November 14, 2010:  Refiguring a 14"

A 14" mirror came in for refiguring recently.

I'm trying to show some examples of what I typically see, and this mirror, though not the greatest, was fairly straightforward to fix.  Yet another undercorrected mirror, the method to fix it is simple - add correction.

Minimum peak-to-valley

However, that view is somewhat optimistic - it minimizes the peak-to-valley error, but in general it is difficult to remove the "high" area near the edge of the mirror.  To try to do so could turn the edge, roll the outer zone, or both.  So, we can adjust the radius of curvature.  This changes the parabola that we are trying to match, and this causes the center of the mirror to rise, while the outer zones flatten out.

Flattening the edge

Thus, there is more polishing to do, but the result will be a better mirror.  From this point, I can carefully polish away the glass inside the edge of the mirror to produce a superb figure that is good to the edge.

I also tested the secondary, and found an approximately 1/4-wave rolloff on one end (photo below, left) and some convexity.  After refiguring, much of the slope error was eliminated (photo below, right).

Secondary beforeSecondary after

So how did the primary turn out?  Let's just say the remaining error on the primary was difficult to measure - I measured better than a 1/20th wave wavefront error, a transverse error of less than one, and a Strehl of 0.99.  Figure of revolution testing showed a very good figure of revolution.  While measurements are not perfect, together these all indicate a VERY good mirror.

These numbers are based on an actual set of measurements, so they are not perfect, but they are also not up for debate.

Not convinced by my testing?  Here's what the owner said about the mirror after refiguring:

"OK, it was two below zero and I got the scope out.  The results confirm my belief that no one should observe at temperatures below 20F :).  My hands got really frost bitten, I cursed metal and its low temperature properties, but in the end, the mirrors look pretty darn good.  Star images are definitely tighter than they were before refiguring but the real test came from the brilliant moon."

"Mike, I have seen some darn good views in the past of the moon in scopes as large as 30 inches, but what I saw tonight was totally unbelievable!"

"When the seeing finally settled and the mirrors cooled down enough to use, the scope handled from 527x to **836x** on the moon (*59.7x per inch* of aperture) which is as high as I can go!!!"

"I was seeing things I have *never* seen on the moon. Hadley Rille's area showed incredible detail (the 2 km wide shallow craterlet St. George was visible at the turn of the rille at the base of the Hadley Delta mountain).  There were a huge number of tiny secondary impact craters from Thebit to its west on the nearby maria that I have never noted in images, let alone seen visually...."

"....I won't be getting the scope back out until things warm up a lot (and much of our snow melts), but for now, I am pretty darn satisfied as to what kind of optics I have.  Thanks again for picking up (the other company's) fumble and running it back for a touchdown."

I think that says more than any number or set of numbers possibly could.

Please check back for future installements of "In the Shop".

Mike Lockwood
Lockwood Custom Optics

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