Re: [ATM] Computing surface from Ronchi image?

[Nils Olof Carlin <nilsolof.carlin@telia.com>]

John, Dale,


>>>I think Nils Olof is right when he says:
>Using a linear, common Ronchi grid, you can compute a profile 
>along the diameter (perpendicular to the lines)

For example if the grating is vertical, you'd measure the horizontal 
diameter. But I am certainly no expert. It appears to me that the best 
is to have the grating inside the roc, and have a bunch of lines 
showing. Outside the roc with a fast mirror the lines loop around, 
probably becoming worthless.

I like to think that if the grating is well enough inside the roc, you 
are performing a "backwards" LWT. Instead of moving the wire in tiny 
increments, you alreay know the distances between the wires. Instead of 
moving the wire to a zone in a mask, you measure the zone each wire 
falls in off of a photo. >>>
----
That's more or less my idea, only that with a Ronchi grid, you have 
the extra complication that you are looking through a diffraction grid!

I think Sixtests will do the necessary calculations, if you (Dale) 
like to do some pilot studying of this. You (John) wrote IIRC that the 
diffraction effects were less with the point grid - this may be true, I 
can't figure with any certainty. Steve K who is inordinately clever 
with such things may perhaps run a simulation or two, to get an idea of 
what problems to encounter...

>>>So a person testing a mirror could use a spot test to look for 
astigmatism, and if none exists, can take a Ronchigram and let your 
program compute the surface.
----
My simulations suggest that astigmatism in a spherical mirror can be 
seen well with a spot test or similar, but only with the greatest of 
difficulty in a paraboloid mirror of "relevant" dimensions.


Nils Olof
_______________________________________________
ATM mailing list http://www.atmlist.net/