Re: ATM Ronchi Question

["Bill & Lois Kelley" <wekelleylwk@sedona.net>]

Roger,
Sure sounds like the common up-down edge. With TOT and long strokes COC and
a few W's, the TDE goes away first and you see no trace of it outside of
focus.
More of the same and the remaining ridge polishes off.
Once in a while , if I'm reallly careful and fussy, I can end up with a
clean 2 or 3 bar test and a perfect Foucault null ( as best as I can tell
with 84 year old eyes).

By coincidence I am leading a friend through this with a 10" F5 sphere he is
making for flexing - just as described above.

Best regards,
Bill Kelley


----- Original Message -----
From: "Roger Ceragioli" <rogerc@as.arizona.edu>
To: "atm" <atm@shore.net>
Sent: Sunday, February 10, 2002 5:26 PM
Subject: ATM Ronchi Question


>
> Hello, All.
>
> I'm having some difficulties with the edge of a mirror and thought that
> your collective experience might be able to help.
>
> It's just a plain-Jane 10" f/5 paraboloid, but even so, it's still
> giving me fits.  The problem is that I can't tell decisively whether the
> edge has a small turn down.  Interferometry is not an option, or I'd
> know for sure.  So, I'm using the good old knife-edge and Ronchi tests.
>
> By the knife-edge, I'd say that the edge is good:  when I null the
> return and just cut it off, a bright diffraction ring appears all the
> way around the surface and is almost equally bright on right and left
> (knife edge enters from the right).  Also--I'm using a slit-source--when
> I move inside focus several inches and cause the shadow of the knife
> edge to blot out about 2/3 of the return, I can see the classic
> diffraction lines to the left of the knife-edge in the still-bright
> area, as described by Texereau.  The ends of the lines look straight or
> perhaps with just a hint of a turn, I'm not sure.
>
> So according to these tests, I should have a good edge, though maybe not
> quite perfect.
>
> Now here's the question.  In Ronchi testing with a 100 ln/in. grating,
> the bands look straight almost to the edge.  Then there's just a hint of
> a TDE, perhaps.  Outside focus right at the edge of the mirror the band
> tips expand and turn outward slightly.  Inside focus, they narrow and
> turn inward.  Having tried for perhaps 6 hours now to get rid of this
> presumed TDE, I'm making no headway.  And since by the diffraction ring
> test using the knife edge, I get a bright ring around the periphery, I'm
> not sure what to think.
>
> I know from a lot of interferometry, that many--perhaps most--spherical
> optics have at least a very narrow shallow TDE, if not all the way
> around the optic, at any rate around parts of it.  And I can't ever
> remember seeing an optic that in the Ronchi test didn't appear to show
> small deformations of the band tips right at the edge when inside focus.
>
> So my question is this:  given a perfect edge, on a really critical
> examination should I expect to see perfectly uniform Ronchi bands right
> out to the edge?  Has anyone of you actually seen them?  Or is there
> some diffraction effect that destroys band perfection at the edge?
>
> Actually, what I'm seeing looks like a shallow up-down edge, where the
> extreme outer zone turns slightly up and then down to meet the edge.
>  This effect is common on aspheric optics, and the more aspheric the
> worse it becomes in general.  So maybe I'm answering my own question,
> but I'd like to hear from experienced mirror makers who use the Ronchi
> test.  Do you ever get perfect bands at the edge of the optic?
>
> Thanks a lot,
> Roger Ceragioli
>
>
>
>