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Star Testing Astronomical Telescopes Second Edition

Posted by Ed Moreno   12/29/2009 00:00:AM

Over the years, there has been a great deal of controversy around the validity of the star test. It has often been stated that star testing is really only valid for “Simple” systems (Newtonian) and that it was not really a reliable test for “Complex” telescopes.

My own experience was VERY much different. Telescopes that star tested well (including SCTs and MCTs, where were by many considered “Complex” telescopes, and therefore not inclined to test properly according to the myth on this topic) seemed to me to always work well. This seemed ESPECIALLY true of the SCTs. If they gave soft images, they ALWAYS gave a poor star test, and if they gave crisp images, they ALWAYS tested much better and ESPECIALLY when spherical aberration was highly corrected. In the designs with large secondary obstructions, I have always felt that the combination of poor spherical aberration correction (which I consider to be the PRIMARY shortcoming in most mass produced scopes) and a big central obstruction was simply too much to overcome, even with lots of aperture. My own experience seemed to be 100% in correlation to the work of Mr. Suiter’s first edition.

Anyway, this debate simply refused to die. As a result, I often found myself defending a position that, to many people, seemed to be specious. It was with great eagerness that I looked forward to the publication of Star Testing Astronomical Telescopes, Second Edition. It was rumored that Mr. Suiter would provide a more definitive answer to the ultimate question of whether star testing was indeed a valid testing method for ALL scopes.

This book is a masterpiece work. Make no mistake; if you want to know how to test telescope systems, this is THE book to buy. What makes it so invaluable is that it is presented in a way that just about anyone can use without having to learn how to use any special tools or to do any math, though for those that want to know the physics, there ARE formulas included, but you do NOT need to even GLANCE and any of these to still get a complete tutorial on how to evaluate your telescope.

The book is organized into fifteen chapters. These chapters start with a brief summary of star testing and the goals of star testing, then proceed through a systematic instructions on how to conduct the test, and includes a chapter on each kind of telescope optical error (including things like alignment and secondary support diffraction) and how that particular error affects the performance of the optical system. Along the way, he presents an incredible amount of good to know details about telescope design and performance.

Not only are the different errors all covered in fairly comprehensive detail, they the chapters also include instructions on how to estimate the amount of error being introduced into the system, and how that error will degrade the performance of the telescope, and even what it MEANS to say that the contrast is degraded (Refractor people somehow seem inclined to believe that the sky is blacker in a refractor because of the contrast and Suiter indirectly addresses this issue). Mr. Suiter is very careful to say repeatedly that the errors are presented as individual errors and cautions us that it is VERY rare to see a SINGLE type of error displayed, and estimating errors becomes a bit more challenging if more than one type of error exists because the errors can “Mix” making it hard to give a clear, comprehensive assessment.

Of particular interest to me was “Chapter 15.4 - The Myth of the Complex Telescope.” In this sub-section Mr. Suitor is absolute in his conviction that once the optics are finished processing the incoming signal, what is left is a “free wave propagating in space” and either the beam converges toward a stellar point, or it does not. He says “The test neither knows nor is sensitive to the complexity of what processed that beam.” As a re-enforcement, he provides a model of the conic slice generated by a perfect 8” SCT, and no surprise to me, the slices are IDENTICAL inside and outside of focus. In particular, the secondary shadow breakout (which is the primary method of judging spherical aberration, and this is important because SA is the most common “Serious” error in most mass produced SCTs) for the perfect model was PERFECTLY BALANCED. And I have owned only two SCTs that have showed this almost perfectly balanced breakout (1/7th on my current C14, and 1/8th wave on Gwen, the last C8 that I owned and stupidly sold). I have owned and tested many more compound telescopes that did not test this well.

And likewise, when I test refractors, I use the 30% obstruction method. I personally don’t think I can reliably judge spherical aberration in refractors without obstructing them. The obstruction just makes it much easier to see the error. In early 2008, I had one individual contact me and express concern about his very expensive APO. He was concerned that his telescope was not delivering contrasty images. Now this was an instrument from a PREMIUM telescope maker, and he had owned other scopes from this manufacturer, and in his heart, he felt that something was not right. He used an unobstructed test and basically said that the rings didn’t look identical, but he was unsure if they should (because of the “MYTH, no doubt). I asked him to obstruct the system and take a picture. When the picture arrived it was CLEAR that there was a (for a PREMIUM telescope) substantial amount of what looked like spherical aberration present. Now at the same time, and unknown to me, the individual was also in contact with the owner of another company that makes premium telescopes. To my utter delight, that individual also basically said “Send it back to the importer” and concurred that there was something pretty seriously wrong (for a premium scope). Now the other gentleman has a business interest to protect, and I think it would have been dangerous legally for him to offer an estimation of the severity, but I easily estimated that the error showed about 1/5th wave of SA. EASY to see with a 30% obstruction, but not easy to see visually even using filters (as the owner told me he did). But the obstruction made the error really jump out. Armed with this information, he was able to contact the importer and return the scope and receive a replacement. Now many people might have simply looked at the unobstructed pattern and said “Well, the rings look pretty similar, so it must be OK, because XYZ wouldn’t sell me a bad telescope and complex telescopes don’t star test the same as simple telescopes, so that must be it.” And I am SURE that they didn’t INTENTIONALLY sell him a bad scope. Either it somehow got though their quality control (Hubble did.. LOL), or perhaps there was some assembly error. I don’t know. I simply know that the star test EASILY showed a condition that SHOULD NOT HAVE BEEN PRESENT. Mark up a BIG win for the star test. When you pay over $3000 for a small refractor, to me, you should be able to DEMAND that the optics test well for SA using the 30% obstruction method. In this case, it did not, and the owner was armed with a very powerful piece of evidence that all was not right. The new telescope (same model) passed the breakout test. Hmmm.

Mr. Suiter does however make the following observation, and I believe that this is where the myth has its roots. Mr. Suiter says that this test is best used with a medium amount of defocus, and I believe that higher order spherical aberrations CAN INDEED make an otherwise perfect telescope with perfect SA correction appear to not test PERRECT when TO LITTLE DEFOCUS is used. This I think may occur when the telescope has a very small secondary mirror and the tester does not use a 30% obstruction (which is what Suiter recommends). The reason for using the 30% obstruction is to de-sensitize the test so that these small defocus amounts are avoided. Mr. Suiter re-enforces this point by saying that you should not complain to a manufacture about a residual aberration of no harm that was detected when very small amounts of defocus were used. This I believe is the fire behind the myth is fanned. When used with very small amounts of defocus this test is amazingly sensitive and even the tiniest amount of aberration will show up. And when I say tiny, I mean so small as to be absolutely meaningless. THIS I believe is where the myth started. Mr. Suiter is quite clear stating that for this test to be used reliably, you have to use a reasonable amount of defocus (not to little, not to much) and the 30% obstruction method was (I am sure) selected by Mr. Suiter to provide the right balance of sensitivity so that these kinds of errors can be avoided.

To conclude the review, the appendix sections touch on topics like other test methods (and in particular debates the validity of the Ronchi test, and I agree, the test is very hard to use reliably based on my own experience) further details on the technical background around the star test, and most interesting of these, Appendix G, which gives details on several designs referenced in the main text, including the aluminized spot MCT and the 8” SCT. This is a particularly interesting chapter that the “MCT vs. SCT” crowd should find enlightening.

As you can see, my personal commentary has been intermixed with the book review, but so far, I have only included commentary more or less specifically tied to quotes from the author.

Here is the real personal commentary. Why is star testing so important, and why do I encourage all serious telescope hobbyists (because it seems that many reading these articles and reviews enjoy the equipment hobby almost as much as the observing and I myself am clearly in that camp) to embrace the star test?

The reason is simple. An educated consumer makes it harder for manufacturers to get by with producing barely passable telescopes. And the more knowledgeable we are as consumers, the more likely that the manufactures will learn that to compete, they have to strive to keep the quality not just consistently “Passable,” but rather consistently very good to excellent (and my own experience is that it truly is hard to tell a very good telescope from an excellent one, but rather easy to see the difference between an excellent one and a barely passable one). In a large community like ours, when a manufacturer figures out that we know what we are talking about when we provide meaningful evaluations, they start to become more interested in ensuring that their products are REPEATEDLY and RELIABLY testing well.

I must have re-read the original edition of this book 15 times, and refer it constantly. And to answer the “Should I upgrade to Second Edition?” question, I would say if you consider yourself to be an accomplished tester, then maybe not. The second edition does not give anything “New” with respect to doing what you have been doing. It does however give a VERY strong conviction to the validity of the methods, and if you have truly been testing as suggested, you can feel confident that your assessments are reasonably solid. I am happy I have the second edition though, and a great deal of clarity has been lavished on this work, so you still might find it worth the price. I got it as a gift, but had I not, I would have purchased it anyway, and now having read it, know that I would have felt it to be money well spent had I purchased the upgrade.

If you are only just getting interested in testing though, Second Edition is a masterpiece work. It dispels the myths that surface in many forums and provides a very comprehensive and easy to understand body of knowledge on how you can tell for yourself if your telescope is performing acceptably for the design and price point. If you think that it is not, and the star test validates your position, you can also speak intelligently to the manufacturer. Remember, you get what you pay for, but as long as we continue paying for telescopes with “Passable” optics, vendors will continue to sell them. And there is NOTHING WRONG with this. Passable means that the telescope should perform OK. LOL.. Like that is what I want…. The smart vendors (Intes-Micro for example) know that their TRUE market position is not in just producing more specialzed designs, but rather in consistently high standards of execution. Every Intes-Micro scope I have ever start tested has tested among the best of all the telescopes I have personally tested. Interestingly, I have tested a couple of 8” SCTs that tested ALMOST A GOOD! And I have tested many more SCTs and MCTs that tested from OK to downright bad. Wouldn’t you like to KNOW if your telescope is at least of acceptable quality?

This is NOT an equipment “Must Have” book, but if you REALLY care about optical quality in the instruments you own, I don’t see how you can sleep without it. I personally put it at the top of my list of recommended “accessories” though.

This book is better than the first, which was already one of the most important books in my library, and receives my highest possible recommendation. Star Testing Astronomical Telescopes Second Edition is the amateur’s bible of assessing optical quality. If you REALLY care, you should buy this book and READ IT THREE TIMES!!!!

And thank you, Mr. Suiter, for caring. I believed in your first work, and you have given me a strong conviction that the tests I have done in the past were reliable. Personal thanks for adding the amazing amount of clarity and important detail to this definitive work.

The amateur community should hoist you onto their shoulders for your efforts in educating us. And if you ever find yourself in Austin Tx, Dinner and drinks are on me.