Re: [ATM] [atm_free] Detecting Astig in Ronchi images

["Mel Bartels" <mbartels@bbastrodesigns.com>]

> Dale Eason wrote:
> > Mel, you truly must be a master ronchi reader.  You are the first
> > to quantify it and orient it.  Your wave rating is in agreement
> > with my interferometer for wavefront.  Your orientation is an exact
> > match.


> Mel, could you comment on how you arrived at those conclusions?
> Could you explain what features you identified to get the orientation?
> How did you estimate the amount?  Just an estimate from experience?


Years ago I did what Jerry mentioned - traced the Ronchi bands' changing
angles around and deduced the actual mirror surface profile vis-à-vis a
particular desired mirror profile.  I would do this three times, for a long
RoC, a 'just-right' RoC, and a short RoC.

Eventually I realized that I can directly picture all three mirror surfaces
in my mind by looking at a Ronchigram.  It simply materializes after a spot
of time like one of those 3D puzzles.  I just need to know the grating
frequency, the mirror diameter and the mirror focal ratio.

To say what I key off of is difficult because it is not a conscious process.
I could point you to the bend in the Ronchi bands that smell like 1/2 wave
on the surface error (in this case, not zonal therefore must be higher order
'stig; furthermore, play with a Ronchi plotting program and see what a 1/2
wave surface error looks like, then project that error into a single banding
wave), and I could point you to several spots in the image that indicate 45
deg angled axes.  But I don't know how useful because it's a post-hoc
explanation of what I 'see' in the back of my mind.  I'm happy to continue
this dialogue if you want to zero in on specifics.

Mel Bartels


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