SV: ATM Alt Bearing Size

["Nils Olof Carlin" <nilsolof.carlin@swipnet.se>]

Jon,

I think anything between the mirror diameter and twice it will be fine.

(the proportionally largest I've tried is 11" for a 6" f/5 OTA, but it
had a very nice action.) The f/ ratio is of less concern.

Using the same diameters for alt and az bearings, and the common
configuration with the alt teflon pieces 45 deg from vertical, the
total friction of the alt bearings are proportional to sqrt(2) times
their load, so if the weight of the rocker box is about 40% of the
total (OTA+rocker box), the alt and az frictions should be about equal.

I've heard many complaints about too small alt bearings with too little
friction, but I have yet to hear complaints about too large bearings.
Has anyone on the list had trouble with too large alt bearings?

Since building my first Dob mount, my favorite bearing material is a
large, disused aluminium cooking pan. I cut off the side, leaving the
bottom with a 1" rim, then I cut along a diuameter to get two D-shaped
sections. I have covered the rim with laminate or vinyl tape, the kind
of laminate used to dress the cut edges of chipboard may be nice.

To see an example, you can check Jeff Medkeff's page,
http://shutter.vet.ohio-state.edu/saa.htm
and go for the S.A.A Photo Gallery to look for my name. 
Disregarding myself, you can see the 13.1" Dob, with the original 12.5"
aluminium pan (the largest I found at the flea market), each section
fastened to the OTA with 2 wing nuts + bolts. (You can also see the
neat adjustable observing seat, a standard item from Sweden's IKEA
chain for appr $15. With this, I can observe at any altitude sitting
comfortably).

Nils Olof
 
> On a Dob mount, what's a good way to determine the best altitude
bearing 
> diameter for a given aperture and f/#  (8" f/8 in my case)?
> 
> Jon