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.

Minimum peak-to-valley

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

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