Grating Spectroscopes and How to Use Them

By Ken M. Harrison

Grating Spectroscopes and How to Use Them is written for amateur astronomers who are just getting into this field of astronomy. Transmission grating spectroscopes look like simple filters and are designed to screw into place on the eyepiece of a telescope for visual use, or into the camera adapter for digicam or CCD imaging. Using the most popular commercially made filter gratings – Rainbow Optics (US) and Star Analyzer (UK) – as examples, this book provides the reader with information on how to set up and use the grating one needs to obtain stellar spectrograms. It also discusses several methods on analyzing the results. This book is written in an easy to read style, perfect for getting started on the first night using the spectroscope, and specifically showing how the simple transmission filter is used on the camera or telescope. No heavy mathematics or formulas are involved, and there are many practical hints and tips – something that is almost essential to success when starting out. This book helps readers to achieve quick results, and by following the worked examples, they can successfully carry out basic analysis of the spectra.

• Language: English
• Publisher: Springer
• Release date: Mar 2, 2012
• ISBN: 9781461413974

Book preview

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Ken M. HarrisonPatrick Moore's Practical Astronomy SeriesGrating Spectroscopes and How to Use Them201210.1007/978-1-4614-1397-4© Springer Science+Business Media New York 2012

Patrick Moore's Practical Astronomy Series

For further volumes: http://www.springer.com/series/3192

Ken M. Harrison

Grating Spectroscopes and How to Use Them

A215455_1_En_BookFrontmatter_Figa_HTML.png

Ken M. Harrison

Wezembeek-Oppem, Belgium

ISSN 1431-9756

ISBN 978-1-4614-1396-7e-ISBN 978-1-4614-1397-4

Springer New York Dordrecht Heidelberg London

Library of Congress Control Number: 2012932210

© Springer Science+Business Media New York 2012

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Preface

The development of the spectroscope has contributed more to the science of astronomy than any other telescope accessory. It has been said that 75% of all astronomical discoveries have been made with the spectroscope. If you’ve just bought, or are thinking of buying, one of the popular filter-sized transmission gratings, then this is the book for you.

The popularity of these gratings as a first spectroscope has been growing over the past few years, and these simple devices provide a good entry point for budding amateurs interested in astronomical spectroscopy. They are ideally suited to low resolution stellar spectral imaging.

The basic challenge facing the novice is where to start. What other equipment will you need? How do you process the CCD image? How do you analyze your first spectrum? These questions and more are addressed in this book. It provides up to date information on filter gratings and processing methods available to the amateur, and more importantly, the how to….

This book has been written specifically for first time users and keeps the mathematics to a minimum. Where some mathematics is necessary, a worked example or look-up table is provided. It should be possible to image your first spectra on your first night.

The low resolution and lack of an entrance slit limit the type of spectroscopy that can be done, but this should not be seen as a negative. By using the telescopes, mountings and CCD cameras currently available to the amateur, this book will show how, with the addition of a simple transmission grating, we can observe and record spectra that reveal the nature of the stars. Many amateurs have successfully obtained spectra showing the temperature, age and chemical fingerprints of the stars as well as recording the elements in bright nebulae and the redshift of fast receding quasars! This is the beginning of a journey into the unknown realms of amateur astronomy. You should be excited to be among the few who will be able to record the wonders of the universe for themselves and see what stars are really made of.

As you practice and gain experience you may want to increase the resolution of your spectroscope, contribute to the ever-growing list of amateur and pro-am projects, or even construct your own spectroscope. A more complete overview of the theory, use and design of advanced amateur spectroscopes is covered in depth in the companion volume to this one, Astronomical Spectroscopy for Amateurs .

This is a new challenging field for amateurs. With even the most basic of equipment your activities can be interesting, thought provoking and most of all fun. Enjoy!

Ken M. Harrison

Acknowledgements

Without the help and assistance of the amateur spectroscopy community this book would not have been possible.

I would like to thank in particular Christian Buil and Robin Leadbeater for all they have contributed and their continued support to many amateurs around the world.

Contents

1 Some Background and Basics 1

Why Spectroscopy? 1

What Is a Transmission Grating? 2

How Does a Grating Work? (The Short Version!) 3

Dispersion 4

Resolution 4

Diffraction Gratings 5

Grating Resolution 7

Is a Grating Good for Visual Observing? 7

Which Telescope to Use? 8

Can I Use Any Camera? 9

Camera Resolution and Pixel Size 10

What Else Do You Need to Get Started? 10

Image Capture Software 11

Seeing Conditions and Their Effects 11

Summary 12

2 Imaging a Spectrum with the Grating 13

How Do We Use the Grating? 13

Using the Grating in a Converging Beam 14

Imaging Your First Spectrum 16

Signal-to-Noise Ratio (SNR) 20

Image Size on the CCD Chip 20

Drift Enlarging 21

Summing Up 22

Objective Gratings on the Front of Your Camera Lens 22

Objective Gratings with DSLR Cameras 27

CCD-Type Cameras 29

Webcams and Video 29

Quick Bit of Theory: Dispersion/Plate Scale and Resolution 29

Dispersion 29

Plate Scale 30

Resolution 30

Summary 30

3 My First Spectrum: What Else Can I Record? 33

A Brief Introduction to Star Classification 35

Stellar Temperatures: Great Balls of Fire! 38

Star Stuff 39

Emission Stars (Be and Shell Stars) 40

Wolf-Rayet Stars (WR) 42

Variable Stars/Nova and Supernovae 42

Nebulae 45

Redshift/Quasars 47

Planetary Spectroscopy 48

Meteor Spectra 48

What About the Sun? 51

4 Processing Spectra 53

Software Overview 53

Preparing the Raw Image File for Processing 54

Using IRIS to Pre-process 56

The Pixel Profile Graph Using an Image Profile and Spreadsheet 57

Spectral Processing Software Options 60

Visual Spec (VSpec) 61

Standard Stellar Spectra 61

Standard Element Lines 61

Obtaining a Pixel Profile Graph Using VSpec 68

Wavelength Calibration 69

Wavelength Calibration Using the Zero Order Image 69

Wavelength Calibration Using Two Lines 72

Wavelength Calibration Using One Line and a Known Dispersion 72

Instrument/Camera Response 73

Correcting Spectra Using the Instrument/Camera Response Curve 75

Identification of Lines and Features 75

Resolution 76

Normalized Spectrum 77

Continuum Removal 77

Creating Additional Stellar Reference Profiles 77

Signal to Noise Ratio (SNR) 77

5 Improving Your Grating Spectroscope 79

Adding a Slit for Solar Spectrum/Reference Lamps 79

The Slit Tube 80

Alternative Solar Slit: The Reflective Needle 80

Solar Spectrum 81

Astronomical Filters 84

Adding a Slit for Deep Sky Objects 85

Using the Grating Behind an Eyepiece (Point and Shoot Camera?) 88

Other Types of Transmission Gratings 89

Making a CCD Adaptor for a Camera Lens 92

Other Grating Spectroscopes 93

Weight and Length 94

The Size of the Collimator Lens 94

The Size of the Grating 95

Camera Lens 95

Summary Points 96

Adding a Slit 97

Measuring the Slit Gap 98

Living Without a Zero Order Image 99

Watkis Transmission Spectroscope 100

Other Transmission Spectroscope Designs 101

Guiding Slit Spectroscopes 102

6 Some Technical (Nice to Know) Stuff 105

Star Magnitudes and Star Brightness 105

Harry Nyquist, Claude Shannon and Bryce Bayer 108

Pixel Size 109

CCD Chip Size 110

Quantum Efficiency (QE) 110

Instrument and Camera Response Versus Recorded Spectra 113

Signal to Noise Ratio (SNR) 114

Grating Theory (The Heavy Bits!) 115

Dispersion, Plate Scale and Resolution (The Long Answer) 116

Reducing Aberration: Adding a Grism 120

Field Curvature 121

Spectral Coma 121

Non Linearity of the Spectrum 124

7 Spectral Analysis: A Bit of Theory 127

Kirchhoff’s Laws 127

Blackbody Radiation 128

Quantum Theory 129

Forbidden Lines: Ionization 131

Doppler/Redshifts 133

Stellar Classification: What You Need to Know 134

HD Classifications 134

The MKK Spectral Sequence 135

Luminosity Classes 136

Suffixes and Prefixes Used in Spectral Classifications 136

The H-R Diagram 137

Standard Spectral Lines and Reference Spectra 138

Reference Spectra 138

Standard Spectral Lines 139

Other Useful Spectral Reference Lines 141

Fraunhofer Lines 141

Nebula Emission Lines 142

O 2 and H 2 O Atmospheric Telluric Lines 142

Common Light Pollution Lines 142

Conclusion 143

Appendices145

Appendix A: The Greek Alphabet145

Appendix B: The Brightest Stars146

Appendix C: The Brightest Be Stars147

Appendix D: The Brightest Wolf-Rayet (WR) Stars148

Appendix E: The Brightest Red and Carbon Stars149

Appendix F: Suppliers of Spectroscope Gratings and Accessories150

Appendix G: Spectroscopy Forums152

For Further Information153

Glossary157

Index165

Ken M. HarrisonPatrick Moore's Practical Astronomy SeriesGrating Spectroscopes and How to Use Them201210.1007/978-1-4614-1397-4_1© Springer Science+Business Media, LLC 2012

1. Some Background and Basics

Ken M. Harrison¹ 

(1)

Wezembeek-Oppem, Belgium

Abstract

The current availability of filter gratings to amateur astronomers has given them the opportunity to become involved in one of the new and challenging aspects of astronomy – spectroscopy. Even with limited time, resources and finances available to him or her, the amateur can still contribute useful data that, although it can’t compete with the professionals using large telescopes located in some of the best sites in the world, can complement their work.

Why Spectroscopy?

The current availability of filter gratings to amateur astronomers has given them the opportunity to become involved in one of the new and challenging aspects of astronomy – spectroscopy. Even with limited time, resources and finances available to him or her, the amateur can still contribute useful data that, although it can’t compete with the professionals using large telescopes located in some of the best sites in the world, can complement their work.

How can an average amateur from his/her own backyard with a relatively small telescope do this? Our time and flexibility is our strength. We can observe what we like, when we like and not be locked into some limited observing time slot like the professionals. Our observations can fill the gaps. Amateurs have already made some significant contributions – monitoring dust shells around stars, finding exoplanets orbiting nearby stars and adding spectroscopic data to the traditional variable star observations. Not that you will achieve all this with a filter grating on your first night, but all the necessary skills and knowledge you will need to be able to do that sort of work in the future starts here.

The majority of our knowledge about the universe has come via spectroscopy. We can share that excitement of finding out what is happening around us, how the star stuff we’re made of came to be and what is going on inside and around the nearby stars. Few amateurs take this opportunity. Think about using the grating as Spectroscopy 101 – the first small step on the journey.

In this chapter we will briefly discuss the tools to be used − the grating, the telescope and the camera – and answer some frequently asked questions by beginners. Once we understand these basics we can quickly move on to the fun part and start obtaining our first detailed spectrum.

What Is a Transmission Grating?

A transmission grating (sometimes called a diffraction grating or a filter grating) breaks white light into a colored spectrum. Sounds simple enough, but a lot of technology has gone into the design and construction of each type of transmission grating. The heart of the transmission filter grating is a section of specially prepared plastic film that has been embossed with a fine series of grooves (called lines).

The shape and spacing of the lines is controlled in a master grating, manufactured under clean room conditions. This master is then used to prepare a number of replica gratings. The replica grating can be produced in a plastic material transparent to light (transmission grating) or subsequently vacuum coated with aluminum to make a reflection grating.

The shape of the groove is important for the efficiency of the grating, and the number of lines per mm (l/mm) defines the ability to separate white light into the various colors. Gratings are available in different l/mm. The Star Analyser grating (SA100) has 100 l/mm and the Rainbow Optics grating (RO200) has 200 l/mm. (Baader also used to produce a 207 l/mm filter grating, but unfortunately this is currently unavailable.) The grating is mounted into a standard astronomical 1.25″ eyepiece filter frame (see Fig. 1.1). The 28.5 mm thread should fit all the 1.25″ filter threads found on eyepieces and adaptors. You’ll find other transmission-type gratings up to 600 l/mm readily available.

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Fig. 1.1

Filter gratings

Gratings are very delicate. Most of the 1.25″ filter-sized gratings are protected by being sandwiched between glass plates. This makes handling them much easier, but they should be treated like all your optical equipment – with care.

How Does a Grating Work? (The Short Version!)

Early experiments and spectroscopic observations were done using prisms. Isaac Newton first showed that a prism could break white light into a colored spectrum, spreading normal sunlight into a rainbow of light from a deep violet through to a rich ruby red. These are the visible limits of the human eye and is now called the visible spectrum. (A single spectrum is spectrum; more than one spectrum is a collection of spectra.) The ability to separate white light into its colors is called dispersion. Prisms disperse the light due to the type of glass they are made from and the shape of the prism used.

Light is a form of electromagnetic energy and has the amazing qualities of acting both as a particle and a wave. Although the speed of light is fixed and unchanging, the wavelength of light can vary between being very short or very long.

As later investigators found, when analyzing the nature of light, the wavelength of the light defines the perceived color. Short wavelengths appear to us as blue; violet and much longer wavelengths look red. These wavelengths can be extended to cover the whole energy spectrum (the electromagnetic spectrum). Shorter wavelengths lead to X-rays, and the longer wavelengths to the infrared (heat) and radio waves (see Fig. 1.2).

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Fig. 1.2

Electromagnetic spectrum

Dispersion

When a spectrum is produced it’s made up from a series of images, one for each wavelength of light. The shape of each of these images replicates shape of the input beam of light. If a circular opening is used in front of a prism (or grating) the resulting spectrum will be a collage of small colored disks