RE: [ATM] Scaling wavefront error

["James Mulherin" <jcmulherin@opticalmechanics.com>]

Mark

I just joined the list a few days ago on Steve Koehler's recommendation.
I'll go back and review the posts on this thread to make sure I am on the
same page. In the mean time...

> Thanks so much for your posting to the atm mailing list regarding 
> wavelength scaling of mirror test results,  I just posted a version of 
> this question a few days ago, because I was looking for a set of 
> reliable rms rating data on good mirrors to use for establishing 
> realistic designs goals for mirror cell induced deformations.

I've participated in mirror cell discussion on other groups in the recent
past, mostly related to edge support issues. As you may know, the most
significant mirror cell deformations are due to the edge supports when the
scope is pointed near the horizon. After some discussion and trial and error
I think most of the participants settled on a cable sling support, carefully
located in the plane of the mirror's center of mass as a practical solution.
There are other solutions but the cable sling is pretty simple and easy to
implement in most Dob applications.

Otherwise, I think it's hard to screw up the back supports to the point
where they would have a detrimental effect on image quality. There are some
good shareware programs out there that do a good job of figuring out where
to place the back supports. If you're using a very thin blank you have to
take care with the back supports but in any case, the edge support is
probably the most critical issue.

> I was going to use the data posted on your web site for this purpose, 
> but did not know what wavelength your measurements referenced.  I 
> suppose I should have just asked you directly.
> 
> Should I conclude that the data currently at the web site is 
> referenced to 633nM?  Will new data be referenced to 550nM?

Yes, all of these measurements were taken and reported at 633 nm. We have
several data sets yet to post that use the same format. We also have quite a
few data sets to post that are measured at 532 nm and will be scaled and
reported at 550 nm. I'm waiting on our web developer to set things up so I
can post the new format to our web site. Things are a bit backed up right
now. [I'm not a programmer.]

How will one tell the
> difference for data posted on the web site?

I'll have our web guy add a note about test/report wavelength to the data.
Once I get some posted I'll let you know. 

So, you should know that what we see in out tests is an extreme case where
the mirror is hanging on it's edge in a cable sling. It can't get any worse
than this. If you're interested I can give you some info on the types and
magnitudes of test sling errors we see and how we deal with them when
analyzing the test data. FYI, I have data for mirrors from 12.5" to 30"
diameter, all 2" thick. 

James

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