Re: [ATM] Active damping (and archery)

[Ross Sackett <rsackett00@yahoo.com>]

--- Ken Hunter <atm_ken_hunter@yahoo.com> wrote:

> Why not make the truss tube large enough that it
> doesn't vibrate in the first place?
> The added weight of the larger truss tube will be
> less than the added "tuned mass
> counter vibrator" and certainly less fuss.

The advice so far on this thread is all sound and
practical (or, at least, visionary and suggestive). 
For large, car transportable scopes increasing the
diameter of truss members and the depth of secondary
cage rings, and encorporating vibration absorbing
materials make great sense, and I've tried to do this
on my bigger scopes.  But in some cases weight and
packing volume are at such a premium that we might
consider some higher-tech alternative.

My 8" f/4.5 2-pole travelscope was designed to pack in
a check-through duffle and still leave enough room for
a week's worth of warm clothing and other gear.  At
this aperture and length the scope performs
gratifyingly well--perhaps just a little more
vibration than your average 8" on a GEM, but not at
all objectionable for a travelscope (as-is it has a
2-5 second settle-down time after a tap, depending on
whether you are touching the scope).  The 3/4"
diameter, 28" long Al tubes are nice and compact and
plenty stiff for what they are being asked to do.  The
problem comes in scaling up the design to larger
apertures--one Stellafane observer said he wanted to
build a similar design around a 12" f/5 mirror and
keep it airline portable.  I was skeptical that one
could do this because the wobble in the poles
increases with the 3rd power of their length, and
you'd have to increase their diameter considerably
(which along with their length would make packing them
impractical).

It was at about this point in the ongoing conversation
that Wayne suggested taking a more active approach to
damping vibration using a tunable slave vibrator
(perhaps a mass on a flexible stalk) to soak up and
dissipate some of the excess vibes.  Our hope (and my
intent in starting the thread) was to find some
approach to damping that didn't involve much in the
way of increased structure that could defeat the
portability goals of such a scope.

Whether or not such a vibrator is practical in this
case depends on its size and weight--if we're talking
about a few ounces of fishing weight wired to 6" piece
of fiberglass kite-rib I think it would be highly
practical, and preferable to beefing up the tubes if
that meant sacrificing volume for other things, like
thick socks.

(At about this point someone, likely Mel, is bound to
point out that we need to distinguish between
vibration and flexure.  The gadget we're talking about
might address the former problem, but do nothing for
the latter.  Tubes with larger sectional properties
would effectively address both problems.)

But wouldn't it be cool if we could just bolt a little
widget to the sky-end of ANY existing scope and get a
little extra vibration relief?

By the way, is this what those cantilevered weights
are for on modern sports bows?  I know bowstrings and
arrows can have a lot of "twang" in them.  Or is that
all about balance (or rotational inertia, or something
else)?  I know there are some archers on the list who
can answer this.

Ross

__________________________________________________
Do You Yahoo!?
Tired of spam?  Yahoo! Mail has the best spam protection around 
http://mail.yahoo.com 
_______________________________________________
ATM mailing list http://www.atmlist.net/