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Help with Mirrors

Started by Craig Burlette, 03/12/2003 03:55PM
Posted 03/12/2003 03:55PM Opening Post
I would like to know what is meant by a mirror being 1/4 wave or 1/8 wave and diffraction limited. Am I right in assuming that a 1/8 wave mirror is better than a 1/4 wave mirror? Is it "twice" as good? Is it reasonable to assume that a mirror that is listed as 1/10 wave could actually be worth $200, or would it be deffective in some other way? Has anyone heard of a mirror that is 1/15 wave for less than $300? What do they mean by saying a mirror is diffraction limited? How would a diffracion limited mirror compare to any of the above mirrors?
Thanks from a Newbie.
Posted 03/12/2003 04:42PM #1
Mirror ratings can be very confusing, for sure. Generally, mirror makers have given what are called peak-to-valley (PTV) readings, which are supposed to be the worst error on the mirror. To add to the confusion, they may give the error on the mirror surface, or at the wavefront after the light has been reflected off the mirror. The wavefront error is twice the surface error, so a 1/8 wave on the surface translates to 1/4 wave at focus.

The problem with PTV is that it may not accurately represent the mirror as a whole. A mirror could be extremely accurate over most of its surface, and have a small region that has a 1/2 wave defect, and still work exceptionally well. A 1/4 wave mirror may be quite rough - have lots of 1/4 wave defects, and perform poorly. Generally a proper test of a mirror considers the entire surface. The RMS (root-mean-square) measurements are one way to do this.

Fortunately, and experienced mirror maker can make a nice, smooth mirror where the PTV readings provide a reasonable quarantee of the mirror's performance. A smooth, 1/4 wave mirror works very well. (Claims of mirrors better than about 1/10 wave from Foucault testing should be treated with skepticism.) If someone generates RMS values from Foucault readings, they are assuming the mirror is smooth and that the zones read are truely representative of the mirror (the same goes for other measurements relating to the entire mirror).

The best guarantee of a good mirror is to buy one from a maker with a good reputation.
Posted 03/12/2003 04:44PM #2
Diffraction limited means the mirror's performance is only limited by the "wave nature" of light. Generally, this is taken as a mirror that is 1/4 wave at the wavefront (1/8th wave surface).