Re: ATM Fold Flat Flatness

["Peter John Smith" <pjifl@bigpond.com.au>]

Anthony,

Very quick  prelimary resulys from Zemax agree with you general conclusions.

Zemax gives if anything Strehl a little higher than yours.

Off axis, if you look at max field at all the 8 'points of the compass' the
Strehl varies slightly.  I had wondered if this accounted for the difference
between vex and cave but no, in general quality is marginally better with a
vex flat.

I am still looking for stupid mistakes to account for the discrepency
between these results and 'myth' - maybe one will yet surface.

I am not quite sure what this means but it sure is interesting.

Maybe there should be some observing trials done with different accuracy
flats in a normal Newt configuration to test this but it would seem that a
1/2 wave flat gives very good images on axis.

One problem.  You would have to stop people examining the defocused images
and confine them to true observing.

Thanks for this post.  I almost always leave out the diagonal in all my
raytracing of Newts to simplify matters.  Is there a lesson there ????

Peter Smith.

PS.  If the fold flat is nearly normal to the rays of course allowable error
in the flat may be in the 10's of waves or more but thats not what this
interesting material is about.