Re: [ATM] How to find Centre of Gravity of an existing telescope

["Donald Good" <donald.good@comcast.net>]

OK, Nils;  How about this one:
You are given an metal OTA 6" F8 with a thin (light weight) mirror that has
a real nice figure (strehl .98).  It used to be mounted on a nice sturdy
GEM, but the previous owner kept the mount for a larger scope.  So you
decide to use it as a dobsonian and build a rocker box and ground board.
Because the OTA is metal and the mirror is light, the OTA COG is near the
middle.  To compensate, the rocker box needs to be taller than normal for a
dob, plus a little more since you plan to put in a battery for a dew heater,
an alt-az encoder, and maybe a goto drive system, but you have not done that
yet.  To keep things as light as possible, you make the rocker box as narrow
as practical and since a narrow box is a stronger box, you can also make it
out of thinner, lighter plywood.  You take it out for first light and every
thing works and LOOKS great (that strehl .98 is fine). 

So you take it to the next club star party which you have been rushing to
get it ready for.  You talk to you friend who has one of those low to the
ground ultra-lights where most of the weight is in the mirror and has a very
low COG.  He had been using an equatorial platform for tracking, but
recently installed drive motors, and is raving about how great tracking is.
And since you were talking about installing drive motors, he suggests that
you try his EQ platform which he is not currently using (he might even be
persuaded to sell it cheap to a friend).

Now you have your poor platform axis/scope COG match; a scope with a high
COG on a platform optimally and well designed for a scope with a very low
COG.  There could likely be more than 12 inches between the scope COG
(rocker box and OTA) and the virtual polar axis of the EQ platform.

As the scope is tracking west, the platform is tracking east along with the
narrow base of the rocker box.  So the COG of the scope is actually moving
west with the scope track and the rocker box base is moving in the opposite
direction.  As long as an imaginary vertical line (plumb line) through the
scope COG remains within the area of the rocker box base, the scope will not
spontaneously tip over.  But if the plumb line moves outside that base in
any direction, it will tip over. It will be east-west, since that is the way
the platform tilts, but the scope could be pointed in any azimuth direction
at the time.  Maybe the OTA COG and/or the rocker box COG did not get placed
directly over the rocker box base center of area.  This would create a
azimuth direction where the combined COG is closest to the edge of the base
even when the base is level.  If that direction is line up east or west, the
scope will tip sooner.

It can tip off to the west toward the end of a tracking cycle.  But it could
also tip east while resetting the platform for a new track.

Even if you have checked it in all azimuth directions at furthest tilt both
east and west, do not feel too secure.  At those points it is still
susceptible to an unexpected gust of wind, an accidental bump, or a pull or
push in the wrong direction while aiming the scope on a new target.

So for what ever the reason for finding yourself with a poor COG match, it
should not be ignored.  Your platform drive mechanism will also thank you
for fixing it.

Clear skies,
Don



-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
Nils Olof Carlin
Sent: Sunday, August 26, 2007 6:04 AM
To: Dale Eason; Bruce MacDonald; atm@atmlist.net
Subject: Re: [ATM] How to find Centre of Gravity of an existing telescope

Dale,

Not often do I disagree with you, but this time I'll make an exception ;-)

> While another has answered the question as to how to find the COG they 
> did not answer what happens if your COG does not meet the table 
> conditions.  It will not create an optical alighnment or tracking 
> problem.  It will create a telescope falling off the table or both 
> tipping over problem.


If the GOG is greatly off, the driver will have to push against gravity half
the arc, and pull the other half, and if there is the slightest play in the
drive (such as with the classical threaded rod with slit+pin arrangement -
there are better ways), somewhere there just may be a hysteresis problem.
Using a friction drive (popular with other designs than the classical
Poncet), a too low friction might just let the platform top slide freely -
but common prudence dictates that you limit the platform movement with hard
end stops anyway. Of course, a hard gust of wind may lift the driven end of
a sector for a moment and thus lose friction - but you're not observing in
such a gale, anyway...

However, the idea that a poor match of the COG would make the telescope fall
is a myth I've long tried to combat - if it topples, it will do so along a
line of rotation through two of the three support points on the alt
bearings, OR the 3 (to 4 in some sector designs such as Jan's, though not
classical Poncet) points that the upper platform rests on. A high COG and a
narrow base will make this more likely, but it has nothing to do with the
platform's axis of rotation.

This said, I feel that the mismatch of COG and axis is a minor problem in
practice - if you use the alt trunnion axis as a substitute, other problems
of design will dominate ;-)

best,
Nils Olof 


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