The VLBI antenna parabolic reflector

The VLBI antenna parabolic reflector of 32 meters diameter
the telescope IRA-CNR in Medicina (BO) -Italy.

The radio telescope is an instrument that can measure and record the flow emitted by radio sources in the sky.
It's composed of an antenna system and transmission lines that convey the received signal to a receiver, from devices for processing and recording of the acquired data. The structure also includes the supervisory and pointing.

Conceptually, a radio telescope is not too different from a normal radio-receiving apparatus, although some special features are specialized to ensure the correct processing of weak signals received.
In the simplest structure a radiometer is equipped with a suitable antenna system, that is a very sensitive wide-band radio receiver designed to measure the intensity of the cosmic noise. Radiometric techniques reveal, in fact, the natural radioemission produced by matter in the light of its thermodynamic state: it manifests the characteristics of a signal inconsistent wide spectrum similar to the internal noise generated by the equipment.
The process of measurement estimate the average values associated with the parameters of interest, such as the received power.

Basic structure of a radio astronomy receiver

 Basic structure of a radio astronomy receiver, in its simplest version (Total-Power Radiometer).

The previous diagram showing the configuration of a superheterodyne receiver, the most widely used: the signal from the radio source is picked up by the antenna and sent to the input of the first stage pre-amplifier RF (front-end LNA: Low Noise Amplifier) characterized by a typical gain of the order of 20-50 dB, with very low noise figure that defines (together with other parameters such as the bandwidth and the integration time) the sensitivity of the radio telescope.

The amplified signal is translated in band (downward) by a frequency converter (mixer + local oscillator) that generates a signal at intermediate frequency (IF) with power directly proportional to the incident radiation.

Most of the gain of a superheterodyne receiver, the order of 60-90 dB, is localized in the IF amplifier chain which, consequently, must possess adequate stability characteristics. In order to prevent any fluctuation of gain and eliminate errors in the evaluation of the power associated to the incident radiation, is avoided systems of automatic gain control (AGC), while a wide dynamic range is obtained using suitable active devices properly polarized and inserting resistive attenuators calibrated to internal IF amplifier chain.

The IF amplifier is followed by the detector stage in quadratic characteristic at the output of which, is measured a signal of amplitude proportional to the power associated with radiation picked up by the antenna: this "information" is further amplified and integrated (by stage amplifier-integrator post-detection) to allow subsequent processing by the display devices and recording (chart recorders, systems of automatic acquisition of data for PC, etc.).
The amplifier-integrator post-detection (with frequency response of low-pass), with the purpose to minimize the fluctuations of the useful signal and to optimize the sensitivity of the system, carries out an average of the detected signal according to a predetermined (and programmable) constant of time, with values ranging fractions of a second to hundreds of seconds.

Like occurs in conventional communication systems, electromagnetic wave received by a radio source is not observed any type of modulation: the signal manifests the characteristics of a "noise", because its intensity varies random over time.

Generally is interesting measuring the energy received and record the intensity of the incident radiation.

Signals processed by a radiometer

 

The figure shows the processed signals from the receiver chain of a radio astronomy receiver (total-power radiometer), highlighting the fundamental theoretical relationship that determines the sensitivity of the receiving system and the parameters on which it depends.

If we study variable sources over time (for example, the Pulsar) will need to maintain the bundle pointing of the antenna on the source and record the changes in intensity over time, while if you want to get the radio equivalent of a photograph will be necessary "sweep" the region of the sky which affects the intensity and record the signal in function of the position of antenna pointing.
Another important difference between the radio and television broadcasts and the cosmic radio emission is that the first transmit on frequencies specified, with very narrow frequency bands (in order to optimize the allocation of frequencies with numerous programs from many different stations), while the second radiating, in general, a continuous spectrum signal simultaneously on all frequencies. Exceptions to this rule are clouds of diffuse matter characterized by widespread emissions to a very narrow band due to atomic and molecular transitions (for example, interstellar MASER).