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

["Nils Olof Carlin" <nilsolof.carlin@telia.com>]

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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