Re: [ATM] WAS: RE: ATM Digest, Vol 30, Issue 21; IS: FEA models of test stand induced deflections

[Dale Eason <atmpob@yahoo.com>]

I can read any format as long as I know what it is, so
don't work too hard on my specific suggestions.

I have measured three mirrors that show test stand
induced astig.  Steve Koehler's 18 x 2, My 16 x 1 F4.8
, and David Davis' slumped 3/8 x 16. 


However, using these to compare to the model could be
problematic.  I almost never measured them balanced on
thier edges.  So there was always some back support. 
I did not record detailed posistions of the back
support.  In most cases I was not able to keep the
stand astig constant between rotations so I could
never really quantify it.

What I found was that it was next to impossible to
apply lateral support exactly at the center of gravity
of the mirror edge.  The latteral support always cased
the edge to rise or be pulled down depending on where
the support pressed in relation to the COG.

The easiest mirror to balance is my 16 x 1.  I could
experiment with that.  In fact it is all I have now in
thin mirrors.

My test stand has two pegs 90 deg apart.


Dale Eason

--- Scott Milligan <starzkey@charter.net> wrote:

> Dale:
> 
> Ok, great.  I will probably need to massage the raw
> output from ALGOR a bit
> to fit one of your desired formats, but I expect
> that Either Excel or
> MathCad can help me with that task.  If you have a
> candidate for a first
> experiment that you would like to investigate, just
> send me the relevant
> information describing the situation and I will
> proceed from there. 
> 
> 
> Scott Milligan
>  
>  
> 
> -----Original Message-----
> From: Dale Eason [mailto:atmpob@yahoo.com] 
> Sent: Saturday, June 17, 2006 11:11 PM
> To: Scott Milligan; 'Stephen Koehler'; 'Wolfgang
> Rohr'
> Cc: atm@atmlist.net
> Subject: Re: [ATM] WAS: RE: ATM Digest, Vol 30,
> Issue 21; IS: FEA models of
> test stand induced deflections
> 
> Scott,
> I can give you several options.
> 
> If you can output the data as the a retangular array
> with the mirrror just
> touching the sides I can import it into OpenFringe
> and display it and
> compute Zernike constants for it.
> 
> Exact format should be one number per line.  The
> first two numbers are the
> row and column counts.  Numbers should be in ascii.
> 
> For example
> 100
> 100
> 1.2
> 2.2
> 
> 
> Would be the first two numbers of row 1 of a 100 x
> 100 array.
> 
> It would help if the numbers were in wavelengths but
> the need not be.  You
> can choose whatever metric you want for them and I
> can accomodate.
> 
> If instead you can only ouput x,y,z version of the
> data inside the mirror, I
> can work with that as well.
> 
> Dale Eason
> 
> 
> --- Scott Milligan <starzkey@charter.net> wrote:
> 
> > Folks:
> > 
> > I own a licensed copy of the ALGOR FEA program,
> and have learned just 
> > enough about using it to generate useful models of
> mirror self-weight 
> > deflection under various support schemes.  I would
> be willing to run a 
> > few test cases to see if these models help
> corroborate what has been 
> > observed on the test stand.
> > 
> > The ALGOR model will give the surface displacement
> values at the nodes 
> > of the mesh; to make these results truly useful,
> however, I suspect 
> > one wants to fit these data to a reasonable set of
> Zernike 
> > polynomials, and do the usual subtraction of
> piston, tilt, & defocus 
> > before reaching conclusions regarding the
> suitability of a given 
> > mounting scheme.  I haven't myself gotten around
> to writing such a 
> > routine (I'm sure that MathCad, R, or similar
> package could do it), 
> > but that is also an area where others in this
> group are strong.
> > 
> > If a routine that resamples the (typically)
> irregular spacing of the 
> > FEA nodes onto a regular grid were to become
> available, then the raw 
> > displacement data can be input to a Zemax model,
> and the usual suite 
> > of image quality analyses then become available. 
> Zemax can also 
> > accept Zernike input, but resampling the raw data
> to a uniform grid 
> > should, I expect, improve the accuracy of the
> Zernike fit as well.
> > 
> > If we go forward with this, what I would need to
> construct successful 
> > FEA models would be something like the following:
> > 
> > 1. Mirror substrate material (Pyrex, Silica, etc.)
> 2. Diameter, ROC, 
> > and thickness.
> > 3. Support scheme description (i.e. 2 pegs @ 90
> degrees, stud through 
> > a central perforation, Sling, etc).
> > 4. An initial assumption describing how the
> support structure actually 
> > establishes contact (i.e. single point in COG
> plane, line contact, 
> > etc.) 5. Orientation of gravity vector wrt to
> surface normal at mirror 
> > vertex.
> > 6. If gravity vector is oriented other than
> orthogonal to surface 
> > normal, then some description of the backside
> support is necessary as 
> > well (i.e. # of support points and their
> locations).
> > 
> > Models with simple geometry (i.e. revolved solids)
> are considerably 
> > less time consuming to set up and debug than are
> models containing 
> > intricate features such as lightweighting ribs,
> socket inserts, etc.  
> > So I suggest we start with something simple.  An
> early lesson learned 
> > about FEA is that simple, "known" cases provide a
> necessary sanity 
> > check on the assumptions used to create the model.
> > 
> > I have successfully executed several models using
> peg mount and center 
> > support mounts, with what I feel is good
> qualitative agreement to my 
> > own interferometric results, but have not yet
> tried to model anything 
> > like a sling.  So if rapid progress counts for
> anything, I suggest not 
> > starting with a sling mount.
> > 
> > Cheers,
> > 
> > 
> > Scott Milligan
> >  
> >  
> > 
> > -----Original Message-----
> > From: atm-bounces@atmlist.net
> > [mailto:atm-bounces@atmlist.net] On Behalf Of
> Stephen Koehler
> > Sent: Friday, June 16, 2006 5:17 PM
> > To: Wolfgang Rohr
> > Cc: atm@atmlist.net
> > Subject: Re: [ATM] ATM Digest, Vol 30, Issue 21
> > 
> > Wolfgang,
> > 
> > > you can help me:
> > > In Germany we had a mirror testing project. 16
> > persons were testing
> > > one astigmatic mirror and the main problem was
> the
> > support of that
> > > astigmatic mirror. The problem is, how we cause
> > any kind of
> > > astigmatism by the support. You can subtract the
> > induced astigmatism
> > > if you can calculate the amount of it. And
> > therefore we need a program
> > > similar like Plop but to calculate the lateral
> > pressure on a mirror. 
> > > Is there anybody who can help?
> > 
> > This seems to directly pertain to my recent post
> to the interferometry 
> > group, where I describe a new (to me) algorithm
> for separating mirror 
> > from test stand aberrations from Zernike
> coefficients from the mirror 
> > at various angles of rotation.  My current
> algorithm works with an 
> > arbitrary amount of rotational data, but can use
> as few as two 
> > rotations.  The rotation angles are arbitrary,
> although some 
> > combinations are not very useful (e.g., rotation
> of 0 degrees or 180 
> 
=== message truncated ===


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