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INTERFEROMETRY - THE MYTH BUSTER I feel that I must better educate my customer and that is the sole intent of this article. If you have searched the internet lately looking for a telescope or perhaps a set of optics you no doubt have come across a lot of information relating to “how good” ones telescope or optics are. Unfortunately a lot of this information is confusing and in some cases down-right misleading. Regardless of how the telescope or optic is tested there is only one way to accurately determine optical quality as expressed in waves or fractions there of, Interferometry. Yes, Interferometry. ....please read on. Interferometry examines the converging wave front in hundreds and sometimes thousands of points and compares them to a wave front of a known quality (high quality) reference, a comparison is made between the two, preferably by software and computer. However, in this case a picture can be worth a thousand words or should we say a thousand dollars. An interferogram photo can be readily dissected by someone who has even limited Interferometry knowledge and even without a computer generated plot, one can make an accurate assessment to the general quality of the optic or telescope. The interferogram is the only way to truly measure the wave front. The entire optic is analyzed at one time and interferometry “takes no prisoners”. The good of this is that with interferometry one can further manipulate the final correction to a level that one could not (with certainty) achieve otherwise. Now I know you probably have read that certain “optical companies” are making some incredible claims about their work and are providing a piece of paper, a “certificate” if you will, to prove it. If you are paying top dollar you do not want to purchase a paper “certificate”, you want and should demand an interferogram photo. In virtually all cases, an optic tested via interferometry will not be “as good” as one tested and plotted on a line via any of the knifedge programs. The reason for this is when an optic is tested by knifedge only a few select zones are measured and compared, these zones are rather large compared to a “point” and we feel that the numbers reported by mirror makers using this procedure actually should be RMS numbers (at best) and not P-V numbers. This is not to say that the knifedge tester is incapable of making excellent mirrors, we know it can be used to do such that. What we really are trying to convey here is it simply cannot be used to compute wave front and RMS claims to the same accuracy as interferometry and certainly not greater. We know that any optic that has computed P-V and RMS values by Focault testing will rarely, if ever, approach the same values when tested by interferometry. We see this over and over again and this is not just my opinion. This places the company who has invested countless dollars into interferometry and software at a very unfair advantage in the competitive consumer market. Thankfully the Government, Aerospace companies and well informed consumers know the truth behind optical testing and what numbers are reality and which are fiction. We do use the Focault tester in our shop, it has it’s place and it is a tool - one of many - that every shop should have. It’s use to see small zones goes without mention. It simply can’t be used alone to generate P-V and RMS numbers. Any optic that comes with “certification” that is not derived from interferometry should be suspect at best, especially if the P-V is better than 1/10 wave. Please consider the fact that ZYGO (as well as a few other reputable companies) sells certifiable and traceable reference spheres & flats (1/10 - 1/20) wave at a cost of $5,000 to $15,000 each!. Please don’t tell us that someone reading a parabolic surface with a knifedge - at radius - in their basement - can do better than that. It just doesn’t happen. What is “certified”? It seems as if a lot of so-called manufacturers are selling optics that they claim are “certified”. I also see this term being used on the “re-sale” sites on the web, but exactly what is certified?.An optic can only be certified if it is tested against a known reference element that is traceable to a bureau of standards that is recognized by the optical industry. These reference elements are used in interferometry. Yes, it is very expensive and time consuming. Yes, it is the only way to know for sure just how accurate an optic or optical system is. Don’t be fooled as there is no substitute!.Data reduction done with the Foucault tester at Radius does not, and never has, “certified” any optic. The P-V numbers attached to these so called “certified” optics done by others will seldom if ever come close to the data results done with interferometry. These exaggerated claims are more of an indication of a complete lack of optical testing knowledge on behalf of the manufacturer and down-right dishonest if you ask me.Our experience has shown that large (more than 12.5") and fast (f/5 or less) mirrors tested at Radius (with a Foucault tester) will suffer from under correction on the edge up to and sometimes exceeding one full wave!. Don’t believe it?, just ask any qualified professional or better yet, have that mirror you paid top dollar for tested by interferometry. You will always receive honest, reliable and accurate data with my optics!.
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