Preview-Of-The-Deep-Sky-Imaging

Welcome Thank you for downloading this sample of The Deep-sky Imaging Primer. The following pages will give you a sense of the book’s content and approach to teaching. I wrote this book to help you maximize your image quality, regardless of your ex- perience, equipment, or skies. There are many decisions to make as you get started: DSLR or CCD? Refractor or reflector? Which software? It’s a challenging road, but you can produce amazing images along the way, and I hope you’ll choose this book to help you make the best choices. If you’d like to purchase The Deep-sky Imaging Primer, you can buy it on Amazon. com or directly from me at http://digitalstars.wordpress.com. Thanks again for your interest in my book, and please feel free to reach out to me with your questions or comments at [email protected]. Best wishes, Charlie Bracken The Deep-sky Imaging Primer by Charles Bracken Copyright © 2013 Charles Bracken All rights reserved. No part of this publication may be reproduced, distributed, or transmitted in any form or by any means, including photocopying, recording, or other electronic or mechanical methods, without the prior written permission of the author, except in the case of brief quotations embodied in critical reviews and certain other noncommercial uses permitted by copyright law. Permission requests should be sent to the author at [email protected]. All trademarks used are the property of their respective owners. Printed in the United States of America ISBN-13: 978-1481804912 ISBN-10: 148180491X First Edition Contents Introduction v I. Understanding Images 13 1 Electronic sensors 14 The challenges of astronomical imaging 14 How electronic sensors work 14 Well capacity, gain, and dynamic range 16 The importance of bit depth 18 Response curves and raw files 18 Creating color images 20 Sensor architecture 21 2 Signal and Noise 23 Signal and noise at the pixel level 23 Shot Noise 24 An example of shot noise 26 Skyglow 29 Thermal signal, hot pixels, and dark frames 30 Read noise and quantization noise 32 Two-dimensional corrections 33 Sensor non-uniformity 33 Uneven illumination 33 Adding signals and noise 34 II. Aquiring Images 37 3 Mounts and alignment 38 Choosing and using a GEM 39 Meridian flip 39 Polar alignment 40 Tracking error 41 Drift alignment 42 4 Cameras 43 DSLRs 43 Dedicated astronomical cameras 43 Sensor size and pixel size 44 Connecting a camera to a telescope 44 5 Optics 46 Telescopes for visual vs. imaging use 47 vii Optical aberrations 47 Refractors 48 Telephoto lenses 50 Reflectors and compound telescopes 51 Telescope quality 54 6 Image scale: matching sensor, and optics 55 Resolution and seeing 55 Sampling 56 Oversampling revisited 58 Field of view 59 Equipment recommendations 60 Focal reducers and field flatteners 61 7 Choosing appropriate objects to image 64 Getting a sense of scale 64 The deep-sky catalogs 65 A survey of object sizes 66 Giant Objects (2° or greater) 66 Really Big Objects (1–2°) 67 Objects from 30-60' 67 Objects from 15-30' 67 Objects from 5-15' 67 Objects from 1-5' 68 Objects smaller than 1' 68 8 Focusing and autoguiding 69 Focusing 69 Achieving critical focus 70 Autoguiding fundamentals 71 Flexure 72 Connecting computer to mount 72 Software and settings 73 Choosing a scope and camera for autoguiding 75 9 Setup and accessories 76 Reducing setup time 76 Other important equipment 76 Dovetails, rings, and other mount accessories 77 Power supplies 77 Dew prevention 78 USB hubs and extension cables 78 10 Filters and narrowband imaging 79 Filters for imaging 79 Light pollution filters 82 11 Taking the exposures 83 Controlling the camera 83 Choosing exposure duration and gain 84 Planning for a night of imaging 85 An image capture workflow 86 Dark frames 87 viii Flat frames 88 Bias frames 89 Dithering light frames 89 12 Atmospheric effects 90 Light pollution 90 Target altitude 91 Local turbulence 93 13 Diagnosing problems and improving image quality 94 Wind or tracking errors 94 Mirror flop 94 Diffraction patterns 94 Focus 95 Halos 95 Tips for better image capture 96 III. Processing Images 99 14 Color in digital images 100 File formats 100 Visual response to color 100 Producing color on print and screen 101 Color management and color spaces 103 LAB color 103 The HSL/HSV/HSB color model 104 DSLR white balance 104 Deep-sky color accuracy 105 Color calibration with G2V stars 105 15 The calibration process 107 Calibration exposures 107 Stacking parameters 110 An example of calibration and stacking with DeepSkyStacker 111 Throwing out problem shots 114 Aligning monochrome images for color combination later 114 The drizzle algorithm 115 Diagnosing defects in calibration output 116 16 Principals and tools of post-processing 118 Selective adjustments 118 Understanding the histogram 118 The curves tool 121 Levels 123 The magic of layers 124 Layer blending modes 125 Layer masks 126 Creating a composite layer 126 Selections and feathering 127 A post-processing workflow 128 ix 17 Stretching: reallocating the dynamic range 129 Non-linear stretching 129 Start with balanced colors 130 Initial development 130 Object-background separation curves 131 Intra-object contrast curves 131 Controlling highlights and boosting saturation 133 Posterization: the perils of over-stretching 133 The effect of stretching on color 134 Digital Development Process (DDP) 135 18 Background adjustments and cosmetic repairs 136 Gradient removal 136 Repairing satellite trails, dust spots, and reflections 138 Correcting elongated stars 139 19 Color synthesis and adjustment 140 RGB Color Synthesis 140 Creating a luminosity channel 140 Narrowband RGB synthesis 141 Further hue adjustments 142 Using clipping layer masks to map color 143 Blending RGB and narrowband data 143 Color balance 144 Enhancing color saturation in RGB 144 Boosting saturation in LAB 145 20 Sharpening and local contrast enhancement 147 Convolution, deconvolution, and sharpening 147 Unsharp mask and local contrast enhancement 148 Selective application with layer masks 151 The high pass filter 151 Other ways to create local contrast 153 21 Star adjustments 155 Star removal and star selection 155 Reducing star size across the image 158 Reducing the brightest stars 159 Correcting star color in narrowband images 160 22 Noise reduction 162 Visual noise, color, and scale 162 Tools for noise reduction 162 Applying noise reduction selectively 163 23 Composition 165 Framing the scene 165 Orientation 165 Color 166 Walk away 166 24 DSLR Processing example: The Witch’s Broom Nebula 168 x 25 CCD Processing example: The Rosette Nebula in narrowband 175 A. Exercise Answers 185 B. Moonless hours 189 Index 195 xi Preview 1Understanding Images This first section briefly covers what’s going on “under the hood” in the electronic imaging process. Understanding how electronic images are cre- ated is uniquely important to long-exposure astronomical images. If you are math-phobic, don’t worry: the concepts are more important than the formulas. These ideas form the foundation of quality images, and we’ll come back to them again and again later as we explore equipment, image capture, and image processing. M31, The Andromeda Galaxy (18 10-minute luminance exposures combined with approximately 11 hours of color exposures) Preview 1 ELECTRONIC SENSORS The challenges of astronomical imaging How electronic sensors work Deep-sky objects include galaxies, nebulae, and star clus- Photons are the fundamental particles that carry light. The ters. Many are stunningly beautiful, and they range in job of the electronic sensors in cameras is to count photons, size from tiny planetary nebulae and distant galaxies only and their sensitivity is amazing. They can register a signal arcseconds across to gigantic nebulae larger than the full discriminate between brightness levels resulting from only moon. All of them, however, are dim, and this is the main a few photons. The individual sensor elements on a sensor challenge of creating deep-sky images. are referred to as photosites, each of which corresponds to a pixel in the final image. “Pixel” is short for picture element, When faced with a dimly lit outdoor scene, a daylight pho- and following the same logic, photosites are sometimes also tographer will use a faster focal ratio or take a longer expo- called sensels. Colloquially, most people ignore the distinc- sure. In our case, we’ll do both, but faster optics can only tion between a photosite and the resulting image pixel-a take us so far. The defining characteristic of deep-sky im- point on the image and a point on the sensor both com- aging is that we must take very long exposures. Instead of monly referred to as a pixel. measuring exposure times in fractions of a second, we will measure them in minutes. Further, we’ll synthesize many images together into one that incorporates hours of total exposure time. None of this was possible before electronic sensors and digital imaging revolutionized photography. The quality of deep-sky images has taken a quantum leap forward as a result, allowing amateurs to create images that were impos- sible in the film era. The process is not easy, however, as there are many challenges to overcome. Aside from being very dim, the sky Figure 1. A typical CCD sensor rotates slowly over our heads, so we’ll require accurate tracking. We’ll also see that while daytime photography is There are several steps between photons striking the cam- relatively forgiving when it comes to optics, the night sky era’s sensor and the image file on your computer. Each (especially stars) seems designed to reveal even the slightest step is an opportunity for inaccuracy, known as noise in optical aberrations.

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