Re: [ATM] data and musings on thin mirror

[Michael Peck <mpeck1@ix.netcom.com>]

At 16:27 2/3/06, Stephen Koehler wrote:
>ATM and atm_free,
>
>
>The next graphic gives a summary of test stand (left) and mirror
>(center) aberrations.  The mirror is an average of all nine
>orientations rotated to line up.  Yikes!  There are 6.8 waves P-V of
>error when spherical aberration is included.  The right frame shows
>mirror aberrations with SA removed.  On this mirror, primary
>astigmatism is large, but SA is larger.
>
>http://www.visi.com/~mkoehler/eason_davis_misc/davis_summary.png

Steve, or Dale:

This was a 16" f/3.45 mirror, is that about right? Which would make 
for an expected spherical aberration of about 7.6 waves at 633nm. I 
assume then that it's 6.8 waves *undercorrected*, or conversely 0.8 
waves overcorrected relative to a sphere?

>One interesting point to take away from this is that at its best,
>Dale's cell angled at 35 degrees from vertical imparted at least 1.5
>waves P-V of error, which is huge.  It seems likely that the large
>error is due to some aspect of the edge support pegs, not the back
>support.  The lesson here is that very thin mirrors are tricky to
>support properly.

Do you suppose there are other lessons for this mirror maker or 
mirror makers in general? I haven't followed these ultrathin mirror 
project threads too closely, but it looks like he took a whack at 
figuring, got about 10% of the way there, in the process polished in 
about 1 1/2 waves of astigmatism (which took heroic amounts of effort 
to detect), and decided that was good enough to send off for coating.

I hope that's not too insulting a summary. I have to wonder if even 
an experienced and talented mirror maker could make a success of a 
project like this.

Mike


_________________

Michael Peck
email mpeck1@ix.netcom.com
Wildlife photoblog http://wildlife-pix.com
Amateur telescope making http://home.earthlink.net/~mlpeck54/astro/astro.html 

_______________________________________________
ATM mailing list http://www.atmlist.net/