Re: ATM Side bearing on dobsonian

[Adam Perkins <scopeguru@yahoo.com>]

--- DeBusk <talkalot@intplsrv.net> wrote:
> I've noticed that on most smaller dobs, people are
> using PVC pipe cut to about 1" in length for use as
> side bearing.  Has anyone tried to attach formica to
> plywood circles as an alternative.  At the Lowes
> hardware store, they have 12" plywood circles that
> are 3/4" thick.  How hard would it be to attach
> Ebony Star formica to these?  Would these be too
> large for a 6 or 8" dob?  If so, how small could you
> go and still attach the formica?  Thanks.
> 
> Wayne DeBusk
> 
Wayne,

My 6" f/8 has 16" dia 3/4" plywood bearings  with
Ebony Star surfacing.  Why so large?  

For one thing a large altitude bearing allows a short
& stubby (stable) sideboard.

My real motivation was this...  The beauty of ebony
star and teflon bearings is not so much the
combination's low moving friction, but the similar
amount of force needed to BEGIN moving -vs- the foce
needed to CONTINUE MOVING.  You cannot tolerate a
bearing that has a high static friction, or you push
harder and harder until the "gription" of the bearing
suddenly lets go, overshooting your target and giving
jumpy movement.

Ebony Star and teflon were my bearing choice due to
this issue.  However, this slippery combination is
prone to sliding around if your tube is unbalanced.  

Picture your telescope tube as a lever.  By increasing
the diameter of the alt. bearing, you reduce the
leverage a heavy eyepiece (for instance) has to push
against your teflon pads.  Large side bearings allow
me to use the low starting friction ebony star/teflon
combination without giving so slippery a bearing that
tiny imbalances move the tube.

My large bearings allowed me to fudge many balance
considerations.  I made the rear of my OTA as heavy as
I pleased and the focuser end somewhat lighter (really
high-tech versus some peoples' balance spreadsheets). 
I found the balance point, mounted my bearings there,
and have never worried about balance problems since. 
I got close enough on the first try to not need any
permanent counterweights.  

I don't really understand the tiny bearings used on so
many commercial dobs.  I can't recommend larger
bearings highly enough.

I can switch eyepieces without any worries without
counterweights.  The motion in the eyepiece is smooth
and easily controlled.

Try it yourself and see...


Adam Perkins
Covington, Louisiana




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