately, while my "Best Fit Conic Constant" is moving closer to a sphere, the depressed zone is not improving. I was hoping to use the normal stroke until I removed the depressed zone and came back to a sphere but unfortunately, I am almost positive that the zone will not go away with my current technique.
Here is some additional information that might help: I am polishing exclusively TOT with a full sized lap and a sheet of cabinet liner underneath the mirror to keep it from moving.
Any seasoned vetrans out there have a minute to help out?
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