Re: ATM balance point in a dob

[Michael Lindner <mikell@optonline.net>]

On Fri, 15 Jun 2001 21:59:03 -0400 (EDT), CharlesMitchard@aol.com said:

> Greetings all,
>  I've just completed renovating a 6" f8 dob.
>  I built a bearing box to clamp to the ota with what I thought was the bearing 
>  axis aligned with the ota centre axis.
>  It balances with the heaviest eyepiece in but only between 20' from 
>  horizontal to within 15' of vertical, ie outside of these elavations it 
>  either continues to vertical or drops to horizontal.

A scope needs to be balanced not only from the front of the tube to the back,
but also from the "top side" of the tube to the "bottom side" of the tube (i.e.
if the tube is horizontal, the top ad bottom, respectively). To look at it
another way, consider the ota on its side. It should be balanced so that it
hangs more or less horizontal if suspended by a single string through the
centers of the altitude bearings.

It sounds like your scope has its center of mass (i.e. balance point) above and
in front of the altitude bearings (with the scope horizontal). When the scope
is 20 degrees to 75 degrees from horizontal, that balance point is close enough
to being directly above the centers of the altitude bearings that the scoe
stays put. Any higher or lower and the center of balance is enough off from the
center of the bearings that the scope moves. In rough ASCII art...

   +------------------------------------------+
   |					     \|
   |			       B	     ||
   |				 A	     || <-- mirror
   |					     ||
   |					     /|
   +------------------------------------------+
   
   A = center of altitude bearings
   B = center of balance of scope

You can try a counterweight on on the bottom of the tube behind the altitude
bearings, or move the bearings up and forward.

-- 
Michael Lindner
http://home.att.net/~mikel
http://www.starastronomy.org
http://www.atmsite.org