Re: [ATM] A Null Test

[Richard <cnc@cncservo.co.uk>]

Hi Richard,

Monday, September 4, 2006, 3:34:23 PM, you wrote:

RFLRS> I put a plot of the longitudinal error of the original two
RFLRS> lens solution.  At:

RFLRS>     http://users.rcn.com/rflrs/offner01-02.jpg

RFLRS> I put a plot of the longitudinal error of the three lens
RFLRS> solution.  It doubles back twice instead of once.  Not too
RFLRS> sure, though, given the performance levels we've seen so
RFLRS> far, that it really matters much in real life.

Oh it matters. You WANT abberation. Specifically, the lens train
needs to introduce the exact opposite of the desired mirror
abberation with respect to a sphere. Any abberation above and
beyond that will show itself as a poor null, and the figures I'm
getting of around 1/1000th or better in the wavefront suggests
it's ok :). The sort of abberations we really don't want are the
ones we cannot gaurantee without spending a fortune on lens, ie.
zonal errors in the elns surfaces.


RFLRS> I've been getting the same kind of numbers (f/2.5 .. f/5.0 and
RFLRS> K=-0.8 .. -1.2).  By switching to the Offner form, I've now been
RFLRS> able to get a single solution sheet for the entire range, even
RFLRS> though not all the solutions have two direction changes.

Excellent. The other part I like is that you don't have to rely
on the lenses having high accuracy on ROC and centre thickness.
As long as you measure what you have and plug them into the
optimisation, then the results should be good.

-- 

Best regards,
  Richard in the UK

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