Re: ATM "Diffraction Limited" a definition

["Richard H. Jarnagin" <afn31987@afn.org>]

Allen Gilchrist wrote:
> 
> Richard,
>   It looked to me as if you were saying that the mirror must be tested to 1/8
> wave in order to produce an error in the wave front of 1/4 wave.  My point was
> that if a mirror tests to 1/4 wave that means wave front error.
> Allen


Allen,

Unless your doing the testing, you would be extrememly naive to think
so. The term 1/4 wave by itself is completely meaningless.  It could be
1/4 wave wavefront P-V, wavefront RMS, surface P-V, surface RMS, +/-
wavefront, +/- surface, etc., etc.  None of these would represent the
same error but all could be called 1/4 wave if indeed that's what the
mirror tested using that particular rating system.  In other words, 1/4
wave what?  Also, the test wavelength should be specified.  In other
words, 1/4 of what wavelength?  Many companies have played the ratings
game with consumers and some still do.

As to my statement about the mirror needing to be at least 1/8 wave P-V
surface to meet the 1/4 wave P-V wavefront criterion, YES that's exactly
what I'm saying.  And as I believe I said, it actually needs to be
better than 1/8 wave to compensate for the additional error that will be
added to the wavefront by the secondary etc.  The 1/4 wave criterion was
meant to be applied to the resulting wavefront from the ENTIRE OPTICAL
SYSTEM at a wavelength of around 550 nm (in the peak of the visible
region of the spectrum), not just the primary.

Richard