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

["James Mulherin" <jcmulherin@opticalmechanics.com>]

> Jay:
> 
> If your mirror is close to spherical the Ronchi test offers a good check
> for astigmatism. If you can move the grating through focus while holding
> one grating fringe dead center in the mirror you can watch for rotation in
> the fringe. Ideally, you will see no rotating in the fringe as it moves
> through focus. If you see rotation this indicates astigmatism.
> 
> You can also check for astig on a star, even with an uncoated mirror. Look
> at the star inside and outside of focus. If the defocused star looks
> elliptical inside and outside of focus this indicates astigmatism. The
> long axis of the elliptical star will change by 90 degrees depending on
> whether you are inside or outside of focus.
> 
> In an 8" F/6 I would say the risk of astigmatism is pretty low, assuming
> you are using a fine anneal Pyrex blank. If you're using plate glass I
> wouldn't be surprised if the astigmatism is real.
> 
> James
> 
> James Mulherin
> Optical Mechanics, Inc.
> jcmulherin@opticalmechanics.com
> www.opticalmechanics.com
> Tel: (319) 351-3960
> Fax: (319) 351-3943
> > -----Original Message-----
> > From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf
> > Of J Killea
> > Sent: Saturday, August 27, 2005 7:47 PM
> > To: atm@atmlist.net
> > Subject: [ATM] Another Astigmatism question, but at the sphere stage
> >
> > 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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