[ATM] Another Astigmatism question, but at the sphere stage

[J Killea <jkillea@yahoo.com>]

As long as nobody tells my family about the trouble I'm having with this mirror, I should be able to keep working on it! 
 
After doing star tests and trying to resolve detail on Jupiter, I decided I had some intolerable combination of TDE and astigmatism in my 8" f/6 blank.  Did some nice, gentle polishing to get back to sphere (an oblate one) and I've got a nice TUE that I can live with for the moment.  
 
HOWEVER, I just did three sets of Foucault measurements along each of 2 diameters and Sixtests (set to "Target b=0" with variable ROC calculation) came back with the ROCs of each diameter differing by about 0.5mm.  That's more delta than what I had when I thought my parabola was done the first time.
 
My question is whether it's too early to judge astigmatism, since I'm only at the sphere stage, and whether Sixtests with Foucault data is even a valid way to judge astigmatism.  I might just stick it back in the 'scope for a star test, and hope that the radical amount of SA I will see will not make it too difficult to discern just the astigmatism.  
 
Is it time to go back to AlOx #15 or 20?  (Please say it ain't so!) 
 
As always, thanks a ton,
 
Jay
 

		
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