The Celestial Tourist Speaks
The problem is, that if you do the test the way that is kindest to the mirror -- that is, that gives the smallest numerical answer -- you get a number that is the better part of ten times smaller than if you do the test the way that is least kind to the mirror. That is, the same mirror will give test results that differ by a good chunk of a factor of ten, depending on where and how you do the test, and on what wavelength of light you use.
The most demanding combination of the things I have listed would be to test peak-to-valley, on the wavefront, in green light. The least demanding would be RMS, on the mirror, in red light. A mirror that tested one-eighth wave RMS on the mirror in red light could be almost a wave off when tested peak-to-valley, on the wavefront, in green light.
Let me give another example to make sure things are clear. A standard for optics that most of us would consider to be good but not great is one-quarter wave, peak-to-valley, on the wavefront, in green. But if someone sells you a "quarter wave mirror", and that result is on the basis of testing via RMS, on the mirror, in red, then it might be almost ten times worse than the actual standard that I just cited. You have to know the details of how a mirror is tested to interpret the results.
For those of you who would like to read a column by me I thought it would be only polite to oblige... :-)
|Jay Freeman; last updated: 1998 Dec 16||Prev Next|