Re: [ATM] data and musings on thin mirror

[Dale Eason <atmpob@yahoo.com>]

Mike,
“took a wack at it”  you cut right to the chase.  Yes
I have been treading lightly around that issue.  I
wanted others to come to the same conclusion I did.

You are the first to pick up on this and post.  Yes
indeed that mirror maker has a lot to learn about
testing.  I asked David before I published this if he
was ok with some of the criticism and ridicule he may
get from posting the analysis.  He said “let it role”.
 He hopes we can all learn something useful about
ultra-thin mirrors.  Unfortunately his effort does not
tell us much about if it possible to succeed.  

I also asked him if I could describe his testing
effort.  He said yes.

He uses a 50 line ronchi to scan for smoothness and
zonyness.  Then he uses the ronchi at ROC as a
Foucault test.  He looks for "shadows to look right". 
He thought this one looked too deep so he thought it
was a little overcorrected. He has a few other
criteria but no different test setup.   Then he sent
it to the coaters.  He also does what he calls “star
testing”.  He has made several mirrors and star tested
each one but he told me he has never seen any rings
when doing that.  He uses what he thought Dobson
described as star testing.  

David is on these lists I will leave it to him to
explain why he was willing to accept the magnitude of
error he did.  When the mirror came back from the
coater he thought the images looked a little soft and
was not happy with the blobby stars.  He only got to
use it a few hrs one or two nights. 

So your analysis “he took a wack at it” pretty much
sums it up.  This is a slumped mirror so the astig
might have been slumped into it by the form.  

This mirror is about the same thickness ratio as the
30 x 1 being done in Hong Kong.  The director of that
project stated that he could see no asymmetry in the
Foucault image so he knew there was no astig in his
mirror.  We have discussed Foucault’s inability to see
astig using that technique before.  I have Foucault
images of David’s mirror and they too are very
symmetrical left to right.  What the 30 x 1 maker did
not notice but I did on his Foucault image is there is
a shift top to bottom of the shadows.  According to
what I have seen using simulated Foucault images that
is a sign of astig.  It could be induced by the test
stand.  It could be on the mirror you can’t tell.  But
it is there in that image.   At least the Hong Kong
group is doing some testing that can measure
correction accurately.


Lessons learned.
1. You have to measure accurately with methods
sensitive enough for the job.
2. Thin mirrors are very hard to measure accurately.
3. Thin mirrors are very hard to support to the
precision required.  See upcoming post.

Finally in my opinion
4. Ultra thin mirrors are very hard to make.  If they
were easy everyone would already be doing it.  No one
wants to lug around a heavy mirror that takes a long
time to reach thermal equilibrium .

Anyone else is welcome to add their observations.  I
want to use the mirror to measure deformation caused
by lateral supports.  PLOP does not help us there.  I
hope I can add to the knowledge base.  Perhaps I
should find a copy of Wilson before wasting my time on
already known issues.

Dale Eason

--- In atm_free@yahoogroups.com, Michael Peck
<mpeck1@...> wrote:
>
> 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@...
> Wildlife photoblog http://wildlife-pix.com
> Amateur telescope making
http://home.earthlink.net/~mlpeck54/astro/astro.html
>


--- Michael Peck <mpeck1@ix.netcom.com> wrote:

> 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/
> 


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