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.
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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
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
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
Do I need to know electronics to do amateur radio
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.
Study Jupiter's noise storms.
Record flares and predict geomagnetic activity.
Detect a pulsar using DSP (digital signal processing).
Detect stronger radio sources.
Look for HEPs (high energy pulses} from the galactic center.
Search for radio correlations to gamma ray bursts.
Study ionospheric scintillation and refraction.
Detect meteors invisible to the eye.
Develop a long base line interferometer.
Learn radio technology.
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.
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