Frequently Asked Questions About Radio Astronomy

Public awareness of radio astronomy lags far behind that of its optical counterpart. The vague image of huge dishes pointed at the sky is perhaps the only connection that many people can make with this new and and highly technology driven science. While, most people can relate on some level to the intrigue of peering through an eyepiece at some distant object, a bump on a graph which might cause great excitement among radio astronomers, does little to stir the public imagination. It is this general lack of familiarity with radio astronomy that gives birth to an array of extremely broad and often difficult to answer questions. Some of those questions will be tackled here, but it would behoove anyone who is new to this subject to seek help at the local library for references which are more in depth. Here is the List:


How is radio astronomy different from optical astronomy?

Radio astronomy and optical astronomy both examine electromagnetic radiation originating from outside the Earth's atmosphere. Where they differ is in the tools used to detect this radiation and in the wavelength or frequency of the waves they study. Note that light and radio waves are both manifestations of the same energetic phenomena. Because radio waves are much longer than optical waves, the telescopes used to detect them must be much larger than optical telescopes.

What is a radiotelescope?

A radiotelescope is basically a very sensitive radio receiver. Whereas communications receivers are designed to extract information which has been intentionally modulated onto a radio wave, radiotelescope receivers are designed to measure the intensity of the radio wave over some band of frequencies. A radiotelescope is essentially an energy measuring device.

Most radiotelescopes use large antennas in order to make their "beam patterns" as small as possible. The beam pattern is the two dimensional area as projected upon the celestial sphere to which the telescope will be sensitive. A small beam pattern endows the telescope with the ability to resolve the level of signals arriving from regions separated only by a small angular distance. Multiple antennas are sometimes combined into "arrays" to enhance resolution. Widely separated antennas may have their signals combined in an "interferometer" arrangement where resolutions can be obtained which surpass those of optical telescopes.

Click here for a tour through a simple radio telescope.

What do radio astronomers listen for?

Actually, radio astronomers very seldom "listen" with their ears to the signals they are receiving. If you do listen to some of the radio telescope output that has been translated to an audio signal you can hear, it sounds just like the static you hear when your television is tuned to a channel where no station is present. It is this broad noise signal that is of interest to radio astronomers who measure it in many ways, but only listen in perhaps to hear what kind of man-made interference is messing up their measurements.

How are pictures made from all this radio noise?

Imagine if your entire view of the world was through a soda straw. If that isn't bad enough, suppose that your soda straw was covered on one end by a translucent substance such as a piece of wax paper. Now, when you look through the straw in any direction you see only a spot of white which varies in intensity depending on where the straw is pointed. Given this situation, you could still build up a rough picture of what the world looks like by sweeping your straw in a regular pattern and recording your impression of the brightness at each point in the sweep. The amount of detail you could create in your picture would be related to the diameter of the straw and your ability to discern small changes in brightness. In a radiotelescope, these parameters would correspond to the telescope beamwidth and sensitivity, respectively. When images or radio maps are being created, a similar sweeping/recording and picture construction scheme is used.

What frequencies are used?

Radio astronomy theoretically concerns cosmic signals at any frequency less than the frequencies of light. A wavelength is the distance traveled through space by the wave during of a single oscillatory cycle. The wavelength, rather than the frequency, (frequency = vibrational rate with respect to time), is often used to describe radio waves above one megahertz or so in frequency. Higher frequencies translate to shorter wavelengths. The relationship between the two measurements is set by the speed of light.

wavelength in meters = 300/ frequency in megahertz.

You will often, for example, hear the molecular hydrogen line frequency region of 1,420 megahertz referred to as the 21 centimeter band.

There are practical limitations to our ability to receive many of these frequencies, especially from our protected position beneath the shield of the earth's atmosphere. Frequencies below 15 Mhz or so, are rarely used due to absorption of these waves by the ionosphere. At the upper end of the frequency range, limitations are imposed by the technology needed to receive signals with such tiny wavelengths. Almost all amateur radio telescopes fall between 18 Mhz and 10,000 Mhz. The exact choice of frequency for a given amateur will depend on the technical abilities of the experimenter, the types of observations being sought, the radio interference pattern in the area, the amount of room available for antennas, and possibly the availability of commercial equipment which can be pressed into service.

How much does a radiotelescope cost?

This is one of those FAQs for which it is nearly impossible to give an adequate answer. The price can range from perhaps $100 up to...well the sky is the limit. Everything will hinge on how much scrounging the builder is willing and able to do. Technical expertise is your most valuable commodity in this endeavor. The more you know, the cheaper you will be able to get by.

Do I need to know electronics to do amateur radio astronomy?

Yes...or have a close association with someone who does. You may hear otherwise from different sources, but my experience has been that people who are clueless about how to use a DVM (digital volt meter), generally give up amateur radio astronomy after wasting quite a bit of time trying to get a radiotelescope operational. There are just too many things that can go wrong, (even when using commercially built modules such as TVRO equipment), to have much chance of success unless you know enough electronics to have a basic understanding of how the equipment functions. You should know a bit about diagnosing problems and how to repair them. This doesn't mean you need to be an electrical engineer. In fact, if you lack basic electronics knowledge, you can hook someone more so inclined into your cause. Try a local ham radio club or ask at your astronomy club meeting if anyone knows electronics. About 40% of people doing amateur radio astronomy are ham radio operators also. You can also take a course in electronics at a local adult night school, or even get one of those 100 in 1 kits and a book on theory to work through. Click here for ideas. Here is another little axiom similar to the one in the FAQ above: The more electronics you know, the greater is your chance of success.

What can I do with a small radiotelescope?

Here are few things that come to mind when asked this. Of course, radio astronomy is like so many other fields of knowledge...the more you know and do in the field, the more you find yourself stumbling upon new and intriguing avenues of discovery.

How do I build a radio observatory?

This is the most FAQ of all the FAQs and yet there is no simple answer! In fact, there are an endless number of very complicated answers to this question. It is best to begin by doing some reading, talking to others who have done this, and developing a set of reasonable goals for a first-time project. "Discover new sulfur based life-forms by submillimeter inspection of dark nebulae." is not an example of a realistic goal. Start with something like "detect the sun" or "record a Jupiter noise storm". You can then search for a suitable frequency band to work on. Pick something you can handle. Use lower frequencies (70 Mhz or below) if you have no experience working with UHF or microwaves. Use a scanner to seach the local airwaves for a nice broad unused portion of the spectrum to work with (good luck on this one!). There is unfortunately a scarcity of detailed plans for building small radio telescopes. This is one of the problems that we hope to help rectify here at Radio-Sky Publishing. William Lonc's new book Radio Astronomy Projects gives good guidance on several approaches using surplus TVRO and other gear. The Radio Astronomy Teacher's Notebook includes plans for two telescopes, one at 38 Mhz and the other at 1.4 Ghz.

Very often, the experimenter will develop her or his own design based on their particular set of needs, abilities, and available resources. Some parts of the telescope will often be home built while others will consist of commercially produced devices such as LNAs (low noise amplifiers), antenna mounts, or power supplies. In order to produce a radiotelescope from a mix of various components, one must still be familiar with the concepts like bandwidth, integration, noise figure etc. A radiotelescope depends on these and other factors in order to extract a usable signal.

How do I set up a SETI observatory?

People interested in joining the search for extraterrestrial intelligence (SETI) have a somewhat different set of criteria to fulfill. SETI is usually approached as a search for a very narrow bandwidth signal of cosmic origin. Some people are using modern communications receivers which can be scanned electronically in frequency. These receivers are tied to personal computers which issue the scanning commands and record the results digitally. Due to the fact that an expected ETI signal is similar in some ways to communications signals used on earth, it is entirely possible that a system can be developed which relies completely on commercially available components. An example may be found at the SETIFOX site.Before you run out and buy a $2000 receiver and install a 24 foot dish, however, you should realize the enormous problems which lie ahead in terms of separating out your "hits" from local interference which arise from a myriad of possible sources. One possible solution would be to coordinate your observations with those of another experimenter in a different part of the country. I strongly suggest that you read Are We Alone? by Frank Drake before you begin your search. While the tone of this book is extremely optimistic, Drake, the first person to ever use a radio telescope to search for ET signals, puts into perspective the enormity of the task.