Re: [ATM] turned edge vs over correction

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

Libor:

> I like this topic because I have problems with both overcorrection and
> turned edge very often,
> mainly with turned edge. Could you recommend me what to do?
> 1) How prevent turned edge during polishing?
> 2) How correct turned edge after polishing if it come up?
> 
> I think that ATM recommends adding TOT between MOT during polishing.
> But my experience is just opposite. It takes more glass from edge and
> makes
> mirror overcorrected and turned edge...?

Sounds like you are working by hand. What size and focal ratio do you
typically work? Small, slow mirrors are readily figure with a full size lap
per the techniques described Texereau's How to Make a Telescope. Larger or
faster mirrors are easier to figure and control working with sub-diameter
laps. The best techniques to use depend on what you're making. Let me know
and I'll offer some general tips.

James

James Mulherin
Optical Mechanics, Inc.
jcmulherin@opticalmechanics.com
www.opticalmechanics.com
Tel: (319) 351-3960
Fax: (319) 351-3943

> 
> Thanks for help,
> 
> Libor
> 
> 
> ----- Original Message -----
> From: "James Mulherin" <jcmulherin@opticalmechanics.com>
> To: "'Wayne Hilliard'" <woodycxd@adelphia.net>; <atm@atmlist.net>
> Sent: Tuesday, November 01, 2005 2:26 PM
> Subject: Re: [ATM] turned edge vs over correction
> 
> 
> > Wayne:
> >
> > In my opinion, yes. Let's imagine your outer 1/2" is over-corrected by
> 1/4
> > wave. No problem, your mirror is still diffraction limited. If your
> outer
> > 1/2" is over-corrected by 1/2 wave your images will still be good but
> they
> > will be a bit softer and you lose some contrast. However, all of the
> star's
> > light still goes into image formation. It's not perfect but it's still
> quite
> > good.
> >
> > A turned edge on the other hand is so steep that it no longer
> contributes
> > light to the focused star image. In stead, some of the light goes to
> making
> > your star images hairy (the hair is outside of the normal image circle)
> and
> > the rest is spread throughout the rest of the field of view, reducing
> > contrast.
> >
> > There may be some argument as to magnitude. In other words, where's the
> > transition between over-corrected and turned down? This is a tough
> question
> > to answer without empirical data and on the sky testing. On the sky you
> can
> > usually recognize a turned edge immediately. It takes some skill in
> > interpreting the star test to diagnose an over-corrected edge.
> >
> > 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 Wayne Hilliard
> > > Sent: Tuesday, November 01, 2005 5:20 AM
> > > To: atm@atmlist.net
> > > Subject: [ATM] turned edge vs over correction
> > >
> > > Is there a difference between a overcorrected 100 percent zone and a
> > > turned
> > > edge?
> > >
> > >
> > > Thanks
> > >
> > > Wayne Hilliard
> > >
> > > --
> > > Wayne Hilliard         "If computers are making the world a global
> > > village,
> > > KA1CXD FN32             and I can't figure out how to use one, does
> that
> > > woodycxd@adelphia.net   make me the global village idiot ?"
> > >                                                      Shoe
> > > _______________________________________________
> > > ATM mailing list http://www.atmlist.net/
> >
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