How to Use a Computerized Telescope

Michael A. Covington

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/9780521007900 © Michael A. Covington 2002

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 2002

ISBN-13 978-0-511-33809-0 eBook (NetLibrary) ISBN-10 0-511-33809-0 eBook (NetLibrary)

ISBN-13 978-0-521-00790-0 paperback ISBN-10 0-521-00790-9 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 xv

PartI

Telescopes in general

1

1

Welcome to amateur astronomy!

3

1.1

Using a telescope

3

1.2

Learning the sky

4

1.3

Is a computerized telescope right for you?

4

1.4

Material you can skip

5

1.5

Does this book cover your telescope?

5

2

How the sky moves

6

2.1

Daily motion

6

2.2

Coordinates

8

2.2.1

Right ascension and declination

8

2.2.2

Declination and latitude

9

2.2.3

Some terminology

10

2.2.4

Other coordinate systems

10

2.2.5

Degrees, minutes, and seconds

11

2.2.6

Distance between points in the sky

12

2.3

Annual motion

12

2.3.1

Why time of year matters

12

2.3.2

Sidereal time

13

2.4

Time of day

13

2.4.1

Solar time and time zones

13

2.4.2

Hints on using UT

15

2.4.3

UTC, ET, and other subtleties

17

2.5

Slow changes in R.A. and declination

17

2.5.1

Precession and epochs

17

2.5.2

How precession works

17

2.5.3

Proper motion

18

3 How telescopes track the stars

3.1 What's inside a computerized telescope

3.1.1 Computer

3.1.2 Motors

3.1.3 Encoders

3.1.4 Digital setting circles (DSC)

3.2 Altazimuth and equatorial mounts

3.3 Site information

3.3.1 Why it's needed

3.3.2 Obtaining site data

3.4 Why compasses don't point north

3.5 Setting up the telescope

3.6 Choosing alignment stars

3.7 Automatic setup with GPS

3.8 Tripods and piers

3.8.1 Steadiness

3.8.2 Other tripod and wedge hints

3.8.3 Observatories and permanent piers

3.9 Pointing accuracy

3.9.1 What to expect

3.9.2 Factors that affect pointing accuracy

3.9.3 Aligning the telescope tube in the mount

3.9.4 The double-GO TO trick

3.9.5 Meade high-precision mode

3.9.6 TPoint Software

3.9.7 What the telescope is calculating

3.10 Computer control

3.11 Electricity for telescopes

4 Using equatorial mounts and wedges

4.1 Why equatorial?

4.2 Must field rotation be eliminated?

4.3 Using an equatorial mount

4.3.1 Setting up the mount

4.3.2 Rough polar-axis alignment without sighting stars

4.3.3 Finding Polaris

4.3.4 Rough polar-axis alignment on Polaris

4.3.5 Initializing the computer

4.4 Refining the polar alignment

4.4.1 Iterating on Polaris and one other star

4.4.2 Fine alignment - the drift method

4.5 Using wedges

4.6 Tracking in equatorial mode 4.6.1 Tracking rates

19 19

28 29 29

32 32 32 34

35 35

39 39 39 43 43

49 49 51 51

Viii

4.6.2 Periodic-error correction (PEC) 53

4.7 Setting circles 53

4.8 Southern declination limits 55

4.9 German equatorial mounts 56

5 Telescope optics 59

5.1 How a telescope works 59

5.2 Upside down and backward images 59

5.3 Light grasp and image brightness 61

5.4 Resolving power 63

5.5 Types of telescopes 64

5.5.1 Refractors, reflectors, and catadioptrics 64

5.5.2 Catadioptric quirks 66

5.5.3 "Fast" and "slow" f -ratios 67

5.5.4 Does the central obstruction ruin the image? 68

5.5.5 Which design is best? 69

5.6 Collimation 70

5.6.1 Collimating a Schmidt-Cassegrain 70

5.6.2 Collimating a Newtonian 73

5.7 Star testing 74

5.8 Buying a telescope secondhand

5.9 Cleaning optics 78

5.9.1 Lenses 78

5.9.2 Mirrors 78

6 Eyepieces and optical accessories 79

6.1 What eyepieces do you need? 79

6.2 Barrel size 79

6.3 Field of view 80 6.3.1 Measuring field of view 82

6.4 Eye relief 83

6.5 Eyepiece designs 83

6.6 New-generation eyepieces 85

6.7 Anti-reflection coatings 86

6.8 Choosing eyepieces wisely 86

6.9 Eyepiece calculations and technical details 87

6.9.1 The exit pupil 87

6.9.2 Limits on low power 88

6.9.3 Limits on high power 89

6.9.4 Field stop and tube size 89

6.10 Eyepiece accessories 90

6.10.1 Diagonals 90

6.10.2 Barlow lenses 92

6.10.3

Focal reducers (compressors)

93

6.10.4

Filters

94

6.11

Eyeglasses

96

6.12

Finders

97

7

Astrophotography

99

7.1

Overview

99

7.2

Attaching cameras to telescopes

100

7.2.1

Optical configurations

100

7.2.2

Brackets and adapters

102

7.3

Two simple projects to get you started

103

7.3.1

Project #1: the Moon, afocal method

103

7.3.2

Project #2: the stars, piggybacking

103

7.4

Equipment for astrophotography

107

7.4.1

Telescope requirements

107

7.4.2

35-mm SLR cameras

107

7.4.3

Other film cameras

109

7.4.4

Digital and video cameras

109

7.4.5

Astronomical CCD cameras

110

7.5

Focal length, image size, and f -ratio

111

7.5.1

Finding the effective focal length

111

7.5.2

Image size and field of view

112

7.5.3

Finding the f -ratio

113

7.5.4

Exposure, film, and development

113

7.6

Focusing and sharpness

115

7.7

Deep-sky techniques

116

7.8

Digital image processing

119

8

Troubleshooting

122

8.1

Electrical and computer problems

122

8.2

Keypad problems

123

8.3

Motor and slewing problems

125

8.4

Optical problems

129

Part II

Three classic telescopes

131

9

Three that led the revolution

133

10

Meade LX200

134

10.1

Introduction

134

10.1.1

Evaluation of the LX200

134

10.1.2

Related products

135

10.1.3

Firmware versions

135

10.1.4

LX200 websites

136

10.2

Electrical requirements

136

10.3

Keypad

137

10.3.1 Direction of movement 138

10.3.2 How to enter negative numbers 138

10.4 Operation without electricity 138

10.5 Motorized operation without alignment 139

10.6 Controlling the slewing speed 140

10.7 Entering date, time, and site information 140

10.7.1 Setting the time 140

10.7.2 Setting the date 141

10.7.3 Entering site latitude and longitude 141

10.8 Aligning the telescope on the sky 143

10.8.1 Altazimuth mode 143

10.8.2 Equatorial mode 146

10.9 Finding objects by coordinates 149

10.9.1 Slewing to a given R.A. and declination 149

10.9.2 Slewing to a given altitude and azimuth 149

10.9.3 Dealing with decimal minutes 149

10.10 How to interrupt a slewing movement 150

10.11 Finding deep-sky objects using the built-in catalogues 150

10.11.1 M (Messier) Catalogue 150

10.12 Finding stars using the built-in catalogues 151

10.12.1 Named stars 151

10.12.2 STAR, SAO, and GCVS numbers 152

10.13 Finding the Moon and planets 153

10.14 More precise pointing 154

10.14.1 How to sync on an object 154

10.14.2 High-precision mode 156

10.15 Training the Smart Drive (PEC) 157

10.16 Cables, connections, and ports 158

10.16.1 Keypad and declination cables 158

10.16.2 CCD port 159

10.16.3 The serial ports 159

10.16.4 Other connectors 161

10.16.5 Internal battery 161

10.16.6 The floating ground 161

10.17 Known firmware bugs 162

10.18 Mechanical and electrical improvements 162

10.19 Menu maps 163

11 Celestron NexStar 5 and 8 169

11.1 Introduction 169

11.1.1 Related products 169

11.1.2 Evaluation of the NexStar 5 169

11.1.3 Firmware versions 170

11.1.4 NexStar websites 170

11.2 Important precautions 171

11.3 Electrical requirements 171

11.4 Keypad 172

11.4.1 Direction of movement 172

11.4.2 How to enter declinations and latitudes 173

11.5 Basic operation without alignment 173

11.5.1 Operation without electricity 173

11.5.2 Motorized operation without alignment 174

11.5.3 Controlling the slewing speed 174

11.6 Entering date, time, and site information 175

11.6.1 Setting the date and time 175

11.6.2 Entering site latitude and longitude 176

11.6.3 Storing an observing site 176

11.7 Aligning the telescope on the sky 177

11.7.1 Altazimuth mode 177

11.7.2 Equatorial mode 181

11.8 How to interrupt a slewing movement 182

11.9 Finding objects with the built-in catalogues 183

11.9.1 Messier, Caldwell, and NGC objects 183

11.9.2 Stars 183

11.9.3 Planets 184

11.9.4 Lists of objects 184

11.9.5 Sky tours 184

11.10 Finding objects by coordinates 184

11.10.1 Slewing to a given R.A. and declination 184

11.10.2 Slewing to a given altitude and azimuth 184

11.10.3 The ''User Object'' catalogue 184

11.11 More precise pointing 185

11.11.1 Approaching with A and ► 185

11.11.2 Backlash adjustment 185

11.11.3 How to sync on an object 185

11.11.4 The controversy over tripod leveling 185

11.12 Cables, connections, and ports 186

11.12.1 Keypad cable 186

11.12.2 Serial (RS-232) port 186

11.13 Known firmware bugs 187

11.14 Menu maps 187

12 Meade Autostar (ETX and LX90) 193

12.1 Introduction 193

12.1.1 Related products 193

12.1.2 Evaluation of the Autostar (ETX-90 and LX90) 194

12.1.3 Firmware versions 194

12.1.4 Autostar websites 195

12.2 Electrical requirements 195

12.3 Keypad 195

12.3.1 Is the computer included? 195

12.3.2 Please type slowly! 196

12.3.3 How to enter information 196

12.3.4 Display adjustments 197

12.3.5 Direction of movement 197

12.3.6 Display modes 197

12.4 Power-on sequence, date, and time 197

12.5 Entering site information 199

12.5.1 Choosing your location from a menu 199

12.5.2 Entering latitude, longitude, and time zone directly 200

12.6 Basic operation without alignment 200

12.6.1 Operation without electricity 200

12.6.2 Land mode 201

12.6.3 Controlling the slewing speed 201

12.7 Aligning the telescope on the sky 201

12.7.1 Checking the finder 201

12.7.2 Altazimuth mode 201

12.7.3 Equatorial mode 204

12.8 How to interrupt a slewing movement 205

12.9 Finding objects with the built-in catalogues 205

12.10 Finding objects by coordinates 205

12.11 More precise pointing 206

12.11.1 How to sync on an object 206

12.11.2 High-precision mode 206

12.11.3 Square spiral search ("box scan") 206

12.11.4 Drive training and backlash adjustment 207

12.12 Cables, connections, and ports 207

12.12.1 The connector panel 207

12.12.2 The serial port 208

12.13 Upgrading the firmware and downloading data 208

12.14 Other advanced features 209

12.14.1 Satellite tracking 209

12.14.2 Sky tours 211

12.15 Menu maps 211

Index 219

Preface

Computerized telescopes are revolutionizing amateur astronomy. Even the least expensive entry-level telescopes are now available with computer-controlled motors to find and track objects in the sky. No longer do you have to search for NGC 1999 or Neptune by carefully comparing the view with a star map - you just tell the telescope what to point at, and it does it.

Do computer controls take all the fun out of astronomy? No more than paved highways take the fun out of the Arizona desert. Professional astronomers have used setting circles to find objects since the time of Tycho Brahe and have always tried to make them as accurate as possible. Amateurs have long had setting circles, but they weren't very accurate. Now, with the advent of computers, professional-level accuracy is within the reach of the amateur, and the computer actually controls the telescope rather than just telling you where it's pointed.

After 30 years of finding celestial objects the old way, I bought my first computerized telescope in 2000 and immediately found myself doing a new kind of amateur astronomy. Suddenly I was spending my time looking at objects instead of for them. No longer preoccupied with "star-hopping", I could spare the time and attention to study the celestial objects themselves.

In fact I realized for the first time that, for all those years, my observing program had been skewed by the fact that some objects are easier to find than others. I regularly viewed M13 and not M92 because the latter is not near any bright stars. I rarely looked at Uranus or Neptune because that would require getting out a special map, updated yearly. Now I can look at anything within reach of the telescope.

At the same time I have become much more aware of, and dependent on, astronomical data sources. If an atlas omits NGC 404 or a star catalogue skips f Ursae Majoris, that's an obstacle I'll bump into and notice. If I use epoch-1950 coordinates on an epoch-2000 telescope, I won't find what I'm looking for. Conversely, the latest data files from the Astronomical Data Center can be put to immediate use with my computer and telescope.

What it all means is that new-style amateur astronomers need a new kind of guidebook. My writing project began as a list of interesting objects that I put together for use at the telescope. Soon I added a concise summary of the Meade LX200 operating manual. Simon Mitton of Cambridge University Press saw my notes and encouraged me to turn them into a book. By the time I finished, I had enough material for two books, How to Use a Computerized Telescope (this volume) and Celestial Objects for Modern Telescopes (the companion volume, which focuses on the sky rather than the equipment).

While I was writing the two books, Scott Roberts of Meade Instruments lent me equipment to try out. The technical support departments at Meade, Celestron, Software Bisque, and Starry Night Software answered technical questions. Daniel Bisque supplied software for testing. Howard Lester, Dennis Persyk, Lenny Abbey, Rich Jakiel, T. Wesley Erickson, Robert Leyland, R. A. Greiner, Richard Seymour, Ralph Pass, Phil Chambers, Ells Dutton, Michael Forsyth, and John Barnes critiqued drafts of parts of the text. Tom Sanford let me try out his Meade LX90 at length. Earlier, Jim Dillard first got me interested in computer-aided astronomy by buying my old Meade LX3 from me and outfitting it with digital setting circles. There are probably others whose names I've forgotten to list, and I beg their indulgence. And I have hopelessly lost track of who helped with which volume!

All along, Melody (my wife) and Cathy and Sharon (my daughters) have patiently endured a living room full of tripods and have even accompanied me on some observing trips. (I keep pointing out that all this is not as expensive as boats or even golf!) I want to thank all of these people, and others unnamed, for their encouragement and assistance.

Please visit me on the Web at http://www.covingtoninnovations.com, where this book will have its own web page with updates and related information.

Athens, Georgia December 24,2001

Parti

Telescopes in general

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