Star Testing Astronomical Telescopes

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Star Testing Astronomical Telescopes Star Testing Astronomical Telescopes

A Manual for Optical Evaluation and Adjustment. Second Edition.

From Richard Berry's Foreword to the Second Edition It is not often that a book opens the eyes of a whole generation of amateur astronomers-but the first edition of Dick Suiter's Star Testing Astronomical Telescopes was just such a book. By giving its readers a simple, sensitive, and reliable test for optical performance, he enhanced the observing experience of every amateur astronomer who took its lessons to heart. Whether you were a novice or expert, Star Testing was important because it told you how to get the most from your telescope.

What makes the star test so great is that it's both very easy and very sensitive. The star test is so simple that you probably use it already without being aware of it. When focusing an instrument you probably roll the eyepiece back and forth a couple times before settling on the sharpest position. If the telescope is well figured, collimated properly, and the air is steady, best focus lies midway between two identical out-of-focus disks. Without even thinking about it, you just know that appearance means good optics.

To perform the test, you simply observe a star with a moderately high-power eyepiece, giving careful consideration to the image on both sides of focus. The patterns you see in the focused and out-of-focus star images tell you whether your telescope is aligned for maximum performance, whether the atmosphere is steady, when the telescope has cooled, and when it is ready to do its best.

I have known Dick Suiter for about twenty-five years, since I met him at a star party in Ohio and subsequently asked him to write a short article about star testing for Astronomy magazine. His ability to meld abstruse theory with practical observing has always impressed me, and now, in this second edition, he has produced once again a spectacular blend of theory and practice.

In the Foreword to the first edition, I told readers how star testing helped me to get top-notch performance from my 20-inch ƒ /5 Dobsonian, back in the days when not many people had experience with telescopes of that aperture. Star testing revealed tube currents, so I added a fan; it revealed a bit of spherical aberration, and I had the primary refigured. With the help of star testing, I tuned up my big Dob to give excellent lunar and planetary images. During the 1988 opposition of Mars, for example, not only did I see Deimos and Phobos for the first time, but I also enjoyed the best views I'd ever had of the planet itself.

In the years since this book first appeared, I think that the increasing number of amateurs who routinely star test telescopes has exerted a powerful but subtle influence on the telescope industry. When the first edition of Star Testing came out, it was not particularly uncommon, at a star party, to accept a proud owner's invitation to look through a new telescope-only to find yourself thinking, "Oh my gosh, does this person realize the telescope has major problems?" Over time, I am sure that amateurs who routinely star test every instrument they look through have given a few proud telescope owners a few bad nights, and that has resulted in bad days for some dealers and manufacturers. But the net result is that today, the astronomical public expects-and gets-a far better telescope than they did some twenty years ago. All because Dick Suiter taught us what to look for in a star image.

What impresses me most about this second edition is that what was already good has become even better. With great care, Dr. Suiter has improved the visual fidelity of the figures by computing effects for the whole range of wavelengths visible to the eye. He has tested differing computational schemes against one another to verify his results. He has carefully compared the complex aberrations found in real telescopes with the pure aberrations found in theory-and he demonstrates that they match.

Star Testing is a resource that should be on the shelf where you keep your most-often-used astronomy books. Toss it into your observing kit along with eyepieces, Oreo cookies, and packs of instant hot chocolate. Read it; absorb it; read it again. Star testing is an integral part of observing.

You will benefit when you become sensitive to your telescope's optical performance. If you find that it has a few problems-and what telescope does not?-it is best to deal openly with them. Would-be astronomers who refuse to acknowledge such problems tend to stop using their instruments, and eventually their interest in astronomy. Once you recognize that a problem exists, you can possibly use the star test to correct it. Often, it's nothing that careful collimation will not solve. If your telescope has properly adjusted and excellent optics, the star test will confirm that fact-and you are free to turn your attention to observing the splendors of the heavens.

Richard Berry Lyons, Oregon

More About this Book Many observers harbor misgivings about their telescope. The manufacturer may have guaranteed accuracy to "one-quarter wavelength" or as "diffraction-limited" but most telescope users have, at best, only a hazy idea of how to personally verifying such claims. Sure, there are ways to check the accuracy of individual components but for many they are hard to understand or require costly reference optics and other test equipment. Besides, telescope users are interested in the performance of the entire optical train, not just the main optical element.

What is really needed is a test that can be used at the observing site, so that all the problems that impact on a telescope's performance can be diagnosed. Isn't there a simpler and more complete way than the complicated shop tests? Yes, the star test is such a method. It uses the entire working telescope. It is not a poor substitute or a work-around that uses bits and pieces of the optical system. It is the oldest and most sensitive of the optical tests-an inspection of the diffraction image itself.

Star-test results apply to the complete imaging performance of the telescope. The star test is lightning-fast and requires only a good high-power eyepiece. It tests the telescope for precisely what it was meant to do. Bad or poorly-aligned instruments fail the star test unambiguously. The star test often allows you to correct the optical difficulty immediately in the field, when you might be frantic to have your telescope perform well to observe a once in a lifetime event.

While the star test has been around for centuries learning it has often been hampered by messy mathematics and its visual nature. Most people who use it have learned it at the elbow of a patient Master. In this book, Dick Suiter becomes your Master. He carefully shields you from difficult diffraction theory and uses advanced computer generated graphics to show you the appearance of each aberration. Again and again, you will look at Dick's graphics and say "I've seen that before. So that's what it was!" The star test is a powerful but inexpensive way of obtaining better resolution and contrast. With this book most observers will find that they don't need a new telescope because they now can test, diagnose and fix the one they have. Using Star Testing Astronomical Telescopes as a guide, your telescope will perform to the best of it's abilities and perhaps it will show images better than you would have believed possible.

From The Reviewers

Tonight's the night. That $800 telescope you've been waiting for has finally arrived. After excitedly setting it up, you center Jupiter in the eyepiece. Excitement builds. You focus and focus again, switch eyepieces, focus again. The view is horrendous! What's causing it? Is it just unstable atmospheric conditions or are the telescope's optics out of alignment? Is it heat rising off the asphalt parking lot you're observing on or is the telescope too warm? Or, horror of horrors-could it be that your new mirror is not up to specifications? And what kind of flaw is it? Rough surface? Spherical aberration? Astigmatism?

Far too many of us have experienced this disturbing scenario. But now, thanks to Star Testing, you can answer these questions easily and probably have the telescope operating in no time. And if there is an optical problem, you will be able to communicate it clearly to the dealer and get prompt action.

Chapter One and Two explain basic optics in a fashion that any motivated beginner can follow. Computer-generated illustrations of defocused star images are so realistic that you can learn a great deal by just looking at the pictures . . . Star Testing is bound to have a big impact on our hobby. Harold Suiter wants to help buyers assess optical quality so that it plays a larger role in purchasing decisions. This, he feels, will give manufacturers added incentive to produce superior products. In my opinion Suiter will succeed-if enough of us buy this book and read it. Its cost is a small price to pay for becoming an informed consumer.

Astronomy magazine

"I'm going to tell you a little-known fact," begins Harold Richard Suiter in his new book . . . "Telescopes are easy to test." It's true. The hard part for most amateurs has been finding out exactly how to do it . . . Now, at last, Suiter has analyzed the star test in book-length thoroughness. He presents a bounty of information and instruction in a clear, practical manner never before available . . . The book displays with perfect clarity all the star test comparison images you'll ever need, illustrating all kinds of telescope aberrations in their pure forms . . . Those are just highlights of this long overdue book. It quantifies almost all the effects it discusses, presents modulation-transfer functions indicating how they affect different types of observing, delves into diffraction theory, and yet is full of advice and experience from real-world amateurdom.

Sky & Telescope magazine

It is very rare to find a book that has such an immediate appeal to the telescope maker, observational astronomer, and theoretical physicist….A first casual inspection of the book indicates that it should reside on the applied optics book shelf of a Physics Department library. Nothing could be further from the truth. Suiter, who is an experimental physicist, has been very successful in using everyday analogies to explain the fundamentals of diffraction optics. There is a great deal of good practical information for those readers prepared to persevere. For those with a more than casual approach to their telescopes, this book will become in the widest sense, a benchmark in astronomical telescope testing literature. Most importantly, it will give some weight to increasing the quality assurance standards of commercial telescopes, from the viewpoint of a better informed user.

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