[ATM] Public Terrestrial Telescope: Focuser Materials & Lubricants?

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

Hi again Folks:

Continuing with my public terrestrial telescope project, I'm now in the
process of deciding what materials and lubricants to use for the focuser
mechanism. To clarify, I'll machine the main focuser tube from stainless
steel. If I make it a helical focuser, it will screw into and out-of a large
threaded female nut. If I make it a slide-focuser or a rack-and-pinion, it
will slide into and out-of of a female bushing.

If I make the nut or bushing from brass (a traditional choice), it will
require a lubricant like a grease, but there's a problem with using
something that has to be greased because this telescope will be mounted
permanently outdoors. As you know, on a commercially-made rack-and-pinion
focuser, there's always a little grease on the exposed parts of the
rack-gear. In a telescope that's stored indoors or an observatory dome,
that's o.k. But in an outdoor environment, any exposed-and-greased parts
will attract dirt and dust that will transfer to the rest of the mechanism
as the focuser it used, and that's not good. A few months of exposure to the
elements will render the grease LESS like grease, and more like
valve-grinding compound.

A bushing or nut made of a self-lubricating material would be ideal, and
Delrin (aka acetal) immediately comes to mind. But when I do a Google search
on the properties of Delrin, I find some references to the effect that it
shrinks with age. In particular, I found some comments by radio-controlled
model car enthusiasts complaining that some of the Delrin gears in their
cars had been press-fit onto aluminum hubs, and that, as the Delrin aged, it
shrank and split. Assuming that what they say is true, that would eliminate
Delrin.

Of course there's Teflon. But Teflon is expensive, it's very soft, and when
stressed, it permanetly deforms, so I'm guessing that a Teflon bushing would
gradually loosen with time.

Then there's HDPE (high density polyethylene). As a "poor man's Teflon",
HDPE has good self-lubricating properties, and it's tougher than Teflon.
Might HDPE be the best choice? Might there be something even better" What do
you guys think. All suggestions will be appreciated.

Dave Sleeter/Moreno Valley, CA






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