Re: ATM aargh! maybe

[Mark Holm <mdholm@telerama.com>]

Nils Olaf Carlin wrote:


> Let me once again plug for the concept of RMS error - any useful data
> reduction program does calculate it. It is by no means less real than
> the P-V, and it is a far better predictor of performance. 
> 


I agree with Nils Olaf.  My physics professors wouldn't speak to me when (and 
if) I go back for reunion if they knew I didn't.  RMS is a better predictor of 
image formation.  As I understand it, RMS essentially weights the magnitude 
(height) of a surface error by the amount of area that error covers.  So, if you 
have two zones of equal height above (or below) the best fit parabola, the one 
that covers more area contributes more to the RMS error.  This also corresponds 
to the amount of light reflected by each zone.

The problem with RMS is that it doesn't give any guidance about where you need 
to have polishing action.  When I am looking at a Foucault test result plot and 
trying to decide what to do next, RMS and Strehl are good for deciding whether 
to stop.  P-V, and, of course, the locations of the high or low zones tell you 
about how much, and where you need to remove glass.  So, it is easy to think, 
"When I get that curve so it all lies within x nanometers, I'm done."  Sixtests 
will do a plot with the surface deviations scaled according to RMS.  It usually 
shows that errors near the edge are more important than ones near the middle. 
That is useful guidance.

> Also, why
> not give it in nanometers instead of the convoluted fractional
> wavefront (wavelength not specified, but 550-560 nm usually) error?
> 

Just habit.  I started my ATM journey reading ATM 1, 2 and 3, Thompson, Howard 
and Texerau.  They mostly used wave values and I haven't shaken the habit yet. 
Not good for someone who usually supports SI units.

I usually use 1 wavelength = 500nm.  According to what I have read, 500nm is 
very near the peak of the sensitivity curve for dark adapted human vision. 
Daylight vision has its peak of sensitivity near 560nm.

http://www.atmsite.org/contrib/Holm/vissens.html

Also, 500 is an easier number to remember and plug into a calculator.  It is 
easier to remember the fractions of 500 too.  1/4 wave = 125nm.  1/10 wave = 50 nm.

Mark Holm
mdholm@telerama.com