The AFU and Urban Legend Archive
Science
Glass Flow
 telescope glass




From: scharle@lukasiewicz.cc.nd.edu (scharle)
Newsgroups: alt.folklore.urban,alt.folklore.science
Subject: Re: Glass flows... [was: Tornado shoves straw through 2x4]
Date: 20 Jun 1995 13:41:34 GMT

People have been making refracting telescopes with large glass lenses for some time. These are precision instruments, and a 50 or 100 year old telescope would surely show signs of glass flow more dramatically than a piece of window glass. I don't know whether this is a problem with old refractors. Perhaps some astronomer or historian of astronomy can tell us.

From: mbartels@efn.org (Mel Bartels)
Newsgroups: alt.folklore.urban,alt.folklore.science
Subject: Re: Glass flows... [was: Tornado shoves straw through 2x4]
Date: 21 Jun 1995 05:46:23 GMT

>>> old telescopes showing glass movement

I am a 25 yr avid student of amateur astronomy and specifically telescope making over the ages. I have never heard of any scope either refractor reflector or a combination that has shown glass movement. Glass movement in the amateur professional world is fairly easily detected to 1/10 wavelength of visible light. In all these years, I have come across only one story of glass changing shape, and that was an anecdotal story of a mirror that sat on a shelf for years that was tested to a different figure than the original maker had described - hardly convincing evidence.

Regards,
Mel Bartels

From: schumach@convex.com (Richard A. Schumacher)
Newsgroups: alt.folklore.urban,alt.folklore.science
Subject: Re: Glass flows... [was: Tornado shoves straw through 2x4]
Date: 21 Jun 1995 20:02:21 -0500

Ahem. Speaking as the resident amateur astronomer:

Glass does not flow. People who think that "non-crystalline = liquid" need to re-read their physics or materials texts. There are telescope lenses and mirrors 150 years old. None of these have changed shape by as much as a nanometer. Any such change due to flow would be obvious, even ruinous.

Glass optics do *sag* enough to degrade images if not they are not supported carefully. But this is not flow; restore the supports and the mirror etc. regains its shape. (A steel rod will sag, too, if not adequately supported. Does anyone call that "flow"?) The size limit for refractor lenses is because they cannot be supported adequately from their edges.


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