Budget Astrophotography: Imaging with Your DSLR or Webcam

By Timothy J. Jensen

Here are clear explanations of how to make superb astronomical deep-sky images using only a DSLR or webcam and an astronomical telescope – no expensive dedicated CCD cameras needed!

The book is written for amateur astronomers interested in budget astrophotography – the deep sky, not just the Moon and planets – and for those who want to improve their imaging skills using DSLR and webcams. It is even possible to use existing (non-specialist astronomical) equipment for scientific applications such as high resolution planetary and lunar photography, astrometry, photometry, and spectroscopy.

The introduction of the CCD revolutionized astrophotography. The availability of this technology to the amateur astronomy community has allowed advanced science and imaging techniques to become available to almost anyone willing to take the time to learn a few, simple techniques. Specialized cooled-chip CCD imagers are capable of superb results in the right hands – but they are all very expensive. If budget is important, the reader is advised on using a standard camera instead.

Jensen provides techniques useful in acquiring beautiful high-quality images and high level scientific data in one accessible and easy-to-read book. It introduces techniques that will allow the reader to use more economical DSLR cameras – that are of course also used for day-to-day photography – to produce images and data of high quality, without a large cash investment.

• Language: English
• Publisher: Springer
• Release date: Oct 25, 2014
• ISBN: 9781493917730

Book preview

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The Patrick Moore Practical Astronomy Series

More information about this series at: http://​www.​springer.​com/​series/​3192

Timothy J. Jensen

Budget AstrophotographyImaging with Your DSLR or Webcam

A303079_1_En_BookFrontmatter_Figa_HTML.png

Timothy J. Jensen

Mebane, NC, USA

ISSN 1431-9756e-ISSN 2197-6562

ISBN 978-1-4939-1772-3e-ISBN 978-1-4939-1773-0

DOI 10.1007/978-1-4939-1773-0

Springer New York Heidelberg Dordrecht London

Library of Congress Control Number: 2014948460

© Springer Science+Business Media New York 2015

This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. Exempted from this legal reservation are brief excerpts in connection with reviews or scholarly analysis or material supplied specifically for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the work. Duplication of this publication or parts thereof is permitted only under the provisions of the Copyright Law of the Publisher’s location, in its current version, and permission for use must always be obtained from Springer. Permissions for use may be obtained through RightsLink at the Copyright Clearance Center. Violations are liable to prosecution under the respective Copyright Law.

The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use.

While the advice and information in this book are believed to be true and accurate at the date of publication, neither the authors nor the editors nor the publisher can accept any legal responsibility for any errors or omissions that may be made. The publisher makes no warranty, express or implied, with respect to the material contained herein.

Printed on acid-free paper

Springer is part of Springer Science+Business Media (www.springer.com)

Dedication

To my wife Mary…

For all her support and encouragement…

in all things…

Preface

Almost everyone that has looked up at the night sky (with or without a telescope), or at a photo taken by the Hubble Space Telescope, and has wondered what it would be like to take a portrait of the stars of their own for all to see. Astrophotography is almost as old as the invention of the camera itself. In 1840, John Drapper made the first known astrophotograph. It took him 20 min to expose a metal photographic sheet to produce an image of the Moon. Today’s cameras, especially digital single lens reflex cameras (DSLRs) have radically changed the photography world, especially in the realm of astrophotography. Capable of high resolution with a wide spectral response and large, sensitive detectors, today’s cameras can reproduce Drapper’s extraordinary feat in as little as one two thousandths of a second… over two million times faster!

The digital photography revolution has changed the paradigm for astrophotography as well. Hour-long exposures with an eye glued to a cross hair eyepiece are a thing of the past. We can now record in minutes what took hours in the past with film without the worry of reciprocity failure (the failure of film to respond to light with increasing exposure time). Computer controlled guiding using a web cam and lap top can completely automate the imaging process. All one needs is a small computer and some software. ¹

By learning a few simple techniques, a whole new photographic world can open up before the lens of your DSLR. That lens can be anything from your standard issue 50 mm, to a 30″ telescope. What makes DSLRs so appealing to the budding amateur astrophotographer is quite simply, price. These cameras are very modestly priced and for an investment of few hundred dollars (assuming you don’t already own one), the wonders of the night sky are yours to explore and capture. Wonderful wide field vistas full of star clouds, dust lanes and nebulae are just a few exposures away. With a telescope, faint galaxies come into reach. You can explore the surfaces of the Moon and Mars, follow the changing clouds of Jupiter and Venus, see spokes in the rings of Saturn, study asteroid light curves, follow the decay of a supernova, measure the elements in a star. All of these fascinating wonders are within your grasp.

There are a myriad of different types of digital cameras on the market today, ranging from simple fixed lens point and shoot, to expensive professional level single lens reflex models. This book will focus on astrophotography using moderately priced digital single lens reflex (DSLR) style cameras as well as simple webcam style video cameras.

Both Canon and Nikon have DSLRs that are suitable for astrophotography. Though Canon is more popular in the DSLR astrophotography field. Which camera is preferable is left to the reader’s personal bias.

DSLRs are not only good for taking stunning portraits. Real science can also be accomplished with these cameras. I hope to introduce you to the world of astrophotography and touch on the both the art, and science, that can be accomplished by the amateur astronomer using these cameras.

This book will provide an introduction to some of the basic theory as well as some of the general techniques required to capture, process, and analyze astronomical images. With a little practice, you’ll soon be hanging eye catching images on the wall for your friends to admire.

A word about software…

There are almost as many different software programs for amateur astronomers to choose from as there are types of telescopes. In the DSLR astrophotography world, a few stand out. Being a Windows person, I use Backyard EOS for DSLR camera control and image capture, ImagesPlus for file conversion, calibration, stacking and initial processing, and Photoshop for final processing. For webcam image processing, Autostakker!2, AVIStack and VirtualDub are essential. I am most familiar with these programs and will refer to their unique features along the way. Macintosh and UNIX users may have different software needs and wants. It is left to the reader to determine what software they prefer. When referring to software command paths, italics will be used to define the path: dropdown menu|submenu|command .

OK, let’s get started…

Timothy J. Jensen

Mebane, NC, USA

Acknowledgements

I would like to express my thanks and gratitude to the people that helped make this work possible through their conversations, suggestions, enthusiasm for astronomy and of course, the use of their images…

(in no particular order):

Barry Clark

Mike Phillips

Paul Jones

Darryl Milika

Pat Nicholas

Contents

1 An Introduction to the Modern DSLR Camera 1

1.​1 The Anatomy of the Sensor 1

1.​2 Chip Characteristics 2

1.​2.​1 Pixel Size 3

1.​2.​2 Chip Size 4

1.​2.​3 Chip Sensitivity 4

1.​2.​4 Linearity of Response 6

1.​2.​5 Noise 8

1.​3 The Meaning of Light 10

2 Telescopes:​ The Basics 15

2.​1 Types of Telescopes 15

2.​1.​1 Refractors 16

2.​1.​2 Reflectors 17

2.​1.​3 Catadioptric 18

2.​2 A Little Bit of Theory… 19

2.​3 Telescope Mounts 22

2.​3.​1 Polar Alignment 24

2.​4 The Telescope and Camera 27

3 Image Capture 33

3.​1 Planning the Imaging Session 33

3.​2 Capturing the Image 35

3.​2.​1 Collimating an SCT 36

3.​3 Calibration 41

3.​3.​1 Bias Frames 42

3.​3.​2 Dark Frames 42

3.​3.​3 Thermal Frames and Scaled Dark Frames 43

3.​3.​4 Flat Frames 44

3.​3.​5 Flat Darks 46

3.​3.​6 Defect Maps 46

3.​3.​7 Improving Signal to Noise Ratio by Stacking 46

3.​4 Camera Positions 49

3.​4.​1 Tripod Photography 49

3.​4.​2 Piggyback Astrophotography​ 51

3.​4.​3 Prime Focus Photography 52

3.​5 Filters 53

4 Image Processing 57

4.​1 Nebula Image Processing 57

4.​2 Preparing the Master Frames and Image Calibration 58

4.​3 Image Enhancement:​ Stretching 63

4.​4 Image Enhancement:​ Layers and Layer Masks 68

4.​4.​1 Blending Modes 68

4.​4.​2 Layer Masks 69

4.​4.​3 Adjustment Layers 76

4.​4.​4 Enhancing Images Using Blending Modes 79

4.​5 Mosaics 81

4.​5.​1 Manual Mosaics Using Photoshop 81

4.​5.​2 Mosaics with Photomerge 84

4.​6 Aligning Moving Objects 87

4.​7 Galaxy Processing 89

4.​7.​1 Processing a Galaxy 89

5 Webcam Imaging 93

5.​1 Planetary Imaging with a Webcam 93

5.​2 Types of Cameras 96

5.​3 Processing AVI Images 97

5.​4 Creating Tricolor RGB Images 101

5.​5 Imaging the Individual Planets:​ Tips and Tricks 103

5.​5.​1 Mercury 104

5.​5.​2 Venus 105

5.​5.​3 Mars 106

5.​5.​4 Jupiter 107

5.​5.​5 Saturn 108

5.​5.​6 Uranus 109

5.​5.​7 Neptune 109

5.​5.​8 Pluto 111

5.​5.​9 The Sun 112

5.​5.​10 The Moon 114

5.​6 WinJUPOS 117

5.​6.​1 Using WinJUPOS to Align RGB Frames 118

6 Advanced Imaging 125

6.​1 Spectroscopy 125

6.​1.​1 Assembling the Spectrograph 127

6.​1.​2 Calibration 128

6.​2 Photometry 131

6.​2.​1 Photometry with a DSLR 132

6.​2.​2 Data Reduction 133

6.​2.​3 Mechanical Measurements 134

6.​2.​4 Electronic Measurements 134

6.​2.​5 Photometric Corrections 135

6.​3 Astrometry 143

6.​4 Lunar Measurements 144

6.​5 Narrow Band Imaging 145

6.​5.​1 Narrow Band Imaging and Processing 146

6.​6 Satellite Imaging 151

7 Advanced Processing Techniques 153

7.​1 Star Removal 154

7.​1.​1 Photoshop 154

7.​1.​2 Images Plus 157

7.​2 Noise Filters 158

7.​3 Enhancing Nebula Contrast with Narrow Band Data 158

7.​4 Light Gradient Removal 160

7.​5 Adjustment Layers and Clipping Masks 164

7.​6 Debayering 165

8 Software 167

8.​1 Image Capture 167

8.​1.​1 DSLR Capture Control 167

8.​1.​2 WebCam/​Planetary Camera Control 168

8.​2 Image Processing 168

8.​2.​1 AVI Stacking Software 168

8.​2.​2 Stacking Software DSLR Images 169

8.​3 Image Processing 169

Gallery171

Appendix235

Glossary239

Bibliography243

Index245

Footnotes

1

Different software programs will be referred to throughout the course of this book. The author is not affiliated with any software manufacturer, developer or distributor. If a specific software package is mentioned, or used as an example, it reflects the author’s personal choice and should not in any way be considered an endorsement.

© Springer Science+Business Media New York 2015

Timothy J. JensenBudget AstrophotographyThe Patrick Moore Practical Astronomy Series10.1007/978-1-4939-1773-0_1

1. An Introduction to the Modern DSLR Camera

Timothy J. Jensen¹ 

(1)

Mebane, NC, USA

Unlike the cameras of old, which relied on a photochemical reaction to create an image on film, today’s modern DSLR cameras use silicon chips to create images. These solid state chips are based on two basic technologies, the CCD or Charged Coupled Device, or the Complementary Metal Oxide Semiconductor (CMOS). CCDs are somewhat more sensitive to light than their CMOS cousins (though this gap is closing), they are also slightly more expensive to construct and their use is generally restricted to high end expensive cameras. As a result, CMOS sensors are more commonly used in today’s DSLR cameras.

1.1 The Anatomy of the Sensor

The detector chip of a DSLR is composed of discrete sites called pixels. It is here that the formation of the image begins. Regardless of the detector technology used, the image is generated in basically the same way: an incident photon generates an electron in the chip’s picture element or pixel. The number of electrons generated is directly proportional to the amount of light that strikes the pixel. The electrons are then collected and counted by the camera’s onboard circuitry (the analog to digital convertor or ADC), and a voltage is generated for that pixel. This voltage value is the analog to digital unit or ADU and its value determines the brightness of that pixel in the captured image.

The sensitivity of the camera can be changed by adjusting its ISO setting. This is the digital equivalent of using different ISO speed film. Changing the ISO changes the amplifier gain and hence, a higher value is assigned per electron so the resulting image is brighter. The higher the ISO, the brighter the image produced. However, this increased brightness comes at a price. As the gain is increased each pixel gets brighter, regardless of the source of the signal, be that actual photons from the target or just the noise in the image and to make matters worse, the dynamic range of the image decreases. For normal daylight photography, where exposures are measured in fractions of seconds, and the image is generally quite bright, this noise is usually not noticeable. However, in an astrophotograph whose exposure time is measured in minutes, and the overall image brightness is small, the noise can become quite obvious.

CCD and CMOS detectors generate electrons, they don’t actually produce an image. That is done by the camera’s circuitry, and the image that is generated is actually in black and white since it is generated based on the voltage assigned to each pixel and voltage values have no color information. To make a color image, another feature must be added. This is called a Bayer matrix. The Bayer matrix, or more accurately, a color filter array, consists of an array of red, green and blue filters placed over the imaging chip (see Fig. 1.1). The light that then strikes a pixel is first filtered through the Bayer matrix. It takes four pixels (two green, one red, one blue) to make one color element in the image. There are more green pixels than red or blue to balance the image color tones. This is because the human eye is more sensitive to green and the intensity of white sunlight actually peaks in this region of the visible spectrum. The camera then takes the pixel intensities and combines them to produce a color. Because four pixels are used to produce one color, the camera electronics must interpolate the intensities based on the 4 color elements available, then assign an intensity to generate the color. A common false assumption is that since there are 4 pixels required to generate the color information, that the resolution is one forth the total pixel count (you are using 2 pixels in each dimension after all and 2 × 2 is 4). But in reality, each pixel is read, the color data interpolated, and a new intensity as well as color information is assigned to the pixels by the camera’s circuitry. So you haven’t lost the resolution, but rather the color data is spread out over 4 pixels, which sort of blurs the color a bit. The original, and worse interpolation method for this process was the nearest neighbor method. More advanced algorithms are now available which can produce more accurate and sharper colors, so depending on the algorithm used for the interpretation, the end result could be of varying quality.

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Fig. 1.1

The Bayer array. The incoming light passes through the Bayer matrix filters. This changes the intensity that is registered at the underlying pixel in the camera’s sensor. The camera’s electronics then combine these different intensities and assign a color. One color element (or sensel) is comprised of light intensity information from four individual pixels to make a single color point in an image

1.2 Chip Characteristics

There are several factors that one should consider when looking at an imaging sensor, such as pixel size, overall chip size, sensitivity, linear response, and noise. All of these are interrelated and have an impact on the final image. Let’s have a look at them in turn and see how they contribute to making a picture.

1.2.1 Pixel Size

The picture element or pixel is the smallest element in the imaging sensor and determines the final resolution of the image. So, the obvious first conclusion is that smaller is better, since more pixels per millimeter equals more resolution.

Well, yes and no… resolution in an image also depends on the aperture of the lens (or in the case of astrophotography, the telescope). Small pixels do allow for a higher resolution, you can’t resolve two objects as being separate if the light from both falls on the same pixel, but because their surface area and overall size is smaller, they require more light to fall on that smaller area to produce an image and tend to have a lower full well capacity. The full well capacity is a term that refers to the pixel’s ability to collect, measure and convert the collected photons into electrons before saturation occurs and the pixel response is maxed out. The larger the full well capacity, the more electrons can be collected and the larger the linear response of the camera will be. Larger pixels are more sensitive, require less light to generate a signal, and have a deeper full well capacity, but… the larger the pixel, the lower the potential resolution.

1.2.2 Chip Size

The size of the imaging chip determines how much sky will be covered in the final image. This is the field of view or FOV. The field of view is normally expressed in degrees. Another related expression is the pixel scale, which refers to the number of degrees of sky an individual pixel covers. Since pixels are so small (usually between 4 and 10 μm in DSLRs), the scale is usually in arcseconds (arcsec or arcs) per pixel (there are 3,600 arcsec per degree). Knowing the field of view is important since this determines if an entire object will fit in one image or require several overlapping frames (a mosaic) to be stitched together.

Today’s larger chip sizes also mean that vignetting can be an issue. Vignetting refers to uneven illumination across the image and occurs when the light cone from the telescope is narrower than the imaging chip and the light intensity falls off toward the edges. This causes an unevenly illuminated field of view with the center bright and the edges dark. This can be corrected by using flat fields. We’ll cover the application of flats in the image calibration section.

1.2.3 Chip Sensitivity

Chip sensitivity refers to the efficiency of conversion of the incident light to electrons that can be measured by the camera and is a combination of the design of the detector and the camera’s circuitry. There are two types of CCD detector constructions, front illuminated and back illuminated. For front illuminated detectors, the light must pass through the lens elements and then the electronics layer before hitting the detector chip. This is a little counter intuitive based on the name, but the name refers to the placement of the chip electronics, in front of or behind the detector. In the case of back illuminated detectors, the circuitry is on the backside of the detector, which allows more light to impact the photo sites. This can increase the sensitivity from 60 to 90 %. Modern DSLR cameras employ backlit technology.

The sensitivity of the detector is