[ATM] Bearing smoothness limit

["Don Clement" <don_clement@charter.net>]

Andy Saulietis wrote in The standard aluminium coating: How much scatter?:

" I once had the opportunity to examine an aluminized but not overcoated
 mirror with a scanning electron microscope. The surface appeared like
 a cobblestone street, with facets of various numbers of sides. The tops of
 the facets were convex, with a curvature about 1/10 the width of the facet.
 The facets were a few angstroms in size, so should not contribute much
 scatter unless they were regularly spaced, so they could possibly act
 like a reflective diffraction grating. This was just one sample, not from a
telescope mirror."



If the surface of an aluminized highly polished mirror looks like "a
cobblestone street" in a scanning electron microscope then what does the
surface of precision ball bearing race look like? There is a limit to how
smooth  mechanical movement that depends on sliding or rolling surfaces can
be. Flexural bearings don't seem to have the same limits on how smooth
mechanical movement can be. What happens when sliding or rolling bearing
surfaces are exposed to grit and dust contamination, as would be the case
when hauling one's scope with exposed bearing surfaces out to an outdoor
dark location? The grit covered fields up at Camp Oakes  RTMC  are an
extreme example. How many precision mechanical telescope movments are not as
precision after being exhibited at RTMC?



Alexander Slocum "Precision Machine Design" p521:

 "Sliding, rolling, and fluid film bearings all rely on some form of
mechanical or fluid contact to maintain the distance between two objects
while allowing for relative motion between them. Flexural bearings (also
called flexural pivots), on the other hand, rely on the stretching of atomic
bonds during elastic motion to attain smooth motion. Since there are
millions of planes of atoms in a typical flexural bearing, an averaging
effect is produced that allows flexural bearings to achieve atomically
smooth motion. For example, flexural bearings allow the tip of a scanning
tunneling microscope to scan the surface of a sample with subatomic
resolution*

*See, for example, G. Binning and H. Rohrer, "Scanning Electron Microscopy",
Helv. Phys. Acta, Vol. 55, 1982.pp 726-735





Don Clement

Running Springs, California
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