Re: "Grain" in Pyrex (was ATM astigmatism with a sling?)

[Adam Perkins <scopeguru@yahoo.com>]

--- Stathis Kafalis <s.kafalis@planet-interkom.de>
wrote:
> 
> Can anyboddy on this list confirm, if Pyrex has an
> orientation, or let's say
> different strength in different directions?
> 
> I had the understanding, that annealed Pyrex should
> be free of such
> inhomogenities, especially if it is precission
> annealed.



Hi,

I recall reading several years ago a Galaxy optics
statement that sheet pyrex has a bias running along
the long axis of the sheet.  Something to do with
differential cooling due to the sheet having a long
and short axis (rectangular as it is produced from the
machine.)  Seems weird to me, precision annealing is
***supposed*** to eliminate this type of stain, right?

If I recall correctly, Galaxy was making this
statement to justify their move to molded blanks,
supposedly free from this type of strain.  I believe
all of their older mirrors (how old I don't know) were
made from cut sheet stock.

Am I absolutely sure of my memory of this?  No.
But I think it was printed in either Astronomy or S&T
maybe 6-8 years ago.

Take my recollection as just that, an unsubstantiated
claim.  

Now that I've contributed my share to the rumors, I'll
be quiet.

Adam Perkins
Covington, Louisiana
USA

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