Star Testing Astronomical Telescopes

Star Testing Astronomical Telescopes
A Manual for Optical Evaluation and Adjustment
By: Harold Richard Suiter

I was surprised to see that no review of this book has previously been posted on Cloudy Nights. This volume is, in my opinion, an absolute MUST read (Must Own?) reference book for anyone that has even the most casual interest in how to judge optical quality of telescopes. In fact, the book is really much more than that. Reading this book will do more to convince owners of the importance of proper care and alignment of their telescopes than anything else I can even remotely imagine. It will also give you a very comprehensive understanding of how challenging getting great performance from ANY telescope on any given night can be.

Optical evaluation is both simple and difficult. I have learned over the years that good optics are easy to recognize with star testing. When optics aren’t good, it is easy to see that using a star test, but determining the exact nature of the optical aberration can be more difficult. This book will teach you to do both.

There are 15 chapters in this book, and I would be lying if I said that everything in it was easy to understand, but the material is handled in such a way that you can often gloss over the extremely technical explanations and still easily come away with the important points clearly understood.

The book starts with a general introduction in Chapter 1, followed by An Abbreviated Star Test Manual in Chapter 2. Many people would think that they can capture the essence of star testing by studying Chapter 2 alone, but you would be doing yourself a great disservice not to continue into the more substantial explanations of the effects of various aberrations, which follows later.

Chapter 3 is a complex chapter that deals with the various filters that interpose themselves between the observer and their target. I think that this chapter by itself does more to explain how challenging it really is to get a decent view of celestial objects. If you have never seen the filter chart as presented by Suiter, you can’t realize how much distortion images are subject to. Suiter does an excellent job of explaining each filtering effect, how severe it is, and if there is anything that can be done to overcome the effects. This chapter should be studied carefully.

Chapter 4 deals with diffraction, and while the reading here can be tedious, it does lay a framework for explaining WHY different aberrations affect images the way they do. The good news is that in my opinion, it is not so essential to understand the physics. You can still do an excellent job of evaluating optics after reading this book, even if you don’t invest time in understanding the complete physics. Skipping the in-depth physics won’t detract from your ability to use the material in the later chapters to good effect.

Chapter 5 deals with how to perform a star test, and includes several example scenarios. Of particular interest is the material dealing with the creation of artificial stars. My own personal experience has led me to believe that except for very small telescopes, the only way to get really reliable quantification of error on a star test would be to use an artificial source. This chapter will tell you exactly how to go about creating these artificial stars and how to calculate the distances you would need to use to place them. You will also learn the amount of de-focus required for telescopes of different apertures and focal ratios. Finally, the simplest method of calculating the magnitude of over/under-correction if it is present is provided.

Chapter 6 provides a comprehensive manual for optical alignment, with particularly good treatment for aligning Newtonian optics. While you might be thinking, “I know how to align my telescope,” what you might not really understand is the enormous effect that even small amounts of misalignment can have on an image. Reading the beginning of this chapter will scare you into aligning your optics before you go to bed every night! Not to give away the big surprise, but I think that Harold Suiter makes a case that this is one area that most amateurs can make the biggest improvement in the performance of their reflecting telescopes. As an aside, my personal experience is that he is correct. I used to do the “Big doughnut” school of alignment, and after reading Suiter’s chapter on alignment, I came to believe that big doughnut alignment was insufficient. After numerous comparisons of images where alignment was exact to images where alignment was “close”, I have come to appreciate that only EXACT alignment will allow your telescope to perform anywhere NEAR its full potential.

Chapters 7 through 14 deal with specific problems, and provide excellent images to instruct you on how to recognize and diagnose the severity of each problem. De-focused images are used to show the effect of each aberration on the fresnel ring pattern of a defocused star image, and how a star would look in-focus. This is the meat of the book, and each chapter is easy to understand and concise in the treatment of each various aberration.

Read the chapter on Turbulence twice. Then read it again. Turbulence is the root of all optical evil. If your telescope doesn’t show images like in people’s pictures, this chapter will explain why. I have only had two or three nights of perfect seeing in all of my observing years. Believe me, even a marginal telescope can show astounding detail when seeing is good. Anything less than PERFECT seeing makes you telescope fall far short of whatever potential it can deliver. This chapter will explain how severely even moderate seeing can degrade the image at the eyepiece.

Chapter 15 is an eye-opener. It basically puts all of the aberrations together and shows you how the cascade of small problems can accumulate to contribute to a severely degraded image. It also offers some simple circumvention for dealing with optical problems, such as getting a larger telescope! In fact, I will confess that after reading this chapter, I was able to more easily understand why people are willing to pay $6000 for a 6” APO refractor for planetary observation.

I will mention Appendix A only because of its treatment of Ronchi grating test. I purchased one of these gratings many years ago, and based on this test, thought every telescope I ever tested with it seemed to have perfect optics, even though visual evaluations suggested differently. This appendix explains what this test is a questionable way to evaluate optics.

This book is in most library systems, so if you can’t afford to own it, you defiantly should check out a copy and read it. If it is not in your system, your library probably has agreements with other regional systems to share. Request it.

For the rest of you, if you have any interest in the hardware of observational amateur astronomy, I would recommend this book as a Must-Have. Everything you see in your telescope is affected to some extent by practically everything in this book. Until you commit to reading it, you can’t really understand TO WHAT EXTENT your images are being affected. Maybe, for you and your equipment, it is as good as it can get. And maybe not. Aren’t you curious to know?

Ps. As an aside, it is easy to obsess on optical quality. My personal experience is that most optics produced today by the major manufacturers are reasonably good. I have owned perhaps 12 telescopes in the last 10 years (I own 5 now) and all but two have been pretty good optically, and of those, one was a fast achromatic refractor, which is a very challenging design. The bad one was really bad, and I was quite surprised (and upset) when I obtained it (in fact, this is an ongoing situation and I have stretched my patience trying to resolve the problem with the vendor). With practice, you can get pretty good at quantifying the amount of problem, but in actual use, it is not as easy to see the differences between 1⁄4 wave and 1/6 wave of spherical under-correction when viewing a planet as it is when performing the star test. All other things being equal, most people would struggle to see a noticeable difference in these two spherical correction errors when viewing planets! Obsess over alignment, because you can control that, but don’t obsess over a sixth of a wave spherical correction difference between you and your buddy’s otherwise identical telescopes. If you can’t help but obsess over that small of a correction error difference, consider consulting a health care specialist. The difference between psychiatrists and psychologists is that the former can prescribe drugs. I am including that tip in case your obsession is extreme! Also, consider living in outer space. It is the only place where your telescope can perform to its full potential most of the time (remember…turbulence is evil).

My warm regards,
Ed Moreno
eddgie@hotmail.com

Accurate quantification of error for some aberrations such as spherical correction or surface roughness can be greatly complicated by poor seeing. While it is possible to detect these errors easily in less than stable sky conditions, it is much more difficult to make the accurate estimates of the severity of the aberration in average seeing conditions. Light is moved around between rings during less than perfect seeing conditions and the intensity between rings can vary moment by moment. Smaller apertures are less susceptible to these effects. Over about 6”, accurate estimation becomes challenging, and at 10” it becomes almost impossible, unless conditions are Pickering 8 or better.