RE: [ATM] Active damping

["Richard Schwartz" <richas@earthlink.net>]

Mark has provided half of the solution: increase the stiffness of the
telescope structure.

The other half of the solution is to REDUCE THE MASS of the structure.
This will also raise the resonant frequencies and make the vibrations dampen
more quickly.

. . . Richard

-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
Mark D. Holm
Sent: Friday, August 12, 2005 6:39 PM
To: atm@atmlist.net
Subject: Re: [ATM] Active damping

Quoting Ross Sackett <rsackett00@yahoo.com>:

> Hi, all,
>
> While showing my 8" f/4.5 travel scope on a 3-point
> hybrid dob mount at the Stellafane mechanical
> competition I had an interesting conversation about
> active damping to reduce vibration.  I don't know much
> about this.  It seemed to involve adding a mass that
> would vibrate near the same fundamental frequency as
> the rest of the scope to soak up the excess vibes.
> Some electronic or mechanical black box controls it, I
> imagine.  I know something similar is done on big
> buildings in quake zones.  Can anyone point to an
> ATM-friendly source on this?
>

Being a chemist, I can't help pointing out that there are materials that
damp
quite well.  No batteries required, and the principles required to use them
effectively are no more, and probably less, involved that for active
damping. 
Those damping pads sold by various commercial telescope houses for putting
under tripod feet are supposed to work pretty well.  Sorbothane, though a
lot
more expensive in small quantities than it ought to be, has very high
damping. 
The foam used in noise damping ear plugs damps like crazy at audio
frequencies
and is also awfully lossy at lower frequencies.

Lots of times, a structure that vibrates has more of a stiffnes problem than
a
damping problem.  Boosting the stiffness, if it can be done without adding
much
weight, pushes resonant frequency up.  This is good, both because it is
usually
harder to excite vibration at higher frequencies, and because damping is
often
higher at higher frequencies.  Telescopes, both commercial and amateur,
often
suffer from designed in stiffness problems.  One of the beauties of the
Dobsonian design is that it is easier to get good stiffness with it.  Even
so,
plenty of Dobs have been built with stiffness boo boo's.

Mark Holm
mdholm@telerama.com


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