ATM "Diffraction Limited" a definition

[Jack Schmidling <arf@mc.net>]

I don't claim to be an expert but computers are powerful tools and
allow one with only superficial knowledge to grunt through some pretty
heady stuff.

Since using Suiter's Admir program to reduce Focault test data, the
definition of diffraction limited seems pretty straight forward.

Diffraction limits the resolution of a telescope (among other things)
to the size of the Airy (otherwise knows as diffraction) disk.  If the
optic is capable of producing an image that diameter or less then it
is diffraction limited.

The program calculates the Airy disk size for the specified mirror and
compares that with the data provided by the Focault test.  If the 
number is less than one that it is diffraction limited if not it aint.
The parameter is called "Max Transverse Abberation Related to the Airy
Disk" in the program or TA for short.

The interesting fact is that it is not directly related to the wave
front error in an overall sense.  For a specific example, since I
started refiguring my 16", I have barely broken the 1/2 wave barrier
but the TA has gone from 10 to about 2 and at around the 1/2 wave
region can vary from 2 to 6.  Again, that's two to six times the Airy
disk size.  Pretty gross but much better than Orion's version of
"diffraction limited" of over ten. 

The reason this is so is because the closer to the edge one gets the
more the error contributes to mucking up the image.  You can have
1/4 wave error in the center and have a fine mirror but if you have
a 1/4 wave error at the edge, it is a disaster.

So it appears to me that to specify wave front error is just fluff and
there should be little doubt what diffraction limited means and that
is what people want, not a numbers game.  To mix the two in a spec only
serves to generate long postings like this.

js

p.s.  I know understand why the 10" F6 had so much latitude in the
Focault readings.  Taint so with this guy.  Unlike the F6, only the 
center zone allows an error of .01".  The rest have to be within
.005" and near the edge, only one with that error can be allowed.
This is really pushing it for me.  I consider my readings at best,
about .005" repeatable.

jjs 

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