Astronomy (2008-05-17)



Table of Contents

1. Personal Context

I had to completely rewrite this essay after reading carroll2007. It is a masterpiece of story telling. I read it cover to cover in 2 weeks (and had to take several days vacation to do it). I'm still not enthused to go get a telescope, but I do want to look into magnetohydrodynamic simulations.

2. Observation

2.1. Observables

Astronomy is the study of the world external to the Earth. Since we cannot go there, do experiments, or bring back samples (except a few places in the solar system), we depend on observations of phemomena which arrive here at Earth.

Those observables are various forms of energy-matter: light, radio waves, high-energy particles, magnetic fields, solar wind, meteors, etc. For any phenomena from far away, the phenomena need to have traveled at or close to the speed of light, and thus are mostly electromagnetic radiation (light, radio, X-rays) and neutrinos.

These in turn can be halted or diverted many places along the path:

  1. Core of star. So hot that photons bounce around randomly without escaping.

  2. Photosphere. Optical depth is thin enough the photons can escape.

  3. Interactions/scattering due to electrons, which in turn may be coupled to/controlled by magnetic force fields.

  4. Partial absorption by or reflection by other objects, including gases and dark matter. This can be selecting so that the spectrum of frequencies observed is changed. Includes increased or reduced flux and thus emission lines and absorbtion lines.

  5. Total absorption by black holes.

  6. Redirection by gravitational lensing from massive objects (gravity as curved spacetime).

  7. Redshifting or blueshifting due to velocity of source relative to Earth.

  8. Absorption, redirection (shimmer, twinkle) in Earth's atmosphere.

  9. Swamped by natural radio or light sources (radio noise from solar flares, sun's visible light).

  10. Swamped by man-made radio noise or "light pollution".

  11. Inadequate aperature of collector (need big telescopes or big arrays to capture enough photons or waves to get a usable picture).

  12. Imperfections in collectors

A whole world of theory (backed by lab and field experiment) takes these distortions into account in order to arrive at a good estimate of what happened "out there". The atmosphere-induced problems push us to orbiting telescopes (like Hubble Space Telescope HST), or to mountaintops. The man-made-noise problems push us to isolated places on Earth. And finally, the distortions themselves have turned out to be some of the most intriguing puzzles of all.

2.2. Amateur Observation of Visible Light

See dickinson2002. An outstanding guide to tools and tricks of the trade, from $50 to $50000.

First step is to get a good pair of binoculars, pick a clear night (not so easy around here), lie down, and stare at the sky. Use a fieldguide or the "stellarium" software package to identify objects in the sky. This works better if you are not near civilization with its light pollution.

Next up, get a good telescope for eyeball observation. A Dobson can provide big-aperature for the price. But it can be hard to find objects in the first place, and once found they don't stay put. So consider a "Go to" motorized mount, or full equatorial mount (but make it sturdy). Either of these can push you to a portable telescope with folded optics. That is ok for eyeball observation.

For astrophotography, the recommendations are:

  1. Don't. It is tedious, frustrating, and expensive. Instead download spectacular HST images from NASA.

  2. If you persist, then you need a "fast" lens, which means refractors with high-quality optics.

  3. Start with the camera "piggy-back" (just attached to your telescope but not looking through it). Use this to solve mount aiming, tracking, and motor drive stability. Consider large-format camera, using film.

  4. Then move to "prime focus", camera-through-the-telescope.

  5. Then move to CCD cameras and image manipulation.

2.3. Amateur Observation of Radio Waves

Instead of sticking to visual light, explore radio waves and high-energy particles. Doing Earth-Moon-Earth (EME) amateur radio can get you involved in those issues.

Amateur Radio has had to deal with astronomic phenomena from the beginning. Even the simplest high-frequency bounce depends on solar-induced ionization.

See arrl_handbook2008 and arrl_antenna2002

2.4. Professional Observation of Visible Light

First, gather more photons. Observatories on mountaintops or on satellites. Very large (and very expensive) optics. Adaptive optics, where the system detects atmospheric distortions and bends parts of the mirror slightly to compensate. Arrays of telescopes, so carefully synchronized that they can be treated as one large telescope.

Take phtographs, and compare them. In the film era, rapidly switch between two images, so that changes (e.g., a comet or a supernova) appear to flicker. In the CCD era, do this computationally.

Do spectrum analysis. Look for signature patterns of emission and absorption lines. These indicate what caused the original energy emission, or what was in the paht on its way to Earth. The frequencies are characteristic of electron-shell changes, so we learn about atomic makeup (how much hydrogen, helium, carbon, iron, etc.)

2.5. Professional Observation of Radio Waves

First, gather more energy. Very large single-dish antennas, and very large arrays of antennas are used.

Collect and amplify and sample it with very-low noise electronics. Then treat it much like visible light: Store it, compare over time

Do spectrum analysis. The frequencies are characteristic of molecular changes, so we learn about simple molecules and radicals (CO, OH, etc.)

It is useful to computationally mix-and-match images from different wavelengths. In particular, it is useful to match radio sources with visible light sources and view the whole in a "false color" image -- often the visible light structure is only a part of the story.

3. Theory

3.1. Theory, Simulation, and Observation

Observation is necessary but insufficient. Deciding what to look for and how to interpret the results requires theory. Theories, in turn, are mere conjectures or hypotheses until tested by independent observations.

As far as I can tell, carroll2007 is *the* text for getting you into the game. It is encyclopedic, very readable, and reality-based. It uses real math and physics, not hand-waving popularizations. Mostly it works from gravity, speed of light, blackbody radiation, and ideal gas law. It introduces special relativity, general relativity, and quantum mechanics as needed.

However, carroll2007 makes quite clear that current research cannot be done with just these mathematical tools. Repeatedly, there are cautions that this or that phenomenon requires simulations of magnetohydrodynamics (MHD) or more carefully analysis using quantum field theory (again with simulations).

Simulations allow researchers to explore the consequences of subtle shifts in parameters, with known start points and no intervening observation distortions. Of course, once a simulation hints that a hypothesis is correct, one must check that the theory stands up to real-world observations.

Unfortunately for the amateur, even if you get up to speed on the theory, state-of-the-art simulations tend to take world-class supercomputer clusters. One can only hope that as costs for multicore CPUs and RAM come down, the amateur astronomer (or the astronomy club) can build Beowulf clusters and get in the game. Even now, we can do a lot with home computers. carroll2007 provides some programs, and others are available as Open Source Software (OSS).

4. Exploration

4.1. Getting there

You aren't likely to develop a rocket which can escape Earth's gravity, but you can at least investigate the engineering task. This is a problem in rocket propulsion, plus some guidance and control surfaces. Thus a math package and a Finite Element Modeling tool are needed. See Engineering

4.2. While there

If you want to experience manned flight (and don't have $10M spare change), you can try SCUBA diving in a dry suit. You get neutral bouyancy, slow motion, and access to a strange new world. You also get cumbersome attire, limited range of motion, and life-threatening crisis (with no help from outside) if anything goes wrong. Combine this with a weeklong outing with 4 unwashed buddies in a mini-van and you can see that humans are really not evolved to do space travel.

The alternative is to send robots. Given the very long communications delay, the robots have to be self-directed. This opens a whole world of Earth-bound experimentation with small robots which can crawl around in hostile conditions (e.g., a sandbox), do useful experiments, and report results back to home base. Thus mobile robotics, artificial intelligence, "agent" modeling, manipulators, and low-power radio communications are all fair game.

This may seem far removed from the field of astronomy, but autonomous robots are the rate-limiting-step right now. Hackers doing "combat robots" probably did more for the Mars mission than a whole flock of Astronomy PhDs.

5. References


Mark J. Wilson, ed. "THe ARRL Handbook for Radio Communications 2008" Amateur Radio Relay League, 2007. ISBN 0-87259-101-8.

Sections on space communications and sections on propagation are relevant.


R. Dean Straw, ed. "The ARRL Antenna Book", 19th ed. Amateur Radio Relay League, 2002. ISBN 0-87259-804-7.

Sections on space communications are relevant (e.g., helical and parabolic designs).


Constantine A. Balanis. "Antenna Theory", 3rd ed. Wiley Interscience, 2006. ISBN 0-471-66782-X.

While the ARRL books are for amateurs using low-cost materials, this text is for professionals (e.g., designing communications satellites or military communications systems). See section 13 (Horn Antennas) and section 15.4 (Parabolic Reflector).


Bradley W. Carroll, Dale A. Ostlie. "An Introduction to Modern Astrophysics" Pearson/Addison-Wesley, 2007. ISBN 08053-0402-9.

If you have had a course in calculus and a course in physics, you can read this book -- it provides the rest. It is math-and-physics based, but uses back-of-the-envelope approximations "yielding 80% of the understanding for 20% of the effort." Sometimes after a result has been derived, they note that more rigorous analysis gives a slightly different formula or slithgly different numerical result. In a few cases they note that magentohydrodynamics simulations are needed, and provide the results from those studies. Worst case they note that no-one yet has a clue but that work is underway.


Terence Dickinson, Alan Dyer. "The backyard astronomer" Firefly Books Inc, 2002. ISBN 10552090597-X,

Excellent and highly readable guide to selecting telescopes, using them, doing astrophotography, and becoming part of the star-gazing subculture. Has an associated website, with additional links:


Paul Horowitz, Winfield Hill. "The Art of Electronics", 2nd ed. Cambridge University Press, 1989 (reprinted 1999. ISBN 0-521-37095-7.

A true classic. By far the best text to go from intelligent-but-ignorant, to designing and building special purpose electronic scientific instrumentation.


John H. Moore, CHristopher C. Davis, Michael A. Coplan. "Building Scientific Apparatus", 3rd ed. Perseus Books, 2003. ISBN 0-8133-4006-3.

Designing and building scientific instruments. See optics sections, and spectroscopes.

Creator: Harry George
Updated/Created: 2008-05-19