Re: [ATM] Active damping (and archery)

["matt" <electro_optic@bellsouth.net>]

if you need stiffer truss tubes, before spending the money (and adding the
weight and complexity) on active vibration cancellation methods, replace the
aluminum tubes with carbon fiber tubes.
For the same weight as aluminum, you'll get much stiffer tubes . They will
cost more than aluminum but far less than the active devices and will not
add any mass.
Please note that I said for the same weight as your current thinner aluminum
poles .
Carbon/epoxy would be around 60% the density of aluminum , therefore
allowing a tube that's 60% larger in diameter for the same mass. So 1.6
times larger diameter, plus modulus that's a lot higher, results in much
stiffer poles.
If you want to improve something, replacing the aluminum poles is THE place
to start IMHO.
A second option is make the thing as fast as you dare and then some. Buy a
Paracorr or the much cheaper Baader coma corrector . I'd rather deal with a
more exact collimation mechanism than with a long vibrating undersized
structure.

best regards,
matt tudor

-----Original Message-----
From: Ross Sackett <rsackett00@yahoo.com>
To: atm@atmlist.net <atm@atmlist.net>
Date: Sunday, August 14, 2005 2:32 PM
Subject: Re: [ATM] Active damping (and archery)


>--- 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
>
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