By Flavio Falcinelli
The radio astronomy observation "par excellence" (the easiest one) consists in determining how does the intensity of the signal received during the "transition" of a radio source (such as the Sun or the Moon) vary in the "field of view" of the antenna (the so-called registration to transit). You orient the telescope at the sky area where the passage of the radio source is foreseen, in its apparent motion, and wait for the formation of the classic "bell" track in the capture software.
The lunar transit observation with an amateur telescope based on our RAL10 receiver.
The next step, a bit more complex and laborious, contemplates the recording of signal intensity received from different directions of the sky. Slowly and methodically collecting a series of measures, you can fill in a "radio-map" of the observed sky region. Obviously "tracking" observations of the radio sources are possible, such as, for example, when you want to monitor solar activity. This requires motorized and automated equipment for the management of the orientation of the antenna system.
Generally, these are the first questions that every amateur radio astronomer asks himself:
- In which frequency band is it better to work?
- What are radio sources that can be observed with a small telescope?
- Are there special requirements in the choice of the instrument installation site?
You cannot answer any question independently from the others.
The mechanisms that explain the emissions of radio sources are complex, linked to their chemical-physical characteristics. As a first approach will be enough to catalog the most intense radio objects in the sky and discover how does their emission vary at different frequencies (radio source spectrum).
At amateur level, taking into account the limitations in sensitivity of the instruments due mainly to poor effective area of the antenna, a reasonable first choice seems to favor the frequencies where the radio sources are more intense and numerous. As we see from the following chart, besides the Sun and Moon that behave more or less as blacks bodies in the radio band (at least for what concerns the emission of the quiet Sun), other radio sources radiate with greater intensity for frequencies below 1 GHz, with an increasing mechanism (called non-thermal) with decreasing frequency.
The graph shows how the intensity of emission varies in the radio spectrum band from 10 MHz to 100 GHz.
However, we need to consider the radio "crowding" in the area where we will install the telescope, due to the presence of various interferences. The artificial noise, very intense in urban and industrialized areas, are the real "plague" in radio astronomy observation: the radio spectrum is practically saturated with signals and spurious emissions of various kinds.
The most common natural sources of interference are the lightnings, atmospheric electrical discharges, radio emissions produced by charged particles in the upper atmosphere (ionospherical disturbances), emissions from atmospheric gases and hydrometeors.
Artificial interferences are caused by disturbances produced by the distribution, use and power transformation of electricity, by radar transmissions for the control of the military and civil air traffic, by terrestrial transmitting stations used for radio and television broadcasting services, by the transmitters and transponders on artificial satellites (including the Global Positioning Satellites GPS systems), and by mobile phone network and military stations.
As seen from the graph below, the intensity of the artificial and natural disturbances decreases with increasing frequency: it is conceivable the installation of a radio telescope at 10-12 GHz in the "back yard", in urban areas, while it is very difficult the receiving at the lowest frequencies. In the latter case, we must opt for a rural area, electromagnetically more "quiet".
Performance of natural and artificial noise power in function of the frequency. It shows the levels estimated in the range from 100 MHz to 100 GHz (Recommendation ITU-R P.372-7 "Radio Noise").
Indeed, the choices based on the analysis of the spectrum of radio sources are contrary to those deriving from the analysis of the spectrum of disturbances: we have a "pro" and "cons" tie. Decisive will be the technological and economic considerations.
An amateur radio telescope "for all" should be easily achievable, economic and of immediate operation: the "heart" of the instrument should be a module designed "ad hoc" for radio astronomy that integrates the essential parts of a basic radio astronomy receiver.
Around this core, the researcher completes the telescope using commercial parts and modules, economic and easily available. All this is possible thanks to the spread of satellite TV reception in the band 10-12 GHz, and the availability of antennas, amplifiers, cables and a host of accessories, new and recycled, suitable for building a perfect amateur radio telescope.
RAL10: the family of receivers for amateur radio astronomy.
We know that the antenna size greatly influences the final cost of a radio telescope. Also the commercial availability of this critical component plays a fundamental role.
If we consider that, with the same antenna gain (is a measure of its ability to capture weak signals in specified directions of space), its dimensions (therefore weight and size) decrease with increasing frequency, using a common parabolic reflector antenna of 1 meter diameter for TV-SAT 10-12 GHz, we can easily build our radio telescope.
The only drawback is the limited number of measurable radio sources: the Sun and the Moon, with small diameter antennas. However, being their radiation very intense, their study is an excellent "training" to begin to familiarize yourself with the tools and techniques of radio astronomy.
To reveal weaker radio sources (Taurus, Cassiopeia, Cygnus, Virgo ....) you need larger antennas, while keeping the rest of the system unvaried.
These are the reasons that have guided us in developing the family of microwave receivers RAL10 for amateur radio astronomy.