The Dobsonian Telescope: A Practical Manual for Building Large Aperture Telescopes

Description

This book tells how you can build a state-of-the-art Dobsonian telescope using readily available materials and supplies. Every step of construction is detailed in photographs and diagrams, and the underlying ideas are carefully explained. As a result of a three-year collaboration between the authors, experienced and well-known telescope makers, one now has the opportunity to build a high-performance telescope with a 14-inch to 40-inch aperture based on the thoroughly tested designs described in this book. Chapter 1 Large Aperture Dobsonians 1.1 Why Dobsonians Are Newtonians 1.2 How Dobsonians Work 1.3 The History of the Dobsonian Telescope 1.3.1 John Dobson Invents the Dobsonian 1.3.2 The Dobsonian Revolution 1.3.4 The Second Dobsonian Revolution 1.4 The Dobsonian in Your Future Chapter 2 Planning Your Telescope 2.1 Non-Telescopic Telescope Considerations 2.2 Tough Telescopic Considerations 2.2.1 Telescope Aperture Considerations 2.2.2 Are You Obsessed? 2.3 How To Get Started Chapter 3 Engineering the Dobsonian 3.1 Engineering for Performance 3.2 Statics: How to Make a Stiff Telescope 3.2.1 Engineering Terms 3.2.2 The Moment of Inertia 3.2.3 Elastic Deformation 3.2.4 The Strength of Materials 3.2.5 Stiffness of a Truss Pole 3.2.5.1 Flexure in the Rocker Box 3.2.5.2 Flexure of a Telescope Mirror 3.2.5.3 Why Simple Scaling Doesn't Work 3.2.6 Dobsonian Dynamics: How Dobsonians Move 3.2.6.1 Balancing a Dobsonian 3.2.7 Friction in a Dobsonian 3.2.7.1 Friction Defined 3.2.7.2 Dobsonian Friction 3.2.7.3 Curing Stiction 3.3 Materials for Telescopes 3.3.1 Wood 3.3.1.1 Wood Is a Natural, Organic Composite 3.3.1.2 Wood Is Orthotropic 3.3.1.3 Selecting Wood 3.3.2 Aluminum 3.3.3 Teflon 3.3.4 Plastic Laminates 3.3.4.1 "Ebony Star" and "Stardust" Laminates 3.3.4.2 Glassboard 3.3.4.3 Put Car Wax on Laminates 3.4 Protective Coatings Chapter 4 Optics for Dobsonians 4.1 Basic Newtonian Optics 4.2 Choosing the Mirror 4.2.1 Choice 1: What Aperture? 4.2.2 Choice 2: What Focal Ratio? 4.2.3 Choice 3: Make or Buy? 4.2.4 Choice 4: What Glass? 4.2.5 Choice 5: Cut Sheet or Molded Blanks? 4.2.6 Choice 6: Fine or Precision Annealed? 4.2.7 Choice 7: How Thick? 4.2.8 Choice 8: Blanchard Ground? 4.2.9 Choice 9: What Coatings? 4.2.10 Choice 10: Optical Quality 4.3 Thinking About Eyepieces 4.4 Thinking About the Focuser 4.5 Sizing the Secondary Mirror 4.6 To Offset or Not to Offset? 4.7 Testing Telescope Optics 4.8 What To Do If You Don't Like a Mirror Chapter 5 The Primary Mirror Cell 5.1 The Flotation System and the Sling 5.1.1 Flotation in Theory 5.1.2 Flotation in Practice 5.2 Building the Flotation System 5.2.1 Flotation System Design 5.2.1.1 Material for Bars and Triangles 5.2.1.2 Flotation System Pads 5.2.1.3 Build the Cell Subassemblies 5.3 The Tailgate 5.3.1 Evolution of the Tailgate 5.3.2 The Open-Frame Tailgate 5.3.3 Tailgate Layouts 5.3.4 Cutting Tailgate Materials 5.3.5 Drill Holes Before Welding 5.3.6 Weld Using a Jig 5.4 Mount the Tailgate in the Mirror Box 5.4.1 Collimation Bolts 5.4.2 Flotation System Hardware 5.4.3 The Sling and Split Bolts 5.4.4 Side Pins Chapter 6 The Secondary Cage 6.1 Dimensioning the Secondary Cage 6.1.1 The Truss-Tube Advantage 6.1.2 The Inside Diameter of the Secondary Cage 6.2 Constructing the Secondary Cage 6.2.1 Order the Spider and Secondary Holder, and Focuser 6.2.2 Cut the Rings 6.2.3 Make Three Rings for "The Big Ones" 6.2.4 Drill the Strut Seats 6.2.5 Make the Struts 6.2.6 Attach the Struts to the Rings 6.2.7 The Focuser Board 6.2.8 Install the Kydex Light Baffle 6.3 Connecting Ring for "The Big Ones" 6.4 Install the Hardware 6.4.1 Install the Focuser Chapter 7 Building the Mirror Box 7.1 Holding It All Together 7.1.1 Why the Mirror Box is Stubby 7.2 Mirror Box Materials 7.3 Designing the Mirror Box 7.3.1 Height and Width of the Mirror Box 7.3.2 How Balance Affects the Mirror Box 7.3.3 Calculating the Balance Point 7.3.4 The Depth of the Mirror Box 7.4 Balancing the Telescope 7.5 Split Blocks for the Mirror Box 7.6 The Side Bearings 7.6.1 Making the Mirror Box 7.6.2 Cutting the Sides 7.6.3 The Light Baffle 7.6.4 Assembling the Mirror Box 7.6.5 Side Bearing Dimensions 7.6.6 Bonding Flat Panels of Plywood 7.6.7 Constructing the Side Bearings 7.6.8 Placing the Side Bearings 7.6.9 Mounting the Side Bearings 7.7 To Shortcut or Not To Shortcut? 7.8 The Dust Cover Chapter 8 The Truss Tube 8.1 How to Build a Serious Truss Tube 8.1.1 Flexure in Truss Poles 8.1.2 The Diameter of the Truss Poles 8.1.3 The Length of the Truss Poles 8.2 Attaching Truss Poles to the Mirror Box 8.2.1 Overview of Pole Sockets 8.2.2 Wooden Split-Block Sockets 8.2.2.1 How to Make Split-Block Sockets 8.2.2.2 Simplified Split Block Sockets 8.2.2.3 Fancy Cam-Action Pole Sockets 8.2.2.4 Strap and Channel Sockets 8.3 How to Align Truss Pole Sockets 8.4 Attaching Truss Poles to the Secondary Cage 8.4.1 The Crushed Tube Method 8.4.2 The Threaded-Insert Method 8.4.3 The Offset-Bracket Method 8.4.4 The Seats and Wedges Method 8.4.4.1 Making Pole Seats 8.4.4.2 Making Pole Wedges Chapter 9 Bearings 9.1 How Dobsonian Bearings Work 9.1.1 What is Friction? 9.1.2 Friction in the Altitude Bearings 9.1.3 Friction in the Azimuth Bearings Chapter 10 The Rocker and Ground Board 10.1 Sizing the Rocker 10.2 Rocker Construction 10.2.1 Ground Board 10.2.2 Ground Boards for "The Big Ones" 10.3 Constructing the Ground Board 10.3.1 The Azimuth Pivot 10.3.2 How to Make the Pivot Bolt 10.3.3 Installing the Pivot Bolt 10.3.4 Installing Teflon Bearing Pads 10.3.4.1 Calculating the Size of the Altitude Bearing Pads 10.3.4.2 Calculating the Size of the Azimuth Bearing Pads 10.4 A Handy Option: Bearing Locks 10.5 Handles for Portability 10.5.1 "It Rolls On Air" 10.5.2 Attaching Handles to the Rocker 10.6 Handling the Big Ones Chapter 11 Assembly and Troubleshooting 11.1 Last-Minute Preparations 11.2 Installing the Optics 11.2.1 Center-Dot the Primary 11.2.2 Center Dot the Secondary Mirror 11.2.3 Install the Primary Mirror 11.2.4 Install the Secondary Mirror 11.2.5 Adjust the Sling 11.2.6 Collimate the Optics 11.3 Set-Up, Use, and Take-Down 11.3.1 Set Up 11.3.2 Using the Telescope 11.3.2.1 Balance 11.3.2.2 Bearing Surfaces 11.3.2.3 Dew 11.3.2.4 Ventilation 11.3.2.5 Safety Precautions 11.3.3 Take-Down 11.4 Cleaning the Optics 11.5 Troubleshooting 11.5.1 Too Little Focus In-Travel 11.5.2 Sticking Poles and Blocks 11.5.3 Telescopes and Moisture Chapter 12 Using Big Dobsonians 12.1 Eyepieces 12.2 Filters 12.3 Finders 12.3.1 The Telrad® Finder 12.3.2 Finder Telescopes 12.3.3 Get Two Finders 12.3.4 Jumbo Finders 12.3.5 Digital Setting Circles 12.4 Electrical Accessories 12.4.1 Dew Zappers 12.4.2 Heat Ropes 12.4.3 Portable 12-Volt Power 12.4.4 Marine Batteries 12.4.5 Power on the Secondary Cage 12.5 Keeping Warm and Dry 12.5.1 Warm Clothing 12.5.2 Keep Warm with Heat Packs 12.6 Eliminating Stray Light 12.6.1 The Light Shroud 12.6.1.1 Sewing a Shroud 12.6.1.2 Installing the Shroud 12.6.2 Focuser Baffling 12.6.3 The External Light Baffle 12.6.4 The Tailgate Cover 12.7 Telescope Covers 12.7.1 Mirror Box Dust Cover 12.7.2 Hat Box for the Secondary Cage 12.7.3 Secondary Mirror Cover 12.7.4 The Telescope Cover 12.8 Dark Observing Sites 12.8.1 Why You Need a Dark Site 12.8.2 Site Selection 12.8.3 Human Factors Chapter 13 Epilog: Making a Small Telescope 13.1 Materials You Need 13.2 Overview of Construction 13.3 Begin with the Tube 13.3.1 The Primary Mirror Cell 13.3.2 Locate the Mirror Cell 13.3.3 Finish the Tube 13.3.4 Add End Rings 13.3.5 Assemble the Tube 13.4 The Mounting 13.4.1 Build the Tube Cradle 13.4.2 Side Bearings 13.4.3 Locate the Tube Balance Point 13.4.4 Construct the Rocker 13.4.5 Make the Ground Board Appendix A Wood as a Structural Material A.1 The Properties of Plywood A.1.1 Types of Plywood A.1.2 The Stiffness of Plywood A.2 Selecting Plywood for Telescopes A.2.1 Hardwood Veneer Hardwood Core Plywoods (HVHC) A.2.2 Hardwood Veneer Softwood Core Plywoods (HVSC) A.2.3 Softwood Veneer Softwood Core Plywoods (SVSC) A.2.4 Particle Board Appendix B Grinding, Polishing, and Figuring Large, Thin Mirrors B.1 Obtaining a Mirror Blank B.1.1 Porthole Glass: the Classic Choice B.1.2 Pyrex Sheet Glass B.1.3 Choose the Mirror Diameter B.1.4 Choose the Mirror Thickness B.1.5 Choose the Focal Ratio B.2 Grinding Tools B.2.1 Solid and Built-up Tools B.2.2 Segmented Tools B.3 Prepare a Suitable Work Area B.4 Preparing for Grinding B.4.1 Facing the Blank B.4.2 How to Prevent Astigmatism B.4.3 Bevel the Edge B.4.4 Grinding Strokes B.5 Rough Grinding B.6 Fine Grinding B.6.1 Fine Abrasives B.6.2 Getting a Sphere B.6.3 Going on to #220 B.6.4 The Fine Side of Fine Grinding B.7 Preparing to Polish B.7.1 Pitch B.7.2 Polishing Agents B.7.3 Pitch Base B.8 Making the Pitch Lap B.8.1 Grooving the Lap B.8.2 Pressing the Lap B.9 Polishing B.9.1 Judging the "Feel" of the Lap B.9.2 Holding the Lap B.9.3 Completing the Polish B.10 Testing B.10.1 Test Stand and Testing Tunnel B.10.2 Begin Testing Early B.10.3 Testing for Astigmatism B.11 Figuring B.11.1 Goals in Figuring B.11.2 Rating Telescope Mirrors B.11.3 Do the Best You Can B.12 Test Methods B.12.1 Testing the Sphere B.12.2 The Sphere and the Paraboloid B.12.3 Star Testing B.12.3.1 Doing a Star Test B.12.3.2 Interpreting the Star Test B.14.4 Applying a Null Test B.14.5 Reading a Ronchi Screen B.13 The Art of Figuring 24 B.13.1 High, Low, Long, Short B.13.2 Figuring Laps B.13.3 Figuring Strokes B.13.3.1 The Long, Straight, Central Stroke B.13.3.2 The W Stroke B.13.3.3 Tangential Strokes B.13.3.4 Strokes for Small Polishers B.13.4 Correcting a Low Edge B.13.5 Correcting a Turned-Down Edge B.13.6 Correcting High Zones B.13.7 Correcting Low Zones B.16 A Few Final Words Appendix C Digital Setting Circles C.1 A Bit of History C.2 How Digital Circles Work C.3 Accurate Circles Need an Accurate Telescope C.4 Installation C.4.1 Mounting the Azimuth Encoder C.4.2 Mounting the Altitude Encoder C.4.3 Mounting the Computer C.5 General Advice Appendix D Equatorial Platforms Appendix E Resources and Suppliers E.1 Aluminum tubing E.2 Foam tube insulation E.3 Double-sided tape E.4 Specialty tools, knobs and latches E.5 Threaded inserts E.6 Levelers (collimation knobs) E.7 Felt tabs and floor protector pads (mirror cell pads) E.8 HVHC plywood (Hardwood Veneer Hardwood Core) E.9 Sliders or cord locks and sling webbing E.10 Heat ropes E.11 Heat paks E.12 Black Ripstop Nylon (for the light shroud) E.13 Kydex plastic for the light baffle E.14 E.14 Etched virgin Teflon E.15 #1782 Stardust Quarry Finish Formica E.16 #4552-50 Ebony Star countertop laminate E.17 Glassboard (bead board or fiberglass reinforced panel) E.18 Self-tapping threaded wood inserts E.19 Cam levers E.20 12-Volt cooling fans E.21 Ladders (for big telescopes) E.22 Small parts E.23 Epoxy resins E.24 Loading ramps, pneumatic wheels for wheelbarrow handles, and marine trailer jacks E.25 Black aperture f/ratio labels E.26 Plastic grommets for light shroud E.27 Duct tape E.28 Collimation tools E.29 Commercial Components for Telescope Making E.29.1 Optics, focusers, digital setting circles, eyepieces, Telrads E.29.2 Spiders and secondary mirror holders E.29.3 Cast aluminum poles seat, clamping wedges, side bearings E.51 Mirror Making Supplies E.51.1 Pyrex glass E.51.2 Kerr dental plaster E.51.3 MicroGrit abrasives E.51.4 Optical pitch E.51.5 Ronchi screens for mirror testing Bibliography Afterword Index