From: "John Sherman" 
To: 
Reply-To: "John Sherman" 






Hi All,

>Is this map showing the wavefront?
>www.johnspics.com/hi/hudek.jpg

Yes. Divide by two for surface error.

> Is a turned down edge visible?

No. The Interferometer only reaches out to about the 97% zone. The image
shows a drastic rolled off edge area, also known as a TDEZ (turned down
edge zone). This would be the outermost zones, 80 - 95%, and by inference
you can imagine that the extreme edge is also turned down. I automatically
blacked out the very edge, the imagined TDE, before putting it in the
scope. And I still had no rings inside focus.

>The shame about this mirror ( in its original form ) is that most of the
>error is just pure overcorrection ....1.6 waves  in fact,

An assessment agreed upon by almost everyone.

>Another 30 minutes polishing down the 70% zone would have produced
>a much finer result

You might think that when he refigured the mirror that that's what he'd've
done. Yet the second incarnation included a large, 8" diameter hill in
the center, surrounded by a deep trough, and the TDEZ remained. The P-V
error remained about the same. It seemed that he just moved the errors
around without curing them. Because I still had no rings inside focus.
Stopping the mirror down to 20" reduced the spikes, and a faint set of
rings began to be visible through the fuzzball. And the images were a bit
sharper, but still poor. He said that is one of the best mirrors he ever
made, and I suppose I believe him....

John

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