What Does Wave Rating Mean in Telescope Mirrors?

[Artman]

I know that a telescope system is supposed to be at least 1/4 wave or less (for both primary and secondary ratings combined), but I am not quite sure what the wave rating refers to. Can someone explain what this is measuring.

 

[chroot]

A "quarter-wave" mirror is a mirror whose figure is an accurate parabola within one quarter wavelength of a specific color of visible light (green, I believe). In other words, the hills and valleys on the mirror's surface are smaller than about 100 nanometers from the ideal parabolic figure.

- Warren

 

[Artman]

Thanks chroot,
I thought it was something like that, but I have never seen it defined.

 

[Labguy]

As chroot said, the measure is usualy in green light, most sensitive to the eye and at a wavelength short enough to be meaningful. A 1/25th wave mirror measured in red light would most likely be a terrible figure for a telescope mirror.

Actually, most wavelength claims are being ignored in favor of an accurate measure of the "Strehl Ratio", which is much more indicative of a mirror's surface accuracy.

See:
http://www.rfroyce.com/standards.htm

Labguy