RE: [ATM] Astigmatism in round robin mirror C with Foucault.

["Koehler, Steve" <Steve_Koehler@securecomputing.com>]

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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