ATM [Fwd: BOUNCE atm@shore.net: Non-member submission from [CHEN@stars.gsfc.nasa.gov]]

[Mel Bartels <mbartels@efn.org>]

  owner-atm@shore.net wrote:

> From: CHEN@stars.gsfc.nasa.gov
> Date: Tue, 19 Aug 1997 15:52:58 -0400
> To: atm@shore.net
> Subject: Magnetic Secondary Holders?
>
>    I would like to comment on the idea of using magnets to hold a
> secondary
>  mirror.  I have worked on the idea of magnetic levitation for
> telescopes
>  for quite a while.
>        First off, I'd like to commend Mr. Zikovsky for an imaginative
> idea
>  in the good ole ATM tradition.  The reply from Buck Childers was very
>
>  accurate and perceptive.  Neil Pegg's comment, although he may not
> realize
>  it, is brilliant.
>        To set the matter straight, it is not possible for a system of
>  permanent magnets to have a stable equilibrium (a stable equilibrium
>  means that if the system is slightly disturbed, it will go back to
> its
>  initial state.  Think of a ball at the bottom of a bowl).  I recall
>  that there is an old physics principle (Earnshaw's theorem) to that
> effect.
>  If you try to balance a magnet on an opposing magnet, one of them
> will
>  try to flip over or go sideways into the attractive mode.
>        People have made magnetic bearings that use both permanent
> magnets
>  and electromagnets to maintain equilibrium.  The problem is that now
> we
>  require position sensing and active feedback.  Also, as Buck pointed
> out,
>  electromagnets are not very efficient and use a lot of power and
>  generate heat.
>        Earnshaw's theorem, however, does not apply to
> superconductors.  My
>  colleagues and I have developed and demonstrated that one can use
> high
>  temperature superconductors (HTS) to support a whole telescope.  An
> HTS
>  bearing can support tens of pounds and permit rotation with
> practically
>  no friction.  The HTS needs to be cooled with liquid nitrogen and is
>  therefore not very practical for your normal star parties.
>       However, HTS IS IDEAL FOR USE ON THE MOON.   The temperature at
>  night is below that of LN2, therefore no cooling is required.  Having
>
>  no friction means that the telescope can be pointed and tracked to
>  sub-milliarcsecond levels.  And there is no gears or contact points
> to
>  wear out.
>       For those interested in more details, we have a web page at
>       http://snoopy.gsfc.nasa.gov/~lunartel/
>
> Clear Skies to all,
>
> Peter C. Chen



--
Clear skies, Mel Bartels    Programmer/Analyst, amateur astronomer
Eugene, Oregon, USA         homepage: http://www.efn.org/~mbartels
mailto:mbartels@efn.org     atm, atm-digest list-owner
Motorize A Dob: http://zebu.uoregon.edu/~mbartels/altaz/altaz.html