Re: ATM Bearings

[Adam Perkins <scopeguru@yahoo.com>]

--- Tim Dolan <tdolan@javanet.com> wrote:

> Can anybody advise me on what size altitude bearings
> to use on a 25-30
> lb. dobnewt? The tube is about 5' long and the
> bearings will be
> teflon/Ebony Star. 


Tim,

I have 16" dia. altitude bearings on my 6" f/8 dob. 

The big bearings allow a shorter than usual rocker box for less
flexure.

The loss of mechanical advantage (i.e.- the ratio of tube length /
bearing radius is larger with my 16" dia bearings vs. your 7" dia.
bearings) means that the bearings are harder to rotate.  On a small
scope like mine, this is no big deal, you don't necessarily have to
have very low overall friction.  VERY low friction bearings will only
lead to balance headaches, my scope balances well with all of my
eyepieces.  What you want is similar STARTING friction and MOVING
friction, this is determined mostly by you choice of bearing material
(teflon and Ebony Star in my scope).

By using large diameters for your alt. bearings, you make it easier to
match their friction to your Az. bearing (also about 16" in my scope). 
Having equal resistace in alt. and az. is what a good Dob is all about.

I built the most well-behaved 6" dob I've seen yet.  There's a reason
ALL of the bigger dobs have such big alt. bearings.  There is a great
discussion of this and other considerations in Kriege and Berry's book.

Good luck,

Adam Perkins
Covington, Louisiana


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