[ATM] Michelson interferometer
towne56 at frontiernet.net
Wed Apr 28 09:02:03 JST 2010
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 -
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.
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.
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?
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