Re: 1/6 or 1/8 wave

Curtis Crouletwrote:
My source says Star made their Pyrex mirrors out of ordinary
commercial-grade Pyrex. Jones or someone else from Star can come
here to
refute it if they wish. There's no claim here that Star doesn't
make decent
optics -- but, by their own admission, not on Pyrex.
--
Curtis Croulet
Temecula, California
33°27'59"N, 117°05'53"W

Chris, who is your "source?" There is only one grade of
Pyrex - Glass Code 7740. United Lens standard anneal is precision
anneal. Newport Glass Works also supplied precision anneal.

In response to my email to Paul Jones, Star Instruments:

"The facts are that all Pyrex comes out of the same furnace in
England and the glass is used primarily for kitchenware (dishes,
pots, pans, bakeware, measuring cups, etc.). The Pyrex used for
telescope mirror blanks is extruded in *** form from this large
furnace and has problems with homogeneity in the form of stria which
flows in the direction of the extrusion. This problem was brought to
Corning's attention many years ago and they basically said
"Pyrex is not an optical glass so you use it at your own
risk." The quality of the annealing of the glass has no effect
on the stria problem. The stria problem presents itself as
astigmatism as the optic cools down because the coefficient of
expansion is slightly different in the direction of the stria versus
across the stria. This is more prominent in large mirrors than small.
Having talked to other opticians over the years, they have commented
that they have seen the same problems but they do vary from one batch
of glass to the next. The stria can actually be seen with the naked
eye if you polish both sides and pass a bright light through the
blank. It appears as lines flowing in the same direction. Annealing
theroretically lowers the amount of stress in the glass and is only
useful if you plan on doing any mechanical work to the blank, i.e.
coring central hole, edging, etc. Annealing has nothing to do with
the figure of the mirror. The finer the anneal, the longer the cool
down period. It doesn't affect the inherent stria at all. You would
have to remelt and remix the glass to improve the homogeneity of the
glass."

Thought this might be of interest.

Mel
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