Book
Categories:

Star Testing Astronomical Telescopes

by Suiter, 9.00" by 6.00", 376 pages, published 1994, hardbound, 2 Lbs., 9 Ozs. ship wt., $29.95.

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.

Southern Stars

Some Examples

Testing your optics to confirm quality is obvious. What is less straightforward is the way of testing. Telescope makers can use a variety of techniques, but ordinary telescope users find that learning a workshop method is difficult. They have only one mirror that doesn't change, so it is easier to test it on the sky. The star test is a good way of evaluating instruments for one-time users.

You must be careful to test the instrument when it has cooled off and is under fairly tranquil skies, as the following spherical aberration with strong turbulence figure shows.

The following aberration types figure shows some common difficulties with telescope optics.

Table of Contents

Foreword
An Introduction to the Author
Preface
1. Introduction
    1.1. Telescope Evaluation
    1.2. Testing the Surfaces
        1.2.1. Sources of Errors
        1.2.2. Measures of Optical Quality
    1.3. The Star Test—A Brief Overview
        1.3.1. Diffraction Rings
    1.4. The Reason for Star Testing
2. An Abbreviated Star-Test Manual
    2.1. Some Necessary Preliminaries
    2.2. Optical Problems in Turn
        2.2.1. Secondary Mirror Obstruction
        2.2.2. Misalignment
        2.2.3. Atmospheric Motion and Turbulence
        2.2.4. Tube Currents
        2.2.5. Pinched or Deformed Optics
        2.2.6. Spherical Aberration
        2.2.7. Rough Surfaces
        2.2.8. Zonal Aberrations
        2.2.9. Turned Edges
        2.2.10. Astigmatism
    2.3. Concluding Remarks
3. Telescopes Are Filters
    3.1. Perceptions of Reality
    3.2. A Comparison to Audio
        3.2.1. Aperture Diameter/Size of Speakers
        3.2.2. Colored Filters/Equalizer Filters
        3.2.3. Image Processing/Signal Processing
        3.2.4. 3.2.4 Scattered Light/Audio Noise
        3.2.5. Spatial Frequency/Audio Frequency Responses
    3.3. The Modulation Transfer Function (MTF)
    3.4. The MTF in Use
        3.4.1. MTF Associated with Defocusing
        3.4.2. Stacking of MTFs
4. Diffraction
    4.1. The Coordinates of Light
    4.2. The Consequence of Filtering
    4.3. Waves Are Reborn
        4.3.1. Diffraction and Focusing
        4.3.2. Fresnel Zones
        4.3.3. Fresnel Zones with Defocus
    4.4. Nodes and Antinodes
    4.5. Other Aberrations—The Pupil Function
5. Conducting the Star Test
    5.1. Defocusing and Sensitivity
        5.1.1. Focuser Motion Related to Defocusing Aberration
        5.1.2. Sensitivity of the Star Test
    5.2. Artificial Sources
        5.2.1. Distance of Artificial Sources
        5.2.2. Diameter of Artificial Sources
        5.2.3. Using a Reflective Sphere Instead of a Pinhole
        5.2.4. Setting Up a Nighttime Artificial Source
    5.3. Performing the Test
        5.3.1. 8-Inch f/6 Newtonian Reflector
        5.3.2. 16-Inch f/4 Dobson-mounted Newtonian
        5.3.3. 6-Inch f/12 Apochromatic Refractor
        5.3.4. 8-Inch f/10 Schmidt-Cassegrain Catadioptric
6. Misalignment
    6.1. Kinematic View of Alignment
    6.2. Effects of Misalignment
    6.3. The Aberration Function of the Misaligned Newtonian
    6.4. Filtration of a Misaligned Newtonian
    6.5. Aligning Three Telescopes
        6.5.1. The Newtonian Reflector
        6.5.2. The Refractor
        6.5.3. The Schmidt-Cassegrain
7. Air Turbulence and Tube Currents
    7.1. Air As a Refractive Medium
    7.2. Turbulence
        7.2.1. The Aberration Function
        7.2.2. Filtering Caused by Turbulence
        7.2.3. Observing Turbulence
        7.2.4. Corrective Action
    7.3. Tube Currents
        7.3.1. The Aberration Function
        7.3.2. Filtering of Tube Currents
        7.3.3. Observing Tube Currents
        7.3.4. Corrective Actions for Tube Currents
8. Pinched and Deformed Optics
    8.1. Causes
    8.2. The Aberration Function
    8.3. Filtering of Pinched Optics
    8.4. Diffraction Patterns of Pinched Optics
    8.5. Fixing the Problem
9. Obstruction and Shading
    9.1. Central Obstruction
    9.2. Spider Diffraction
    9.3. Shading or Apodization
    9.4. Dust and Scratches on the Optics
10. Spherical Aberration
    10.1. What Is Spherical Aberration?
    10.2. The Hubble Space Telescope
    10.3. Generalized Spherical Aberration
    10.4. The Aberration Functions
    10.5. Correction Error (Lower-Order Spherical Aberration)
        10.5.1. Filtering of Spherical Aberration
        10.5.2. Star-Test Patterns of Correction Error
        10.5.3. Estimation of the Severity of the Problem
    10.6. Testing for Correction
    10.7. Higher-Order Spherical Aberration
        10.7.1. Star-Test Patterns of Higher-Order Spherical\newline Aberration
        10.7.2. Filtering of Higher-Order Spherical Aberration
    10.8. A Compact, Uniform Standard for Optical Quality
    10.9. Tolerable Errors
11. Circular Zones and Turned Edges
    11.1. Causes of Zonal Defects
    11.2. Interior Zones
        11.2.1. Aberration Function of S-Zones
        11.2.2. Filtering of S-Zones
        11.2.3. Detecting Interior Zones in the Star Test
    11.3. Turned Edges
        11.3.1. Aberration Function
        11.3.2. MTF of Turned Edge
        11.3.3. Image Pattern of Turned-Down Edge
        11.3.4. Signal-to-Noise Ratio of a Turned Edge
        11.3.5. The Width of the Turned Edge
        11.3.6. Remedies for Turned Edge
12. Chromatic Aberration
    12.1. Dispersion
    12.2. The Achromatic Lens
    12.3. Residual Chromatic Aberration
    12.4. The Apochromat
    12.5. Testing Refractors for Other Aberrations
    12.6. The Star Test for Chromatic Effects
        12.6.1. Wedge, Assembly Errors, and Atmospheric Spectra
        12.6.2. Star Test for Conventional Astronomical Visual Doublets
        12.6.3. Star Test of Apochromats or Advanced Refractors
        12.6.4. Chromatic Effects in the Eye
        12.6.5. The Eyepiece   
    12.7. Conclusions and Remedies
13. Roughness
    13.1. Roughness Scales and Effects
    13.2. The Terminology of Roughness
    13.3. Medium-Scale Roughness, or Primary Ripple
        13.3.1. The Aberration Function of Medium-Scale Roughness
        13.3.2. Filtering Effects of Medium-Scale Roughness
        13.3.3. Star Test on Medium-Scale Roughness
        13.3.4. Roughness and Turbulence
    13.4. Small-Scale Roughness, or Microripple
        13.4.1. The Aberration Function of Small-Scale Roughness
        13.4.2. Filtering of Small-Scale Roughness
        13.4.3. The Great Unknown
14. Astigmatism
    14.1. Astigmatism in Eyes and Telescope Optics
    14.2. Causes of Astigmatism
    14.3. Aberration Function of Astigmatism
    14.4. Filtering of Astigmatism
    14.5. Star-Test Patterns
    14.6. Identification in Newtonian Reflectors
    14.7. Refractors or Schmidt-Cassegrains
    14.8. Remedies
15. Accumulated Optical Problems
    15.1. Breaking the Camel's Back
    15.2. Fixing the Telescope
    15.3. Errors on the Glass
    15.4. Testing Other Telescopes
    15.5. When Everything Goes Right
A. Other Tests
    A.1. The Foucault Test
    A.2. The Hartmann Test
    A.3. Resolution of Double Stars
    A.4. Geometric Ronchi Test
    A.5. Interferometry
    A.6. How Do Interferometers Work?
    A.7. The Point-Diffraction Interferometer
    A.8. The Null Test
B. Calculation Methods
    B.1. Diffraction Concepts
    B.2. The Fraunhofer and Fresnel Approximations
    B.3. Image Calculations for Symmetric Apertures
    B.4. Image Calculations for Nonsymmetric Apertures
    B.5. The Programs
        B.5.1. Symmetric Pupil Function
        B5.2. Asymmetric Pupil Function
    B.6. Verification of Numerical Procedure
        B.6.1. Comparison of APERTURE and ASYMM
        B.6.2. A Numerical Comparison with an Analytic Solution
        B.6.3. Comparison with Published Patterns
    B.7. Numerical Limitations on Programs
    B.8. Difficulties in Printing
C. Minor Axis and Offset Derivation
    C.1. Derivation
    C.2. Test Case
    C.3. Approximations
D. Labeling of Diffraction Patterns
E. Eyepiece Travel and Defocusing Aberration
F. Glitter in a Shiny Sphere
G. List of Common Symbols
Glossary
Bibliography
Index 

Copyright ©1998–1999 Willmann-Bell, Inc. All rights reserved.