By Flavio Falcinelli
In honor of Karl Jansky the unit of measurement of the flux density of radio sources has been defined:
1 Jy = 10-26 W/(m2 ∙ Hz)
This report helps us to understand what does a radio telescope measure: it is a radiant power coming from the sky, precisely the power that affects the uptake of the antenna surface (m2), included in the receiver bandwidth (Hz).
An alternative way, very convenient to express the signal power "collected" from the antenna, is the so-called brightness temperature: in fact a radio telescope measures the (equivalent) temperature of the scenario "seen" by the antenna. The term "equivalent" will be made clear later. It is possible to demonstrate that, in radio astronomy, the brightness temperature of a radio source is directly proportional to its radiated power.
If we orient the antenna of the instrument in a given region of the sky, in particular to a radio source that "stands out" over the ground, we measure an increase in signal intensity (namely, a power) proportional to the brightness temperature of that object, which will coincide with its physical temperature only if this is a black body, ie a (ideal) material which perfectly absorbs all radiation incident on it, without reflecting it. In nature there are no blacks bodies, but there are objects that approximate very well their behavior, at least within a specified frequency band.
If, as noted by Reber, we consider the telescope like a thermometer, we will have that the temperature measured by the antenna, that is the brightness temperature, will be proportional (not identical) to the physical temperature of the region through a coefficient called emissivity of that region. This is the meaning of the term "equivalent" used above. Emissivity is a measure of that material's ability to radiate energy and is a complex function of the chemical-physical properties of the radio source and the frequency characteristics. A black body emissivity is equal to 1, thus having a brightness temperature coincident with its physical temperature, while a material body (gray body) has an emissivity between 0 and 1, then a brightness temperature lower than its physical temperature.
As mentioned, the technology of a radio telescope is not substantially different from that of a home radio-receiving apparatus (such as, for example, a television, a car radio or a mobile phone): obviously, some features are specialized and performance are optimized to measure the very weak signals from space.
The crucial question is that in radio astronomy you need to highlight the noise from radio sources (useful signal) with respect to the noise generated by the electronics and the environment (unwanted signal): these "hiss" cross, identical to those we hear when in an FM no station is tuned, have the same nature and are, in principle, indistinguishable.
In-depth documents will illustrate the techniques used to solve this problem.