Introduction to Astronomical Observations

Astronomy and astrophysics (there is no practical distinction today between the two) are, in general, an observational and not an experimental science. We cannot build stars and galaxies in the laboratory; experimentation in astronomy is limited to detector development, optical design, and perhaps some zero-gravity experiments in fluid dynamics and convection. We observe the universe by detecting radiation (electromagnetic or gravitational), and then attempt to interpret that radiation in terms of known physics.

As observers, we suffer a fundamental limitation. We did not design the experiment, and we cannot re-run the experiment. Not only that, we can only observe the universe from one direction, and at a specific time. Since all astronomical objects are time-variable, all observations are unique and are potentially irreproducable. In a sense, observational astronomy and astrophysics is a detective show wherein the objective is to determine not only the modus operandi, but also to deduce the nature of the crime. Fortunately, we have lots of clues.

Modern astronomy is a multi-wavelength endeavor. Astronomical objects emit radiation at all wavelengths. Observing in one narrow band of the electro-magnetic spectrum is akin to the tale of the blind men of Serendip: you may describe what you see accurately, but it may have little relation to the whole. An observational astronomer must be proficient at planning and undertaking observations at many frequencies, from X-rays through radio, and must be proficient at reducing and analyzing these data. Astronomers are generalists, not only in regard to wavelengths, but also in regard to objects.

Today astronomy suffers from a glut of data. Modern large-field, high-quantum efficiency imaging detectors return enormous amounts of data faster than they can be completely understood. NASA maintains a public archive of all the data obtained on NASA spacecraft. Much of this wealth of information has never been fully analyzed, and many serendipidous discoveries lie in the archives. Large-scale digital optical and near-IR surveys are underway. This is truly a golden age of astronomy.

This section of PHY 445/515 is designed to provide a taste of observational astronomy (PHY 517, Astronomical Observing Techniques, formerly AST 543, offers a more detailed introduction to the field). There are four laboratory exercises. All exercises involve an archival data component; three involve direct observations. Unlike the physics labs, these are less well controlled in the sense that you do not know in advance exactly what the right answer is.

These exercises require that you do three things:

As with the physics labs, you must keep a notebook. We want to know enough to be able reproduce your analysis.


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