Grating Spectroscopes and How to Use Them
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Grating Spectroscopes and How to Use Them - Ken M. Harrison
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.pngKen 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
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.
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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.
A215455_1_En_1_Fig1_HTML.jpgFig. 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).
A215455_1_En_1_Fig2_HTML.gifFig. 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