November 25,
2013: Bad luck all the way around All text and images Copyright Michael E. Lockwood, all rights reserved.
I receive a lot
of mirrors for testing and possible refiguring. Recently I
received a 12.5" mirror and an elliptical flat for testing.
Usually one or the other has an issue, but can be fixed.
In this case, though, the prognosis for both mirrors was poor.
The primary mirror, even with the coating stripped, had a residue on
the optical surface, likely due to etching of the glass and possible
chemical damage.
There is a pattern in the etching that could be from a paper towel,
cloth, or something else that may have been left on the surface of the
mirror.
The moral of the story here is don't store a mirror without cleaning
it, and keep it in a dry place. Also be very careful about
what you use to protect the coating - acid free paper is what I
recommend.
As it turns out, the anneal of the mirror was a bit suspect, too, as
seen at left below. The dark "cross" pattern is due to strain,
and the other wavy "stuff" seen within the glass are striae, or areas
of glass that aren't quite the same as other areas of glass, something
that is not desirable.
Seen below at right is the interferogram of the flat that came for
testing with the mirror - it is many waves from flat, and has a rough
surface, possibly due to the same type of damage that the primary
experienced.
Bottom line, all of the news for this optical set was bad.
Please check back for future installments of "In
the Shop".
Mike
Lockwood
Lockwood Custom Optics
