Re: [ATM] back to longer FL

[Ken Lowther <hermit@outofoptions.org>]

To me the most accurate measure comes from reflection.  I'd do a flash 
polish and check it that way.  If your tool is hard enough,  you can 
change it enough.  It will also change a little during polish.  I have 
changed the focal length during polishing, but I did lots of stuff way 
too long on my first mirror blank. ;-)  Ken   Quoting Jan van Gastel 
<jhm.vangastel@wanadoo.nl>:  > The FL of the second 12 inch f/5 mirror 
for the binoscope I want to build is > 1.2% shorter at the moment than 
the FL of the mirror I already finished. > This should be less then 1%. 
Getting it to, lets say 0.8% difference means a > shallowing of the 
sagitta of the second mirror with 0.03 mm (0.0012"). I > just finished 
finegrinding with Carborundum 600 and have to do 800 and 1200. > Is it 
possible to to go back in sagitta this far with those two grits, or > 
should I go back to a grinding with a coarser grit? > > > Jan > 
http://home.wanadoo.nl/jhm.vangastel/Astronomy/ > > 
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