To: atm{at}shore.net
From: mdholm{at}telerama.com
Reply-To: mdholm{at}telerama.com


Jerry,

Waldo's Plop instructions get you to an 18 point cell with pretty good
numbers.  I get about 1/250 wave RMS.  No reason to want better than that. 
One might get a little better by letting Plop vary the force ratio between
the inner and outer support points, but since you have 1/250 wave RMS
already, that may be an excercise in gilding a lilly.

Waldo's analysis of the color deformation plot is a little oversimplified.
Plop changes the scale on that plot so that the maximum deformation (high)
is always blue and the minimum (low) is always red.  The middle is green. 
This is true no matter how great the deformations are.  (One can change
this default behavior, but it isn't worth worrying about right now.)  What
really matters is the RMS deformation number.  It is given in millimeters. 
To turn it into waves, divide by .0005 or .00055. (I use .0005, but many
good ATM;s use .00055.)  To see what the denominator of the fraction is
just press the 1/x key.

1/200 wave RMS or less is pretty good.  For many jobs, 1/120 or less is
good enough.  Anything less than or equal to about 1/240 is good enough for
just about any purpose an ATM might put a mirror to.  (These are surface
deformations.  Multiply by 2 to get wavefront deformations.  1/240 surface
is 1/120 wavefront.)  In a real world telescope, there will almost
certainly be other sources of wavefront deviations greater than this and in
any case, you won't be able to see the difference between this and a
theoretically perfect system.

Mark Holm
mdholm{at}telerama.com

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