Re: [ATM] RMS error vs Strehl Ratio

["Richard F.L.R. Snashall" <rflrs@rcn.com>]

Richard F.L.R. Snashall wrote:

> When I first started learning design, I quickly accepted that
> RMS wavefront error and Strehl Ratio were correlated, and even
> that an equation approximated the relationship between the two.

All of this would be pretty worthless, if I weren't trying to go
somewhere with it.  I was unable to find a "best" using N-BK7
and F4/F2.  However, I did find an "au naturel" kind of solution
using N-BaK2 and F2 (150 mm f/15).  This solution is a triple
point: the minimum focus is at the e-line (well... actually, I
missed by about 0.010 nm); both LA and OSC are pretty well nulled
at the e-line [at both the 70% and the 100% zones].

    http://users.rcn.com/rflrs/d03-53.atm
    http://users.rcn.com/rflrs/d03-53.len
    http://users.rcn.com/rflrs/d03-53.txt
    http://users.rcn.com/rflrs/d03-53.zmx

These design files use the F-e-C lines.  The Strehl results,
however, were determined using 11 wavelengths over the 450..670 nm
range.  The Strehl ends up being about 0.788.

This lens matches, longitudinally, [at the 70% zone] 642.1 nm
against the F-line; somewhat bluer than the "normal" C-F correction.
C-F correction with a reasonable null at the e-line is pretty well
achieved in the design in Chapter 4b of Roger Ceragioli's design
page.  Is it possible that the popularity [and hence low price] of
BK7 and F2 glasses is actually, in part, due to the ability to
achieve that C-F correction?

The LA nulling at the e-line can be demonstrated by:

   a) In ZEMAX, zero the 0.171 BFL and look at the wave front error
   b) In OSLO, zero the 0.171 BFL and look at the wavefront error
   c) In ATMOS, zero the off-axis angle (hitting OK) and look at
      the wavefront error.

In each case the indicated P-V is at or below 1E-4.


 
By the way this has nothing to do with Mahajan's approximation
or random wavefront generation;-)

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