Astrophotography for the Amateur

Transcription

1 Astrophotography for the Amateur Second edition MICHAEL A. COVINGTON CAMBRIDGE UNIVERSITY PRESS

2 Preface Notes to the reader Symbols used in formulae xi xiii xiv 3.7 Zodiacal light, Gegenschein, and 3.8 lunar libration clouds All-sky cameras I SIMPLE TECHNIQUES 1 Welcome to astrophotography The challenge of astrophotography Choosing equipment Sharing your work with others Maintaining balance and enjoyment 7 Photographing stars without a telescope Stars and trails BASIC TECHNIQUE 1: Photographing stars without a telescope How long can you expose? PRACTICAL NOTE: HOW to approach formulae ' Choice of camera and lens Slides versus prints PRACTICAL NOTE: Getting good color prints Getting the most out of your film Keeping records PRACTICAL NOTE: Film and false economy Interpreting your pictures scientifically 19 3 Comets, meteors, aurorae, and space dust Comets BASIC TECHNIQUE 2: Photographing a bright comet Meteors BASIC TECHNIQUE 3: Photographing a meteor shower Aurorae BASIC TECHNIQUE 4: Photographing the aurora borealis 28 4 The moon 4.1 Lenses and image size 4.2 Using a telephoto lens BASIC TECHNIQUE 5: Photographing the moon through a telephoto lens Determining exposures PRACTICAL NOTE: What is a "stop"? Afocal coupling to telescopes and binoculars BASIC TECHNIQUE 6: Photographing the moon (afocal method) Films and processing Eclipses Lunar eclipses Lunar eclipse dates and times Lunar eclipse photography Videotaping a lunar eclipse BASIC TECHNIQUE 7: Photographing an eclipse of the moon Solar eclipses - partial and annular Eclipse safety PRACTICAL NOTE: HOW eclipse eye injuries happen BASIC TECHNIQUE 8: Viewing a solar eclipse by projection Safe solar filters Photographing partial solar eclipses BASIC TECHNIQUE 9: Photographing a partial solar eclipse Solar eclipses - total Shadow bands and other phenomena BASIC TECHNIQUE 10: Photographing a total solar eclipse Session planning Videotaping solar eclipses The 1999 total eclipse in Europe

3 Viii CONTENTS II ADVANCED TECHNIQUES Coupling cameras to telescopes Prime-focus astrophotography Telescope types and optical limitations Image size and field of view Afocal coupling Positive projection PRACTICAL NOTE: Measuring s 2 for eyepiece projection Negative projection Compression (focal reducers) Combinations of projection setups Diffraction-limited resolution The subtle art of focusing Camera viewfinders PRACTICAL NOTE: Does your SLR focus accurately? Aerial-image and crosshair focusing Knife-edge focusing How accurately must we focus? Focusing Schmidt-Cassegrains and Maksutovs 91 The solar system Film or CCD? The challenge of high resolution Tracking Vibration Unsteady air Dew The sun ' The moon Planetary photography The individual planets BASIC TECHNIQUE 11: Photographing a planet (afocal method) BASIC TECHNIQUE 12: Photographing a planet (by projection) 112 Deep-sky photography Piggy-backing BASIC TECHNIQUE 13: Piggy-back deep-sky photography BASIC TECHNIQUE 14: Polar alignment procedure Barn-door trackers Lenses for deep-sky work Scale enlargement and edge-of-field fall-off Magnitude limits and surface brightness Guiding PRACTICAL NOTE: What do you mean by 12 volts? Polar alignment accuracy Periodic gear error, PEC, and autoguiding Choice of film Light pollution and nebula filters PRACTICAL NOTE: The campaign against light pollution Deep-sky photography through the telescope BASIC TECHNIQUE 15: Deep-sky photography with an off-axis guider Keeping warm while observing Safety and etiquette at the observing site Mosquitoes and other vermin 146 III PHOTOGRAPHIC TECHNOLOGY 149 Cameras, lenses, and telescopes 9.1 The -mm SLR 9.2 Choosing an SLR 9.3 Olympus SLRs 9.4 Nikon SLRs 9.5 Other SLR makers 9.6 Buying used cameras

4 Contents ix Camera maintenance and repair Some miscellaneous SLR hints Other types of cameras Special astrocameras Lenses Lens quality and performance Lens mounts Buying lenses BASIC TECHNIQUE 16: Testing lenses Lens repair Choosing a telescope PRACTICAL NOTE: Does a lower f-ratio give a brighter image? Telescope quality and performance BASIC TECHNIQUE 17: Star-testing a telescope How to clean optics 1 72 Film How film works Spectral sensitivity The characteristic curve Film speed Reciprocity failure: theory Reciprocity failure: measurement PRACTICAL NOTE: Does film "give up" after a certain amount of time? 184 Hypersensitization 184 Graininess and resolution 186 Some specific films 187 PRACTICAL NOTE: Film: What's in a name? 192 PRACTICAL NOTE: IS "professional" film better? 192 Bulk loading 193 Developing, printing, and photographic enhancement The darkroom Developing black-and-white film Black-and-white printing PRACTICAL NOTE: Color negatives on black-and-white paper? Making high-contrast prints Unsharp masking Processing color film PRACTICAL NOTE: Help! The film is scratched! Slide duplication Rephotography 210 IV DIGITAL IMAGING Computer image enhancement How computers represent images Resolution and image size PRACTICAL NOTE: HOW images get resized File compression File formats Getting images into the computer Scanner artifacts PRACTICAL NOTE: Taking pictures that scan well The ethics of retouching Manipulating the characteristic curve Working with histograms Manipulating color Enhancing detail PRACTICAL NOTE: An example of digital enhancement Combining images Printing out the results Image enhancement theory: spatial frequency PRACTICAL NOTE: Signal and noise Convolutions, 1: smoothing PRACTICAL NOTE: Median filters Convolutions, 2: sharpening The Laplacian operator

5 CONTENTS PRACTICAL NOTE: Convolution or deconvolution? Maximum-entropy deconvolution CCD imaging How CCDs work Video and digital cameras Astronomical CCD cameras Field of view Aiming and focusing Exposure Optimal focal length BASIC TECHNIQUE 18: Imaging the moon or a planet Flat-fielding Calibration frames Deep-sky work Choosing a CCD camera 253 APPENDICES 257 Exposure tables A.1 How exposures are calculated 259 A.2 Obtaining B from photometric brightness 259 A.3 Other systems for calculating exposure 260 A.4 PRACTICAL NOTE: Why don't my results agree with the tables? 260 A.5 Moon and lunar eclipses 261 A.6 Sun and solar eclipses 266 A.7 Planets 269 A.8 Faint objects 273 B.2 Declination drift 276 B.3 Field rotation 279 B.4 Computer algorithms 281 C Plans for an electronic drive corrector 283 C.1 How it works 283 C.2 Circuits and parts list 286 C.3 Adaptation to 240 V, 50 Hz 287 C.4 Drive rates 288 C.5 Line power supply 288 C.6 Other designs 289 D Film data 290 D.1 Kodak Technical Pan film (TP) 291 D.2 Kodak Professional Ektachrome Film E D.3 Kodak Professional Ektapress Films 306 E Photographic filters 307 E.1 High-efficiency yellow, oranges, and reds 307 E.2 Other sharp-cutoff filters 308 E.3 Color balancing filters 309 E.4 Other filters 310 F Organizations and resources 311 F.1 Organizations 311 F.2 Internet resources 312 F.3 Magazines 312 F.4 Manufacturers 313 F.5 Dealers 315 F.6 Camera repairs and modifications 31 7 B Mathematical analysis of polar-axis misalignment 276 B.1 Summary of the most important results 276 Bibliography Index Colour plates facing p. 114