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From: stuart@rochester.UUCP
Newsgroups: net.space
Subject: Re: Star-Wars/Space Telescopes (parabolic mirrors)
Message-ID: <13652@rochester.UUCP>
Date: Tue, 3-Dec-85 17:43:01 EST
Article-I.D.: rocheste.13652
Posted: Tue Dec  3 17:43:01 1985
Date-Received: Fri, 6-Dec-85 06:37:52 EST
Sender: stuart@rochester.UUCP
Organization: U. of Rochester, CS Dept.
Lines: 28

From: Stuart Friedberg  <stuart>

References: <384@ukc.UUCP> <26@sbcs.UUCP> <1124@gitpyr.UUCP>, <11128@ucbvax.BERKELEY.EDU>

  There seem to be at least two differing interpretations of the
original article.  My interpretation is that a disk of film is attached
to a fixed, rigid ring, and then more pressure is applied on one side
than the other.  This yields a caternary, not a spherical section.  The
critical difference between this and the soap bubble interpretation
(which leads to spherical sections) is that the edge of the disk is
FIXED and can not move.

  If you take a soap bubble, draw a circle on its surface and change
the internal pressure, the circle will shrink or grow.  Moreover,
every circle you can draw on the surface of the sphere will change by
the same proportion.

  If you take the anchored disk and change the pressure on one side,
the rim is FIXED, and circles drawn at different distances from the
rim will change by different proportions.

  By referring to the original article, it should be clear what the
proposed situation was.  In any case, unless additional forces
(rotations) are placed on the disk, the surface will be neither
spherical, nor parabolic as originally conjectured, but hyperbolic
(a 3-D caternary surface).

Stu Friedberg  {seismo, allegra}!rochester!stuart  stuart@rochester