What does the output of a radio telescope look like?

[accdd]

What does the output of a radio telescope look like?
I suppose it is not an image like that of a telescope observing visible light

 

[Hyperfine]

Typically a plot of signal amplitude as a function of frequency. So a baseline with a peak, very similar to what a spectrum in any frequency domain looks like.

Often the images of a radio emitting region will be illustrated via contour lines depicting the signal magnitude. As an example from NRAO: Viewing a contour plot.

 

[Baluncore]

What does the output of a radio telescope look like?

The observation is recorded over a period of time.
The bandwidth of the signal is determined by the antenna and receiver selectivity.
The signal voltage is digitized, so it can be analyzed later using signal processing.
In effect, the antenna and receiver is a thermometer and the signal represents a temperature.
The signal sounds like frying white noise.
With an antenna aperture over about 500 m², the brightest pulsar can be heard in real time in the noise.

Accurate radio images are synthesized by recording the same area of sky, with many widely spaced antennas and receivers, simultaneously as the Earth rotates. The recorded signals are later synchronized, then correlated to produce the image.
https://en.wikipedia.org/wiki/Very-long-baseline_interferometry

 

[TheOrionNebula]

Radio astronomical data comes in several forms. For observing spectral lines, for example the well-known hydrogen 21 cm line, the output of the telescope is processed in a spectrometer to produce a spectrum of the kind you have seen before, and can usually be obtained with a single radio dish.

For sensitive observations of the ‘continuum’ emission of radio galaxies, we would normally use many telescopes joined together to form an interferometer, or an array. The data from all of these individual telescopes is joined together in some complexicated electronics (technically called a correlator), where we see the ‘interference pattern’ of the sky image as viewed by the telescope signals when they are brought together. Technically the array of telescopes are measuring the Fourier transform of the sky image, where the instantaneous 'Fourier componebts' are the interactions between the signals of the various telescopes when they are joined together. Then with a simple bit of data processing (an inverse Fourier transform), and a few other tricks, we recover the sky distribution, which is what you see looking like a radio image of the sky.

For observations of transient objects, for example the Sun, or Jupiter, or pulsars, wither an array, or a single telescope can be used, that measures the signal intensity variations, in a similar way to the spectroscopic case above, but in this case [probably] with a broader receiving bandwidth. In the early days of pulsar discovery, this signal would often be fed directly into an audio amplifier, where you could hear the click-click-click of pulsars as they entered the field of view.