Fw: ATM 12.5" f/5 Double-Arched Mirror Blank.

["Galogaza" <elizabeta.galogaza@siol.net>]

----- Original Message -----
From: "Roger Ceragioli" <rogerc@as.arizona.edu>
To: "Galogaza" <elizabeta.galogaza@siol.net>
Sent: 16 September, 2002 21:59
Subject: Re: ATM 12.5" f/5 Double-Arched Mirror Blank.


> Hi, Vladimir.
>
> Well, I wouldn't presume to give you authoritative advise, but one
> technique which opticians use in a case like yours is to make a "pitch
> blocking."  You would cast a pitch layer onto a solid backer (for
> example a thick mirror blank).  Then you gently warm the blocking in a
> bucket of water while your mirror lies directly on the pitch (back-side
> down!).  It will stick to the pitch after a while, which is good.  Then
> you gradually "anneal" the assembled whole by letting your bucket of
> water cool off to room temperature WITH the pitch/glass blocking in the
> water.  Do this annealing process several times, starting with water hot
> enough to soften the pitch, and letting it cool off COMPLETELY.
>
> Once the mirror is firmly attached to the pitch and the assembled whole
> has been annealed, you then grind and polish your mirror just as you
> would a normal thick mirror.  Once your finish, you then chill the
> pitch/glass blocking in the refrigerator and then in the freezer until
> quite cold.  Finally, to free your glass from the pitch you carefully
> protect the front surface of your mirror with a covering of tape, take a
> rubber mallet and begin tapping firmly on the side of the thick blank in
> the blocking.  Persist in this for a long while and gradually you'll
> shatter the pitch layer and your glass will come free.  The figure on
> the glass should not change, if you annealed it carefully before you
> began grinding.  This is a standard optical technique.
>
> Yours,
> Roger
>
> Galogaza wrote:
>
> >Thanks Roger ,
> >
> >After reading lot about problems with mirror supports
> >in telescopes or during manufacturing ( grinding ,polishing and figuring)
> >I developed great respect for stress.
> >
> >What you are saying ("The flat annulus is about 2 cm wide.
> > And yes, the blank rests just on  that. ") contradicts everything
> > that I saw in print or on the list.
> >
> >And yet you wrote  : "And so far I have seen no signs at all of any
problem
> >during
> >grinding/polishing/figuring--or in the telescope itself."
> >
> >I am working on a 11 mm,  mirror 8",  f / 3,  spherical back, intended
for
> >flexing ala Adler. I did not know how to support properly the mirror due
to
> >its
> >spherical back in order to avoid any chance to stress it while grinding
and
> >polishing.
> >
> >After your post I will look at the problem differently.
> >Perhaps one dogma less.
> >
> >Regards
> >Vladimir.
> >
> >
> >
> >
> >
>
>
>