Re: [ATM] [atm_free] RE: data and musings on thin mirror

["vladimir sacek" <vla@toast.net>]

Nils Olof Carlin wrote"
>
> You need to know the distance grating to mirror vertex of course but
> not to extreme precision. Then, like any other zonal test, you know the
> zonal location on the mirror, and you know the relative position of the
> normal at a point near the COC - as you do with e.g the Gaviola test,
> and from that information you can do some integration
> and come up with a best fit curve of some kind (parabola,
> ellipse/hyperble) without knowing the ecact location of the paraxial
> COC - if any.

Well, Nils, which is it: (1) you need to know the COC (grating) location
"but not to extreme precision", or (2) you don't need to know it at all?

Gaviola test is different, because it's based on direct measuerement of
zonal foci, from which the local (zonal) radius is derived. In other words,
it gives you two reference points - zonal ceneter and focus - determining 
the
radius. Then you use the zonal data to piece together a surface profile
approximation.

With Ronchi pattern alone, you only have a zone at the mirror
and corresponding zonal deformation at the grating pattern. If you don't
know grating  location (or aberration), you can't do any meaningful 
projection
to determine zonal radius/focus- you simply do not have a fixed second
reference point to connect with the zone at the mirror.

 If we don't know the location, the only way to determine the aberration is 
to
compare - mathematically or visually (measurement) - actual pattern to a
model pattern obtained from *known* location/aberration figures.
When they nearly coincide, we simply attribute the model pattern aberration
to the actual pattern. In fact, determining surface properties this way does
require known both pattern location and aberration.

Vlad


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