Re: [ATM] Here is what happens when you warm the edge of a 18 x 2Pyrex mirror

[Dale Eason <atmpob@yahoo.com>]

Several have asked for a little more info about what I
recently posted.  They wanted to know what the mirror
looked like when it was cool.  The following link
should help show that.

http://home.comcast.net/~doeason/coutour_cool_then_heated_edge.jpg

There is no scale on this but the computed strehl is
.922 for the cool and .861 for the edge heated
version.

The surface error is computed using Zernike
polynomials and since only 38 are used there is some
error reflected over 180 from the heating as John
thought might be happening.  That is not real but just
an effect of the approximate solution.

It took the mirror about 30 minutes to cool back down
as far as I can remember.  The mirror was on my test
stand in the basement. 

I did not set out to do this experiment directly so I
did not take a lot of data.  However I think I
remember the mirror warmed from 69 to 70 deg.  So you
are seeing the effect of a 1 deg warming.  I used a
non-contact infra red thermometer to measure it.  What
I was really trying to measure was how was the
orientation of the mirror represented in the results
of my interferometer analysis program.   I needed some
way to make the mirror have a known difference on one
of its edges.  Interferograms tended to hide any
visual marker put on the mirror so I decided to try
heating it a little.

What I was trying to do with the mirror was find a way
to measure its surface and remove any test stand
induced astig.  I have discussed this project in the
past a little on the yahoo atm_free list.  Bottom line
is at 18 x 2 inches thick it is vary flexible and
creates a lot of self weight induced astig when held
horizontal for measurement (compared to its true low
rms surface error.)  I have tried many edge support
schemes but all were extremely dependant on the
balance of the mirror and were not easy to repeat.
That is another story that I can go into if any are
interested.  Note this is mostly a problem when trying
to measure a mirror using an interferometer and not
when using the scope as intended.


Dale Eason

--- John Sherman <atm@mail.johnspics.com> wrote:

> 
> Hi Dale,
> 
> >3D wire frame image 
> >http://home.comcast.net/~doeason/hand_heat.gif 
> 
> Good stuff, thank for sharing it with us. I notice
> that there is a high edge halfway around the mirror
> from the edge you heated. Was that edge there
> initially? Most of the edge looks low. I suspect
> that lower-left edge looks a bit high because the
> software is trying to average the surface into a
> "best-focus". Which would mean, apparently, that the
> heated edge is actually higher than it appears. 
> 
> Thanks,
> 
> John
> 
> 
> 
> 
> 

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

> Good stuff, Dale.   Do you have any data showing how
> the overall RMS error
> changed after heating, or how long it took for the
> figure to relax after the
> heat was initially applied?  
> 
> It looks like Steve K has a pretty nice mirror, (but
> does he ever find time
> to use it in a telescope? Grin)
> 
> Scott Milligan
> 
> 
>  
> 
> -----Original Message-----
> From: atm-bounces@atmlist.net
> [mailto:atm-bounces@atmlist.net] On Behalf Of
> Dale Eason
> Sent: Monday, January 16, 2006 9:18 PM
> To: atm@atmlist.net
> Subject: [ATM] Here is what happens when you warm
> the edge of a 18 x 2
> Pyrexmirror
> 
> I thought you might like to see the results of
> warming
> the edge of a 18 x 2 Pyrex mirror with my hand and
> then taking interferograms and analyzing them. I
> placed my palm on the upper right edge.  The mirror
> started at 69 deg F and I placed the palm of my hand
> on the edge for 1 minute. It raised the front
> surface
> of the edge by about 45 nm.
> 
> 3D wire frame image
> http://home.comcast.net/~doeason/hand_heat.gif
> 
> 
> It took about 5 minutes to take all the
> interferrograms. Then the
> results were averaged together. The mirror was
> cooling
> during that time.
> 
> Color 2D contour map with Zernike terms.
>
http://home.comcast.net/~doeason/contour_map_hand_heated_edge.gif
> 
> 
> A profile right to left through the diagonal showing
> surface error
> in nanometers.
>
http://home.comcast.net/~doeason/profile_hand_heat.jpg
> 
> Dale Eason
> 
> 
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