[ATM] Public Terrestrial Telescope: Re: Replies Regarding Focuser Bearing Material

["David Sleeter" <d.sleeter@roadrunner.com>]

Hi again Folks:

And as always, thenks for your replies to my previous post. In that post I
asked for advice on a material that might be used to machine the female
bushing or thread on a focuser that's exposed to the elements. Bob Mohr and
Richard in the UK suggested Nylatron, but I need a solution that would last
for many years, and alas, Nylatron is Nylon-based, and like Delrin, Nylon
shrinks with age.

I've machined many things from Nylon in years past, and I've learned through
trial and error that Nylon undergoes significant shrinkage as it ages. For
example, in a part of one of my businesses I work on microscopes. The black
1960s-70s vintage Nikon Model S scopes use nylon gears pressed onto
stainless steel hubs in their fine-focus mechanisms. In virtually all of
these scopes now, these gears have shrunk and split and separated from their
hubs. These gears are out of production and no longer available, so the fine
focus mechanisms in these Nikon model S scopes are essentiall impossible to
repair.

And Don Clement suggested Turcite, another polymer-based self-lubricating
plastic. But alas, Turcite is made of Delrin and molybdenum sulfide, so it
comes with the shrinkage problems common to Delrin.

BUT I MIGHT HAVE A SOLUTION. It occurred to me last night that, since I'm
going to put a sealed optical window over the objective, I could make the
window fixed; then mount the lens in a sliding cylinder inside the tube,and
focus the scope by moving the lens-cylinder assembly back and forth like the
primary mirror in a Celestron C8. Like a C8, focusing would be accomplished
by turning a thumbscrew on the back of the tube assembly. With the exception
of the small thumbscrew and its O'ring seal and bearing, all moving parts
would be sealed inside, and this would be a GREAT solution to the problem.

My original tube assembly design doesn't have room for the thumbscrew, but
as of this morning, I THINK I can redesign so that a thumbscrew
configuration would work. I'll keep you posted.

David Sleeter/Moreno Valley




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