January 3,
2011: Refiguring
a 24"
A 24" mirror came in for refiguring recently.
This one was just plain bad - ~400 nm overcorrected on the glass, for
nearly 1.5 waves of error on the wavefront. Yuck.
There
is simply no excuse for this amount of error. The engraving
on
the side of the mirror clearly said "Classical Cassegrain primary", so
a parabola was intended, but perhaps the primary was matched to a
particular secondary. As usual, I won't identify the source,
but
let's just say I expected more given the reputation of the shop.
The
piece of good news was that the elliptical flat was reasonably good,
and I sent it back to its owner before the primary was shipped to the
coater.
This mirror was a conical with a central hole, which was
mis-centered by more than 1/16" - this means the owner must be careful
when using a laser to collimate the primary, because the center of the
post is not in the center of the mirror!
I guess the glass
machinist was having a bad day. Fortunately the blank showed
only
a little strain around the central hole, which is normal, and not
really an issue.
Following refiguring, let's just say the mirror measures more than ten
times more accurate than when I received it.
I
look forward to hearing from the owner when he gets the scope back
together, and it better be available for use at the Okie-Tex Star Party
later this year or he will have some explaining to do.... :)
Please check back for future installements of "In
the Shop".
Mike
Lockwood
Lockwood Custom Optics
