[ATM] Michelson interferometer

[Nathan Towne]

I've been working toward mirror testing with interferometry lately and 
have had some success.  I noticed that a lot of people are using 
interferometry now, usually of the Bath type.  The one I built is of the 
Michelson type with spherical wavefronts.  I've uploaded a sketch of my 
particular scheme - 
http://www.atmlist.net/contrib/towne56-at-frontiernet-dot-net/MichelsonInterferometerDiagram.pdf.  
I was able to get good fringes from the mirror so I think the scheme is 
viable.  Very good contrast. 

I wanted to avoid having optics in line with the mirror (aside from null 
correctors).  Instead, I set up the interferometer so that the laser 
beam is converging as it goes through the beam splitter, focuses at the 
radius of curvature of the mirror on the mirror branch, and on the 
reference branch the focus is at the reference mirror.  The two beams 
are diverging as they are recombined and projected onto a screen or 
camera.  I use a 4-mm eyepiece to diverge the laser beam and a 
plano-convex lens to focus the beam. 

I tested it on an eight-inch f/4.5 roughly spherical mirror that was 
machine polished.  The mirror is slightly oblate with zones by the 
Foucault test, so the fringes have structure.  Some astigmatism.

http://www.atmlist.net/contrib/towne56-at-frontiernet-dot-net/Interferometer/20100425Interferogram8F4.5PyrexMirrorInterferogram3742.jpg
http://www.atmlist.net/contrib/towne56-at-frontiernet-dot-net/Interferometer/20100425Interferogram8F4.5PyrexMirrorInterferogram3743.jpg
http://www.atmlist.net/contrib/towne56-at-frontiernet-dot-net/Interferometer/20100425Interferogram8F4.5PyrexMirrorInterferogram3744.jpg
http://www.atmlist.net/contrib/towne56-at-frontiernet-dot-net/Interferometer/20100425Interferogram8F4.5PyrexMirrorInterferogram3745.jpg

There's a photo of the laser and interferometer assembly.  Don't laugh 
too hard and hurt yourself.  It is very crude.  Everything is moved by hand.

http://www.atmlist.net/contrib/towne56-at-frontiernet-dot-net/Interferometer/MichelsonInterferometer3751.jpg

The eyepiece seems to introduce a lot of intensity modulation into the 
beam that can be seen on the interferograms.  Anyone know why that is 
and what to do about it?  A better lens I suppose? 

Nathan