Practical Amateur Astronomy Digital SLR Astrophotography

Michael A. Covington

Cambridge

UNIVERSITY PRESS

CAMBRIDGE UNIVERSITY PRESS

Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, Sao Paulo Cambridge University Press

The Edinburgh Building, Cambridge CB2 8RU, UK

Published in the United States of America by Cambridge University Press, New York www.cambridge.org

Information on this title: www.cambridge.org/9780521700818 © M. A. Covington 2007

This publication is in copyright. Subject to statutory exception and to the provision of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press.

First published in print format 2007

ISBN-13 978-0-511-37853-9 eBook (NetLibrary) ISBN-13 978-0-521-70081-8 paperback

Cambridge University Press has no responsibility for the persistence or accuracy of urls for external or third-party internet websites referred to in this publication, and does not guarantee that any content on such websites is, or will remain, accurate or appropriate.

Soli Deo gloria

Contents

Preface page xiii

Part I Basics 1

1 The DSLR revolution 3

1.2 Choosing a DSLR 6

1.2.1 Major manufacturers 6

1.2.2 Shopping strategy

1.3 Choosing software 8

1.3.1 Photo editing 9

1.3.2 Astronomical image processing 9

1.3.3 Freeware 9

1.3.4 Judging software quality 10

1.4 Is a DSLR right for you? 10

1.5 Is film dead yet? 12

2 Main technical issues 14

2.1 Image files 14

2.1.1 File size 14

2.1.2 Raw vs. compressed files 14

2.1.3 "Digital film" and camera software 15

2.2 Focusing 15

2.3 Image quality 16

2.3.1 Combining images 16

2.3.2 Overcoming sky fog 16

2.3.3 Dark-frame subtraction 16

2.3.4 The Nikon "star eater" 17

2.3.5 Grain 17

2.4 Sensor size and multiplier (zoom factor) 19

2.5 Dust on the sensor 19

2.6 ISO speed settings 21

2.7 No reciprocity failure 21

2.8 How color is recorded 22

2.8.1 The Bayer matrix 22

2.8.2 Low-pass filtering 23

2.8.3 The Foveon 23

2.9 Nebulae are blue or pink, not red 23

3 Basic camera operation 26

3.1 Taking a picture manually 26

3.1.1 Shutter speed and aperture 26

3.1.2 Manual focusing 26

3.1.3 ISO speed 28

3.1.4 White balance 28

3.1.5 Do you want an automatic dark frame? 29

3.1.6 Tripping the shutter without shaking the telescope 30

3.1.7 Mirror vibration 30

3.1.8 Vibration-reducing lenses 32

3.2 The camera as your logbook 32

3.3 Limiting light emission from the camera 33

3.4 Menu settings 33

3.4.1 Things to set once and leave alone 33

3.4.2 Settings for an astrophotography session 34

3.4.3 Using Nikon Mode 3 35

3.5 Determining exposures 35

3.6 Cool-down between long exposures 36

4 Four simple projects 38

4.1 Telephoto Moon 38

4.2 Afocal Moon 39

4.3 Stars from a fixed tripod 40

4.4 Piggybacking 44

4.5 Going further 45

Part II Cameras, lenses, and telescopes 47

5 Coupling cameras to telescopes 49

5.1 Optical configurations 49

5.1.1 Types of telescopes 49

5.1.2 Types of coupling 50

5.2 Fitting it all together 53

5.3 Optical parameters 55

5.3.1 Focal length 55

5.3.2 Aperture 56

5.3.3 f -ratio and image brightness 56

5.3.4 Field of view 58

5.3.5 Image scale in pixels 60

5.3.6 "What is the magnification of this picture?" 61 5.4 Vignetting and edge-of-field quality 61

6 More about focal reducers 63

6.1 Key concepts 63

6.2 Optical calculations 64

6.3 Commercially available focal reducers 67

6.3.1 Lens types 67

6.3.2 Meade and Celestron f /6.3 67

6.3.4 Others 69

7 Lenses for piggybacking 70

7.1 Why you need another lens 70

7.1.1 Big lens or small telescope? 70

7.1.2 Field of view 71

7.2 Lens quality 73

7.2.1 Sharpness, vignetting, distortion, and bokeh 73

7.2.2 Reading MTF curves 74

7.2.3 Telecentricity 75

7.2.4 Construction quality 76

7.2.5 Which lenses fit which cameras? 76

7.3 Testing a lens

7.4 Diffraction spikes around the stars 78

7.5 Lens mount adapters 80

7.5.1 Adapter quality 81

7.5.2 The classic M42 lens mount 82

7.6 Understanding lens design 84

7.6.1 How lens designs evolve 84

7.6.2 The triplet and its descendants 86

7.6.3 The double Gauss 87

7.6.4 Telephoto and retrofocus lenses 87

7.6.5 Macro lenses 88

8 Focusing 89

8.1 Viewfinder focusing 89

8.1.1 The viewfinder eyepiece 89

8.1.2 The Canon Angle Finder C 90

8.1.3 Viewfinder magnification 92

8.1.4 Modified cameras 92

8.2 LCD focusing 92

8.2.1 Confirmation by magnified playback 92

8.2.2 LCD magnification 94

8.3 Computer focusing 94

8.4 Other focusing aids 95

8.4.1 Diffraction focusing 95

8.4.2 Scheiner disk (Hartmann mask) 96

8.4.3 Parfocal eyepiece 96

8.4.4 Knife-edge and Ronchi focusing 96

8.5 Focusing telescopes with moving mirrors 98

9 Tracking the stars 99

9.1 Two ways to track the stars 99

9.2 The rules have changed 100

9.3 Setting up an equatorial mount 102

9.3.1 Using a wedge 102

9.3.2 Finding the pole 103

9.3.3 The drift method 104

9.4 Guiding 106

9.4.1 Why telescopes don't track perfectly 106

9.4.2 Must we make corrections? 106

9.4.3 Guidescope or off-axis guider? 107

9.4.4 Autoguiders 108

9.4.5 A piggyback autoguider 110

9.5 How well can you do with an altazimuth mount? 110

9.5.1 The rate of field rotation 111

9.5.2 Success in altazimuth mode 114

9.5.3 What field rotation is not 115

10 Power and camera control in the field 116

10.1 Portable electric power 116

10.1.1 The telescope 116

10.1.2 The computer and camera 117

10.1.3 Care of Li-ion batteries 117

10.1.4 Ground loop problems 118

10.1.5 Safety 118

10.2 Camera control 119

10.2.1 Where to get special camera cables 119

10.2.2 Tripping the shutter remotely 119

10.2.3 Controlling a camera by laptop 122

10.3 Networking everything together 124

10.4 Operating at very low temperatures 125

11 Sensors and sensor performance 127

11.1 CCD and CMOS sensors 127

11.2

Sensor specifications

129

11.2.1

What we don't know

129

11.2.2

Factors affecting performance

129

11.2.3

Image flaws

131

11.2.4

Binning

133

11.3

Nebulae, red response, and filter modification

133

11.3.1

DSLR spectral response

133

11.3.2

Filter modification

135

11.3.3

Is filter modification necessary?

135

11.4

Filters to cut light pollution

138

11.4.1

Didymium glass

138

11.4.2

Interference filters

138

11.4.3

Imaging with deep red light alone

141

11.4.4

Reflections

141

Part III Digital image processing

143

12

Overview of image processing

145

12.1

How to avoid all this work

145

12.2

Processing from camera raw

146

12.3

Detailed procedure with MaxDSLR

147

12.3.1

Screen stretch

148

12.3.2

Subtracting dark frames

149

12.3.3

Converting to color (de-Bayerization, demosaicing)

151

12.3.4

Combining images

154

12.3.5

Stretching and gamma correction

155

12.3.6

Saving the result

158

12.4

Processing from linear TIFFs

159

12.4.1

Making linear TIFFs

161

12.4.2

Processing procedure

163

12.5

Processing from JPEG files or other camera output

163

13

Digital imaging principles

165

13.1

What is a digital image?

165

13.1.1

Bit depth

165

13.1.2

Color encoding

166

13.2

Files and formats

166

13.2.1

TIFF

166

13.2.2

JPEG

167

13.2.3

FITS

167

13.3

Image size and resizing

167

13.3.1

Dots per inch

167

13.3.2

Resampling

168

13.3.3

The Drizzle algorithm

168

13.4 Histograms, brightness, and contrast 169

13.4.1 Histograms 169

13.4.2 Histogram equalization 169

13.4.3 Curve shape 170

13.4.4 Gamma correction 170

13.5 Sharpening 172

13.5.1 Edge enhancement 172

13.5.2 Unsharp masking 172

13.5.3 Digital development 173

13.5.4 Spatial frequency and wavelet transforms 173

13.5.5 Deconvolution 175

13.6 Color control 176

13.6.1 Gamut 176

13.6.2 Color space 177

13.6.3 Color management 177

14 Techniques specific to astronomy 178

14.1 Combining images 178

14.1.1 How images are combined 178

14.1.2 Stacking images in Photoshop 181

14.1.3 Who moved? Comparing two images 183

14.2 Calibration frames 183

14.2.1 Dark-frame subtraction 183

14.2.2 Bias frames and scaling the dark frame 183

14.2.3 Flat-fielding 185

14.3 Removing gradients and vignetting 188

14.4 Removing grain and low-level noise 189

14.5 The extreme brightness range of nebulae 190

14.5.1 Simple techniques 190

14.5.2 Layer masking (Lodriguss'method) 191

14.6 Other Photoshop techniques 193

14.7 Where to learn more 195

Part IV Appendices 197

A Astrophotography with non-SLR-digital cameras 199

B Webcam and video planetary imaging 202

B.1 The video astronomy revolution 202

B.2 Using a webcam or video imager 202

B.3 Using RegiStax 206

C Digital processing of film images 207

Index 209

Preface

Digital SLR cameras have revolutionized astrophotography and made it easier than ever before. The revolution is still going on, and writing this book has been like shooting at a moving target. New cameras and new software are sure to become available while the book is at the factory being printed. But don't let that dismay you. All it means is that we'll have better equipment next year than we do now.

This book is not a complete guide to DSLR astrophotography; the time is not yet ripe for that. Nor does space permit me to repeat all the background information from my other books. For a complete guide to optical configurations and imaging techniques, see Astrophotography for the Amateur (1999). To get started with a telescope, see How to Use a Computerized Telescope and Celestial Objects for Modern Telescopes (both 2002). All these books are published by Cambridge University Press.

What I most want to emphasize is that DSLR astrophotography can be easy, easier than any earlier way of photographing the stars. It's easy to lose track of this fact because of the flurry of technical enthusiasm that DSLRs are generating. New techniques and new software tools appear almost daily, and the resulting discussion, in perhaps a dozen online forums, thrills experts and bewilders beginners.

My goal is to save you from bewilderment. You don't have to be a mathematician to get good pictures with a DSLR, just as you didn't have to be a chemist to develop your own film. I'll concentrate on simple, reliable techniques and on helping you understand how DSLR astrophotography works.

The people who contributed pictures are acknowledged in the picture captions. (Pictures not otherwise identified are my own work.) In addition, I want to thank Fred Metzler, of Canon USA, and Bill Pekala, of Nikon USA, for lending me equipment to test; Douglas George, of Diffraction Limited Ltd., for help with software; and all the members of the Canon DSLR Digital Astro, Nikon DSLR Astro, and MaxDSLR forums on YahooGroups (http://groups.yahoo.com) for useful discussions and information. As always I thank my wife Melody and my daughters Cathy and Sharon for their patience.

If you're a beginner, welcome to astrophotography! And if you're an experienced astrophotographer, I hope you enjoy the new adventure of using DSLRs as much as I have. Please visit this book's Web site, www.dslrbook.com, for updates and useful links.

Athens, Georgia March 7, 2007

Parti

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