Re: ATM Re: Diagonal Flatness Specification

[Bill & Tracy Marriott <btk@pacbell.net>]

MikeW wrote:
> 
> How is flatness measured and reported for the small elliptical diagonals
> used on Newts?  Flatness of large optical flats can determined by counting
> interference fringes across the diameter of two flats in the presence of
> monochromatic light.  Does the same sort of measurement apply across the
> major or minor diameters of an elliptical?  What are the minimum necessary
> specifications for satisfactory performance of an eliptical diagonal in a
> Newtonian?
> 
> I'm trying to make sense of vendor claims about their diagonals.
> 
> Regards,
> 
> Mike


Mike:

This is indeed how diagonal mirrors should be measured.  One should 
hope for 1/10th wave, across both axis. And that equals to 1/5th of a
fringe curvature, which is noticeable curvature. Straight as a board
fringes, edge to edge, is closer to 1/20th wave.  While the major axis
is the longest reach, it may be more typical to see slight amounts of
roll off on the minor axis, due to blocking configuration, though this
may only affect the outside row of a block of parts, but that is where
batch processing comes in, process a batch, good ones go here, OK ones
go there, as previously mentioned in a earlier version of this thread.

If people demand good diagonals, maybe we'll get good diagonals...

Oh yeah, check out my diagonal page at the URL below

Bill Marriott
Woodacre, Ca. USA
btk@pacbell.net

http://www.starlightoptical.com
Optical/ATM projects

http://www.starlightoptical.com/marriott
The Marriott HomePage

And, now, P1800 Volvos
http://www.starlightoptical.com/volvos.htm