It doesn't matter whether your light source is monochromatic or not when you are testing mirrors using a Foucault test.
Really! It doesn't!
Mirrors have a good feature: the angle of reflection is not dependant on the wavelength of the light reflected. Therefore all the different spectral bands will be focussed to exactly the same point.
So, what does this "wave error" mean? Well, generally the wavelength that is used for judging optics is the wavelength in green light which is near the eye's peak sensitivity, around .000022 inches or near 560 angstroms. How do we measure this? We don't. It modifies the tolerance envelope of good readings. Check out the Milles-LeCroix formulation, or Texereaux.
It is interesting to note that the radius of the Airy disk is also wavelength dependant, so in theory the Airy disk should show some color effects as well. I wonder if any of our intrepid readers has ever seen this?
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Mark T. VandeWettering Telescope Information (and more)
Email: <markv@pixar.com> http://webspace.com/markv/
<markv@webspace.com> Clear Skies!