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

[Dale Eason <atmpob@yahoo.com>]

Thank you Nils Olof,

I stand corrected.

Dale Eason
--- Nils Olof Carlin <nilsolof.carlin@telia.com>
wrote:

> 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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