From: "Ed Erbeck Jr." 
To: Roger Chisholm-Batten , atm 
Reply-To: "Ed Erbeck Jr." 


> Roger wrote and asked:

  and I'll answer as best I can between the lines
>
> One matter that I have not seen properly explained in the archives is that as
> a result of the basic Dobsonian design, whilst the altitude turning force
> remains equal wherever the scope is pointing, the rotational or azimuth force
> required when pushing the end of the scope varies from minimal when pointing
> at the horizon to increasingly difficult when nearing the zenith.  Moving the
> tube alone near the zenith tends to topple over the scope rather than rotate
> the tube.

     There are two things going on here 1. Simple leverage - a long "pole"
around a pivot point will always move easier that one pointed straight (or
almost straight up)
>
> Does one accept this as a design quirk of a Dobsonian or are there
get-arounds
> such as a lever/pole sticking out of the base to provide a suitable turning
> force or is Ebony Star & Teflon so malleable that the effect disappears?
> Surely Not?
>
> I noted this a few weeks ago just after my 'First Light' (alas not kosher
> Ebony Star but definitely good grade Teflon).  Looking at Saturn was a joy
> whilst Jupiter near the zenith became a pain after a while.
>
> Roger

    Now for #2. The Center Pivot point gets ignored as a part of a System.
Remember the whole thing has to move and if any part is binding, well the
rest will not work as well as it could. Personally I like Bronze Bushings
for Center Pivot "Bearings". They stand up well to the wear from
the Threads on the Pivot Bolt and there is No maintenance. Also adding a
Center piece of Teflon and shimming it will reduce the force required to
rotate the Scope.
   I built an 18" F5.5 Dob and do not have any tipping problem at Zenith.
   Hope this helps a little.

         Crazy Ed

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