Radio Telescopes

The collection of experimental data is one of the keystones in astrophysics as it is crucial to studying systems that we are not able to replicate on the earth. Although the first telescope was developed by Galileo Galilei at the beginning of the 17th century, the idea that astrophysics needs to measure much more data than what we see with our eyes is relatively new.

It is in this context that specific types of telescopes were developed to gather incoming radiation in many different frequencies (such as gamma rays, x-rays, radio waves, etc.). The relevance of radio telescopes comes from the fact that radio waves constitute a type of radiation that is more efficiently transmitted through the universe and corresponds to specific large objects like black holes or the nuclei of galaxies.

What are radio waves?

Radio waves are electromagnetic waves with wavelengths of more than 1 millimetre, the largest wavelength in the whole electromagnetic spectrum. They are produced by the largest objects in the universe, and it is crucial to collect and study them.

Sources in the universe

The main source of information about the universe is electromagnetic radiation. There is a simple physical connection between this radiation and the material content we observe in the universe: larger objects are associated with larger wavelength radiation and smaller objects with shorter wavelength radiation.

This does not mean that small or large systems do not emit other kinds of radiation but that their most intense emission is of the types mentioned above. This is why the collection of radio waves gives us information about large objects in the universe. The main sources in the universe are the following:

• Black holes: these are objects formed by an accumulation of a very large amount of mass in a small region, which do not allow anything to escape their gravitational attraction, not even light. Their properties can be studied thanks to the processes they cause in their surroundings, some of which are emissions in the radio region.
• Galaxies: these are associations of millions of stars that may orbit a nucleus. The collective effects observed may be associated with scales so large that they can only be studied in the radio region.
• Supernovas: these explosive events, which constitute the death of stars, emit in all frequencies, but due to the characteristic size of these events, there is a relevant emission in the radio region.
• Quasars: these supermassive black holes are the nucleus of a certain galaxy. We have already mentioned galaxies and black holes as independent objects that can emit radio radiation. A quasar is a black hole that concentrates a huge amount of the mass of a galaxy and, as such, emits radio radiation more intensely than smaller black holes or galaxies that do not have a black hole in their nucleus.
• Cosmic microwave background: a remnant spectrum of radiation from the first stages of the universe, which mainly peaks in the microwave region (the second largest-wavelength region). However, there is also plenty of relevant information in the radio region that allows us to study the origin of the universe.

Transmission and extinction

The collection of astronomical data happens after the radiation has travelled very large distances between the sources and our telescope, which cannot be very far from the earth (because of time and technological constraints). If, however, we know how to correct the bias and issues produced by observing from a large distance, there should be no problem with using these measurements. However, there is an unavoidable problem associated with all kinds of radiation: extinction.

It turns out that this effect obeys a similar rule to that for the association of wavelengths with sizes: smaller wavelength radiation interferes more with smaller objects while larger wavelength radiation interferes more with bigger objects. Since most of the structures in the universe consist of components of small size, extinction is a phenomenon that affects mainly smaller wavelength radiation. In other words, radio radiation is the most faithfully transmitted radiation in the universe.

What are single-dish radio telescopes?

Single-dish radio telescopes are the most widely used devices to gather information that comes from the universe in the form of radio waves. Usually, they consist of a single large dish necessary to capture the waves in the desired range.

Parts and functioning

The main parts of a single-dish radio telescope are the following:

• The dish or reflector: like most modern telescopes, radio telescopes are reflecting telescopes where the dish is in charge of collecting the incoming information and reflecting it towards another part. Due to the characteristic wavelength of radio waves, the dish of radio telescopes is very large (more than 100 metres in diameter), which means that they have a good ability to collect radiation.
• The antenna: this is the part towards which the radiation is reflected and which is in charge of collecting and transmitting it to the devices that carry out the analysis of the information.
• The amplification and analysis system: this is the part where the information is amplified in intensity if needed and analysed to extract conclusions about the spectrum properties.

The reason we refer to single-dish radio telescopes is that, usually, radio telescopes are part of a larger complex that includes multiple single-dish radio telescopes. These complexes allow us to amplify further the collecting power by combining the telescopes instead of building a huge dish of several hundreds of metres (or even kilometres). The first image ever detected of what seems to be a black hole was obtained thanks to seven radio telescopes located all around the earth, which allowed to amplify their power.

Figure 1. First image of a black hole. Source: Event Horizon Telescope, Wikimedia Commons (CC BY 4.0).

Advantages and disadvantages

The radio telescopes have many advantages because the radiation they measure is subject to almost no extinction. The size of the telescopes ensures a great collecting power and a huge capacity of resolution, allowing to separate different objects at very large distances. The main disadvantage is that the economic cost of these telescopes is high, but the development of telescope complexes has brought those costs down.

Figure 2. Very Large Array (VLA) of radio telescopes. Source: Hajor, Wikimedia Commons (CC BY-SA 2.0).