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RE: [ATM] Astigmatism in round robin mirror C with Foucault.
Exciting to see the holographic test put to use. I look forward to the
pictures! Regarding to which accuracy you have to cut the fringes, it's
more or less so that if one wants the null to be accurate to lambda/10
on the wavefront, the cut out fringes has to be accurate to 1/10 of the
distance from one fringe to the nest.
/Mauritz Andersson
On Thu, 2005-01-27 at 21:58, Koehler, Steve wrote:
> Group,
>
> I said:
>
> > However, it's is pretty easy to detect qualitatively in a null setup
> > with the Ronchi test. Just center a Ronchi line and move it
backward
> > and forward through focus. If astigmatism is oriented at 45 degrees
> > from vertical, the Ronchi line will first lean one way, then lean
the
> > other way. I have some nice simulations that show how this looks.
> > Rotate the mirror in small increments until the effect is at a
> > maximum, and you can tell the orientation of the astigmatism. This
> > works very well for testing a spherical mirror at COC before
> > parabolizing. I'm wondering if the effect can be seen in a
> > holographic null test, if the slit is replaced by a slit with an
> > embedded Ronchi grating or wire. It seems like this might be
feasible for >
> testing astigmatism on a finished paraboloid. (Dale:
> > perhaps we should try this out.)
>
> I have an update on this. Dale and I performed a holographic null
test on
> mirror C over lunch today. I prepared a low-resolution mask (less
cutting) out
> of plain paper. We used Dales's robotic platform, replacing the green
LED with
> a red one, and replacing the knife edge with a narrow slit. It took a
little
> time to find the null on the correct diffraction order, but once we
found it the
> mirror nulled pretty cleanly. There were some asymmetric features
that stayed
> put on the mirror. I'm not sure, yet, if they are actual features on
the
> mirror, or whether they are due to sloppy mask cutting.
>
> In answer to my question, above, we were able to detect the
astigmatism in
> mirror C. We set up the mirror with the astigmatic axis at 45 degrees
from
> vertical to maximize the effect. I did not attempt to use a Ronchi
grating,
> because the slit was only .1 mm wide. However, using the slit/knife,
we
> observed very clearly that the shadow line tilted first one way and
then the
> other as we moved through focus (total travel distance of .03
inches). Thus,
> 1/4 wave of primary astigmatism is clearly visible under the
holographic null
> test.
>
> I hope to post pictures of both the appearance of the mirror at null,
and the
> tilting of the line, if Dale can capture some good images.
>
> -- Steve Koehler
> steve_koehler@securecomputing.com
>
>
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