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[ATM] Re: Re: Re: Hyperbola and TDE - Tenacious
Mike Lockwood asked:
======================
>>It's an 8" F/6 mirror. What conic constant did you measure? Was it
really -2.513? (I wasn't sure how to interpret the SixTests data in
your previous post.)
How long does it take the pitch channels to close when you are doing
normal polishing?
How much pressure do you exert (downward) when polishing?
=======================
My responses:
The -2.513 got into my Sixtests input files before I really knew how to properly set them up, so it is meaningless - As Jim Burrows points out, I need to go back and change that "b" to a negative 1.0 ! The numbers I refer to in my posts about Sixtests are generally versus the best-fit parabola that Sixtests calculates.
My lap is pretty hard - synthetic pitch in a room at about 68 degrees F. The channels really don't close up unless I do a hot press, which for me is a disincentive to doing hot presses. When I reopen the channels, using even a new utility knife blade, I invariably get some chipping, which is always a little disconcerting. That said, it's probably time for another warm press.
When doing the Bob May trademark short half-stroke to reduce TDE I have been pressing REALLY hard - producing a fairly roughed up surface, but it does take out the TDE quite steadily.
Because of the roughness and zoniness produced by that, I have recently done a couple short sessions of the TOT, short W with the lap edge just over the middle of zone 4 (my most peripheral zone). This has worked to simultaneously smooth some of the roughness as well as reduce TDE.
I am currently thinking of continuing to reduce the TDE using this TOT W method. According to Sixtests, when I use a sphere as my target conic ( b = 0.0 ) I am quite close to that shape except for that TDE. My "b" is -0.385 presently, so maybe if I just go for a sphere I will be able to safely parabolize without a lot of other figure issues getting in the way. Granted, my radius will be a couple millimeters longer than ideal, but I would be happy to have a good f/6.4 mirror!
Thanks,
Jay
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