FW: [ATM] make ronchi grating

[saundby at foothill.net (Mark Graybill)]

On Monday 26 January 2004 06:40 pm, Michael Lindner wrote:
> On Monday 26 January 2004 05:26 pm, Mark Graybill wrote:
> > I made my own Ronchi grating from the postscript code, it worked great.
>
> Are there any concerns with (or has anyone experienced) curling of the
> transparency material? I would think that if the ronchi grating curls even
> a few thousandths of an inch the results would be off.
>
> For that matter, to throw out an idea...the Mobsby grating uses curved
> ronchi lines to perform a null test on a parabola (or other non-sphere).
> Anyone ever consider curving the substrate of a ronchi grating to achieve
> the same result?
>
> (warning, I have not done the math on this - just throwing out an idea).

I avoided the problem myself by securing the transparency material to the 
Foucault tester under the knife edge on one side and under a pair of 
thumbtacks on the other side that held it flat. I viewed through a hole in a 
piece of wood on my tester.

On the film can, I secured the material between a couple of sheets of stiff 
cardstock and secured that to the inside of the film can lid after a couple 
of unsuccessful attempts to use RTV to glue it in. There may be some small 
curve to this grating, but it doesn't seem to affect the test results as far 
as I can tell.

 I don't think the Ronchi test is so sensitive that any slight curvature will 
affect the test strongly. At least it didn't appear to be in my experience. 
Others will no doubt be able to comment on this quantitatively.

-Mark G.