Star Testing Complex Optical Systems

by Roland Christen

I should start out saying that I have the highest regard for Suiter and his book, there are however enough holes in the book vis a vis refractors and catadioptrics that it cannot be used for all occasions. If you want to see some wildly or perhaps mildly asymetrical printouts, contact TEC Telescopes. They can send you some Zemax results of Mak-Cass designs that may want you to start nibbling. Now on to my soapbox: The situation with the star test is more complicated than manufacturers being afraid to be found out now that amateurs have an easy test to evaluate the optics. When the optical system gets more complex than a simple parabolic mirror, then there are inherent aberrations that affect the star test. An example is the Maksutov Cassegrain. This system can be manufactured many different ways, but one popular way is to have all surfaces spherical, which cuts down the need for hand work. Machines exist now that can lay down a 1/20 wave or better spherical surface on a piece of glass without any human intervention. In this pure form, the Mak-Cass has left over 5th order aberrations and, depending on design, these can be less than 1/10 wave on the wavefront. By the way, fast Apo refractors have these same aberrations also. The RMS value will be better than 1/50 RMS and the Strehl ratio will be exceedingly high. In other words, the optic will deliver a very high contrast image, consistent with the high wavefront rating. When tested on the night sky, the inside and outside diffraction patterns will be quite different. Any beginner will see it so and may conclude that the optic is of poor quality. As an example, I have recently finished exhaustive tests of different 10" F14 Mak-Cass systems, some with these inherent aberrations left in, some with them meticulously removed. All the optics tested between 1/10 and 1/12 wave with Strehl ratios of 98% or better. The system with the uncorrected or pure Maksutov curves had the central hole of the donut break out 4 times farther on one side of focus than the other. It would be judged by the Suiter star test method as being maybe barely 1/4 wave, if that. The hand aspherized version, which was no better on the interferometer and had similar Strehl ratio, showed a donut breakout approximately at the same point on either side of focus - in other words textbook perfect. At focus, both scopes showed a tight central Airy disc with a faint first diffraction ring. At focus, it was impossible to tell them apart. The ratio of brightness of the Airy disc and first diffraction ring was essentially identical. Both scopes showed the same high contrast on Jupiter and Saturn. Both scopes could split doubles with equal precision. This same thing happens in my short focus Apo triplet lenses. They also have classical 5th order like Mak-Cass systems. I have seen literally thousands of star patterns on my test setups. I can tell you that the shadow breakout difference is almost entirely due to slight increase in focal length of the very inner zones on these optics. That is one half of the 5th order defect. This causes no problem in definition and contrast for 2 reasons. 1, there is very little energy as you go toward the middle of the optic, and 2, the depth of focus approaches infinity at the very center. However, the shadow breakout is linear, not asymtotic like the encircled energy. This is easily fixed, of course by commercial makers by simply making a large central obstruction. The other half of the defect occurs at the outer zone. If this is left uncorrected, you will see fuzz in the in-focus image which is highly destructive of contrast. The inside and outside patterns will look the same, causing you to conclude that the optic is textbook perfect. It is this outer zone that the competent optician will concentrate on, since this is the place that contributes most to the definition, resolution and contrast of the optical system, even though it has little or no effect on the inside/outside star pattern, particularly where the shadow breakout occurs. In view of the above, I have seen amateurs at star parties wrongly interpret the star test and overestimate the quality of one optic that was really not that good, and underestimate another that was really superb. I think all manufacturers will agree that the star test must be properly done to place a VALUE on the correction. This is not so easy with certain types of defects, and so the star test, when improperly interpreted by untrained individuals, can overestimate and also underestimate the actual quality. If you ask any reputable maker of complex optical systems, they will agree that the interferometer will give the most reliable result. Since Peter Ceravolo will not get on SAA, I will paraphrase what he has told me time and again : You cannot rely on anything but the interferometer to give you a wavefront NUMBER. The interferometer takes no prisoners. In my experience, when an optic measures 1/10 wave AT THE DESIGN WAVELENGTH, then the performance will be nothing short of stunning. Peter's criterion was a bit less at 1/8 wave P-V. Either way, the customer will have an excellent optic. By the way, Ceravolo is my mentor on testing of optics. Even though I have been making optics for a long time, without his guidance in the testing lab, I never would accomplished what I did. And yes, I used to use the star test in the distant past when I made longer refractors. Some of my optics came out good and have surprised me when retested on the interferometer. But for the fast F6 and F7 triplets I make now, the star test falls short. The dilemma for manufacturers then is, should we do our best to produce smooth high contrast optics, or should we please the star test crowd and do some hand aspherizing to get a more pleasing out-of-focus star image? I can tell you that it is easy to do some rough compensation with quick local polishing at several zones to get more equal inside and outside star patterns, but the result will almost certainly be a loss of contrast. Add to that a nice big central obstruction to get rid of the offending inner zones, and presto! you have a nice "fast food" Mak-Cass that doesn't work any better than a typical SCT. In our case, we will do our utmost to produce the closest faximile to the star patterns in Suiter's book, but they will never be exactly equal. The overriding concern will be that the optic has a very smooth and accurate wavefront to produce the highest contrast possible in the final image, which I assume will be in focus. Roland Christen ASTRO-PHYSICS
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