A Practical Guide to Observational Astronomy

M. Shane Burns

• Publisher: CRC Press
• Year: 2021
• ISBN:
  9781000434439
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Astronomy & Astrophysics

A Practical Guide to Observational Astronomy provides a practical and accessible introduction to the ideas and concepts that are essential to making and analyzing astronomical observations.

A key emphasis of the book is on how modern astronomy would be impossible without the extensive use of computers, both for the control of astronomical instruments and the subsequent data analysis. Astronomers now need to use software to access and assess the data they produce, so understanding how to use computers to control equipment and analyze data is as crucial to modern astronomers as a telescope.

Therefore, this book contains an array of practical problems for readers to test their knowledge, in addition to a wealth of examples and tutorials using Python on the author's website, where readers can download and create image processing scripts.

This is an excellent study guide or textbook for an observational astronomy course for advanced undergraduate and graduate astronomy and physics students familiar with writing and running simple Python scripts.

Key Features

• Contains the latest developments and technologies from astronomical observatories and telescope facilities on the ground and in space
• Accompanied by a companion website with examples, tutorials, Python scripts, and resources
• Authored by an observational astronomer with over thirty years of observing and teaching experience

About the Author

M. Shane Burns earned his BA in physics at UC San Diego in 1979. He began graduate work at UC Berkeley in 1979, where he worked on an automated search for nearby supernovae. After being awarded a PhD in 1985, Professor Burns became a postdoctoral researcher at the University of Wyoming. He spent the summer of 1988 as a visiting scientist at Lawrence Berkeley National Lab, where he helped found the Supernova Cosmology Project (SCP). He continued to work as a member of the SCP group while a faculty member at Harvey Mudd College, the US Air Force Academy, and Colorado College. The 2011 Nobel Prize in Physics was awarded to the leader of the SCP for the group's "discovery of the accelerating expansion of the Universe through observations of distant supernovae." During his career, Professor Burns has observed using essentially all of the world's great observatories, including the Keck Observatory and the Hubble Space Telescope.

Companion website for the book: https://mshaneburns.github.io/ObsAstro/

Chapter 1 Astronomical Coordinates and Time

DOI: 10.1201/9781003203919-1

Suppose one night you were looking through your backyard telescope and you discovered what you think is a supernova. If you want to tell others where to look to find this event, you will need to designate the position of the object in the sky. This chapter introduces the standard astronomical coordinates systems that astronomers use to specify the location of objects on the celestial sphere. The celestial sphere is an imaginary sphere on which the astronomical objects appear to be located. We can specify any point on the sphere by specifying two angles. Only two angles are needed, but there are a variety of different and surprisingly subtle ways to define those angles.

To specify any coordinate system, we need to choose an origin for the coordinates. In astronomy, this is usually taken to be at the center of the Earth or the Sun. We also need to specify the reference directions from which to define the coordinates. For example, to define a conventional Cartesian coordinate system, we need to specify the directions of the x, y, and z-axes. Astronomical coordinate systems use two angles of a spherical coordinate system. Spherical coordinates are specified by defining a fundamental plane, a fundamental direction, and the directions of increasing angle. The fundamental plane is a plane which divides the sphere into two hemispheres. The fundamental direction specifies the direction of one of the axes. The traditional definition of spherical polar coordinates defines the x–y plane as the fundamental plane. Points on the surface of a sphere centered on the origin specified by the angles θ and ϕ, where θ is the angle from the +z-axis and ϕ is the angle in the x–y plane shown in Figure 1.1. The angle ϕ increases from the $+x$-axis toward the $+y$-axis.

Figure 1.1: Two angles θ and ϕ specify the location of any point on the sphere. This is a conventional right-handed coordinate system, with ϕ increasing from the $+x$-axis toward the $+y$-axis.

1.1 HORIZON COORDINATES

The horizon or altitude-azimuth coordinate system is the simplest way to describe the position of a celestial object from the surface of the Earth. The origin of horizon coordinates is taken to be the observer. The fundamental plane is a plane tangent to the surface of the Earth at the location of the observer so that the z-axis of a conventional Cartesian coordinate system would point toward the zenith. The zenith is defined to be the point on the celestial sphere directly over the observer. The point directly under the observer is called the nadir. The fundamental direction for horizon coordinates is due north. The x-axis of a Cartesian coordinates would point in this direction. The position of a star is determined by specifying the altitude angle h from the horizon to the star, and an azimuth angle A measured along the horizon from north or the x-axis (Figure 1.2). Notice that for conventional horizon coordinates A increases from north to east, which makes this a left-handed coordinate system. The zenith distance z is the angle between the z-axis and the star so that $z=h-{90}^{°}$. The local meridian is the arc on the celestial sphere from the point on the horizon that is due south, through the zenith to the point on the horizon that is due north.

Figure 1.2: The horizon coordinate system.

Horizon coordinates are useful because they are simple, but have several disadvantages. One is that the origin of the coordinates depends on the location of the observer. This isn't a serious problem for most celestial objects. The stars are so far away that shifting the origin to the center of the Earth wouldn't change the star's altitude and azimuth. A more serious disadvantage is that the directions of the axes depend on the observer's location. The azimuthal coordinate depends on the observer's longitude and the zenith distance depends on the observer's latitude (see Problem 1.8). However, the biggest disadvantage is that celestial objects don't have fixed coordinates. This is due to the Earth's rotation and orbital motion around the Sun.

1.1.1 Diurnal and Annual Motions

As the Earth rotates all the celestial objects rise, transit¹, and set. Their altitude and azimuth are constantly changing. The length of the day is derived from the motion of the Sun around the celestial sphere. The solar day is the time between successive transits of the Sun. The time between transits is on an average 24 hours, but some days are actually be a little shorter than 24 hours and some a little longer. The difference is due to the fact that the Earth's orbit is slightly elliptical. Section 1.5 di...

Table of contents

• Cover
• Half-Title Page
• Title Page
• Copyright Page
• Dedication Page
• Contents
• Preface
• CHAPTER 1 Astronomical Coordinates and Time
• CHAPTER 2 Optics and Telescopes
• CHAPTER 3 Measuring Light
• CHAPTER 4 Charge-Coupled Devices
• CHAPTER 5 Image Processing
• CHAPTER 6 Photometry
• APPENDIX A Spherical Trigonometry
• APPENDIX B Data Analysis
• APPENDIX C Fitting and Graphical Representation of Data
• Bibliography