Re: wind and spiders

[Chuck Dethloff <dethloff@uofport.edu>]

On Thu, 12 Oct 1995, Mel Bartels wrote:

> 2) no one has mentioned this effect yet so I will through it out to see if
> anybody else is seeing this: in the past year, I have gone around to several
> gigantic scopes and observed that the diagonal shakes in moderate wind.
> This is a high frequency shaking, that is stopped by reaching in and
> touching the diagonal holder with the tip of a finger.  The diagonal shakes
> in a rotational manner.  These giant spiders cannot be tightened enough
> without pulling the upper end into distortions, and consequently, the
> diagonal is pretty easy to twist back and forth by hand.  The wind catches
> these big heavy diagonals and sets them to vibrating back and forth.  In a
> continuous wind, this vibration is pretty constant and ruins high power
> observing.

This is a problem that many large scopes including my own 24" suffers 
from occasionally. I find when the wind holds steady at speeds much more 
than 10 mph vibrations induced by it become a factor. My scope when 
placed on a firm surface will dampen out within 1 to 1.5 seconds when 
given the Tom Clark "thunk test". These numbers are standard for any well 
made Dobsonian. But when it is windy enough the dampening time will 
approach infinity!

How tight you can get the spider perhaps depends upon design. I find that 
using tubes with threaded inserts, when bolted with machine screws to two 
plywood rings, will allow for considerable tension to be applied. In fact 
you can achieve enough tension that you can literally pull the spider 
apart. I admit to haveing done this on two occasions! It is not a bad 
idea to use a medium strength anaerobic (sp??) glue such as lock-tight 
once the spider is correctly positioned in the upper asembly (cage). You 
can break the bond if needed, but it will not allow drift due to bumpy roads.

The rotational vibration is generated by the wind hitting the flat sides 
of a conventional spider and generating varying moments. It would seem 
that the better protected your spider is from the wind by your cage the 
less direct effect to your spider. You still have the bouncing OTA to 
deal with though (sigh).

I have found that a thin shock absorbing material, when placed under 
the 4 teflon pads that support the altitude bearings, can also be 
helpful. In scopes from 8" to 30" you can cut the dampening time in about 
half with the addition of these pads. Ace hardware sells small sheets of 
gasket rubber that works well. Make sure you get the thinner of the two 
sizes offered, it is around 1mm I think, and red in color. Sorbothane 
works very well as well. I use etched teflon and glue the rubber to the 
teflon and in turn glue the teflon/rubber pad combination to the rocker 
box bearing supports. It is important to note that thick or spongy 
materials will actually make the situation worse, as they will cause 
back-lash when you move the scope. I am sure other things would work and 
would love to hear from  you if you have any suggestions.

Chuck Dethloff
Forest Grove, OR
<dethloff@uofport.edu>