Robert Preston wrote:
>
> I discovered a quick-and-dirty way to reduce 16" mount vibrations
> last night (saw the vibrations subject line while cleaning out my
> mailbox). The Science Center's pier-mounted 16" is located on
> a fifth floor balcony, where it picks up building vibrations that
> make higher-than-100X difficult to use except when the building is
> closed and no elevators are running. When I pulled downward on the
> housing of the star diagonal with about five pounds force with my
> finger, the vibration amplitude was reduced by at least 95%. The
> main use of the scope at the moment is to show planets or lunar
> features, visually, to visitors at the Center. I think it might be
> possible to solve the vibration problem more practically (for visual
> observing, anyway) by hooking the diagonal to the base of the pier
> with a bungee-cord-and-rope combination that has an appropriate spring
> constant. Or maybe by suspending a five-pound weight on a bungee cord
> on the diagonal to simulate finger pressure.
>
> If it works, I'll send pictures of the device to mapug.com.

You are very quick and alert about this finger damping phenomenon.
When you use a finger to stabilize the vibrations, you are actually
applying damping to the scope. That is, your finger and arm are
absorbing some of the vibrational energy because they are spongy
reactive elements.

Mechanical schemes which do this are called passive vibration dampers
and are used on everything from motors and engines to buildings.

A very spongy, not springy, bungee cord will indeed do this sort of
damping. The problem with passive dampers for moving objects, like
the telescope OTA, is that it is hard to find a mass against which the
damper works. Inertial dampers consist of a mushy spring, that is a
spring with damping, and a mass. The first order damping system is
set to resonate at the same frequency as the scope mount. For the LX
200 fork assembly this is in the neighborhood of 8 to 16 Hz.

I would like to see some people experiment with this scheme. and
report their results in a systematic way. A weight on soft rubber
bands or on a soft bungee cord might be a good starting point. I
might give it a try when time allows.


> This brings to mind the possibility of a dynamic anti-vibration unit.
> I realize one of these is already marketed by SBIG under the name
> "adaptive optics" unit. But maybe an electromagnetic inertial device
> mounted on top of the forks, using feedback from a CCD autoguider,
> would be cheaper than movable mirrors controlled by an autoguider.
> Sort of an automated finger tuned to the vibrations. Doesn't have
> the same panache as a bungee cord on the diagonal, however.

Now you have hit the nail on its head. I happen to have done dynamic
damping systems for the last 20 years. A number of my graduate
students started a company in the early 80s that designed digital
adaptive vibration damping systems. These were applied to a variety
of motor operated things. Large engines in earth moving equipment and
airplanes to mention only two. They are exactly as you describe. You
need an inertial driver and a sensitive pickup (geophone) and the
appropriate electronic circuitry.

I have set up and experimented with such a system on the LX200. It
works well. The problem is that the technology is complex and
costly. It is best applied to earth movers, airplanes and buildings
which cost at lease a quarter million dollars or so.

The system I put on the LX200 would have cost tens of thousands. I
have now retired from this area of research. The system requires a
dedicated microprocessor. Someday, perhaps an affordable system will
appear for modest sized applications. It might be possible to do this
with analog electronics. That would be much less expensive.

> RP
> Pittsburgh PA
>
> P.S. Doc, how about a night-blooming Cereus for your garden? Or
> some of the many other night-blooming species. You could have
> flowers and stars at the same time, if that wouldn't overburden
> your retired heart. ;-)

I had not thought of night bloomers. Very interesting :-)


Doc G

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