Plate
Glass Vrs Pyrex
For most amateur telescope makers Plate or Pyrex becomes an
issue of personal preference as to which of the two materials to use as a mirror
substrate. For the making of, and for general viewing, either of the two materials are
adequate to make a functional mirror.
With the new technologies available, Plate glass has gone through some quality
changes that have greatly improved the structural make-up of the glass itself. Zero
impurities, x-ray approved quality, temperature stability, and tensile strength required
for building construction, are a few of the changes for the new generation Plate glass.
The other main differences between Plate glass and Pyrex is cost. Plate glass is generally cheaper to
purchase than Pyrex. Pyrex is not an optical glass. The manufacturer of this glass has verbally told
me that you couldn’t bend this “stuff” if you put an ocean liner on it. I
have seen several 45 + storey buildings in Montreal that was constructed with this Plate
glass several years ago, with only a light metal channel holding these massive 40-foot
sections together. These buildings withstand
wind, extreme temperature variations (-35’ to +40’ C) and show no signs of
stress or fatigue. I, myself have not carried out any stress tests with this glass.
Plate glass is always being improved upon, where as Pyrex glass has not changed
since it was invented. The new plate glass can be purchased today with “zero
defects/zero impurities, and completely free of defects with x-ray quality.
Last January, 3 of us with borrowed ISO approved test equipment, and conducted
Energy (heat) Transfer Tests on several mirrors for 6 hours at –35’C. We
conducted tests on full thickness Pyrex mirrors, 1” thick Plate, and .75” thick
Plate. The Pyrex mirrors never achieved the ambient temperature, always being 1’F to
1 ½’F warmer than the surrounding air. The Plate glass mirrors had the same
problems, but only remained from ½’F to ¾’F above the outside air temperature.
This internal heat comes to the centre of the mirror, and rises upward onto the viewing surface of the mirror, creating a boundry
layer of warm air that acts as another
coating on your mirror face. This is what we erroneously refer to as heat currents in the tube. The thinner mirrors by
themselves gave clearer viewing due to less energy stored in the mirrors.
These findings were presented to our club at the January 2002 meeting, and
immediately almost every Newtonian/Dob had a side mounted fan blowing air across the face
of the mirror. All production telescopes 10” and larger now have cross blowing fans
as part of the scope’s make-up. Removing that boundry layer of warm air, makes for
unbelievable viewing.
This has encouraged some of our Club members to make large sized telescopes
using thin Plate glass mirrors. The Club President is making an 18” mirror, and Peter
Pekurar is building a 25” Telescope with a 1” thick mirror.
peekr@golden.net
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Technologies