Re: [ATM] How to make a mini golf court

[Mike Lockwood <melockwo@uiuc.edu>]

Oliver,

Oliver Kersten wrote:
> after successfully removing the TDE of my 10" f5.5 (thanks for your 
> help  !), unfortunately I created a deep central hole by
> polishing MOT with maximium overhang ways too long. I think I learned 
> that  lesson. I would like to ask you to have a look at
> 
> http://www.comesso.de/oeli/atm/mirror/mirror.html
> 
> where the latest Foucault images can be seen. Is it okay to work with a  
> star-shaped mask pressed into the lap's center to
> flatten the hole ? Of course, I got a TUE in the meantime...

Once again, excellent Foucault photos!  Everybody makes mirrors 
differently, but here's my advice.....

A star shaped mask will work, just make sure it's significantly 
smaller than the hole.

Personally, I'd just avoid polishing the hole when parabolizing, if it 
wasn't too deep.

In a program such as Figure XP, put in your Foucault readings and set 
the conic constant to 0 (a sphere).  This should give you an idea of 
the depth of the hole.  The error will be plotted with respect to a 
sphere.

Then make the Foucault readings all zero and change the conic constant 
to -1 (a parabola).  Move the ROC slider until the edge is nearly 
flat, and look at the height of the hill in the central part of the 
mirror (which is the glass that needs to be removed for parabolizing). 
  Compare the depth of the hole with the height of the hill.

If the hole is deeper, then you should try to remove some of it.  This 
can be done with the mirror on top, ovehanging so that the "rim" of 
the hill is near the edge of the lap.  This offset is a little 
different for each lap/operator.

If the hole is not that deep, then parabolize, but avoid deepening 
that part of the center by reducing the width of the parabolizing stroke.

	Mike Lockwood


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