[ATM] Controlling focal length

["Paul Bradley" <paul@paul-bradley.com>]

I'm now in 220 with my 8" binocular mirrors which I'm grinding on the same tool. 
I just did a sharpie and the centre ground away in about 30-40 strokes, the edge 
taking another 20 strokes or so to grind off.

I was working exclusively TOT as I had hit my target sagitta and just want to
remove pits and stay roughly spherical, but since the sharpie indicated a lump
in the middle I've now done about 4 wets MOT and the sagitta has gone up 
about 0.007" ! - this seems like a lot of glass to remove so quickly with 220
grit, so I presume I had formed quite a small (diameter) but high lump in the
middle. 

I've done another sharpie and I'm still not in contact (centre still grinding away
20 or so strokes before edge), and my mirrors are now F6.4 (I'm shooting for F7) - 
I don't know whether I should switch back to TOT so as to move the focal length
back up, or stay MOT and grind into good contact first, then go TOT.

Oh, and while I'm asking, I've tested at ROC with a torch by covering the mirrors
in cooking oil, and they seem to be about the same focal length, but it's difficult
to tell as the reflection from the oil is poor and it's difficult to get a uniform film of oil
that doesn't run off when I stand the mirrors up to test them. If I do a sharpie on both
mirrors one after the other and it indicates good contact on both mirrors, can I take
that to mean the focal lengths will be within tolerance for binoculars? (Some people
say 1%, but I'm going to try for 0.5% or better to mitigate eyestrain).

Thanks

Paul
_______________________________________________
ATM mailing list http://www.atmlist.net/