ATM A Beral Coating

[Roger Ceragioli <rogerc@dakotacom.net>]

I recently had my 6" ULE flat that's destined for my folded refractor
coated by Clausing with beral.  While it's quite true that I foolishly
rushed through making the flat and did a less than excellent job, leaving a
large number of small sleaks on the surface, when I examined the wavefront
before coating, it looked pretty smooth.  But when I reexamined it after
coating, I noticed some annoying streaks that looked like dust.  They
weren't dust, but are apparently caused by the coating: they are linear
formations such as I've never seen before, whereas I finished the piece on
a moving spindle.  The spindle left some light circular zones on the ULE,
but couldn't really leave straight chordal streaks.

It will be interesting to see whether the diffraction disk or focused image
in the finished scope shows any signs of the less-than-optimum coating.  I
suspect not, since the streaks are light and the double-pass R-C
arrangement makes even trivial surface deviations look like the Grand
Canyon.  But the light scatter will be increased, alas.

Roger Ceragioli