Who has not looked at the sky in a clear sky night, in a place apart from the city lights and has not asked himself if there is anything more beautiful than a starry sky? It is almost sure that all of us who decided to study the universe started in this way and, in fact, besides being delighted with the beauty of the sky, we started to wonder why everything was like it was and not in another way.
Also, it is almost sure, that we all started to ask our parents for a telescope, that it was always smaller than the one we finally got.
Later on, besides the telescope, we wanted books about how to observe the sky, what objects could be observed, when to observe them and that, in addition they had cool pictures of galaxies, nebulae, globular clusters, and all in all, any object out there.
In those books there were also pictures of telescopes, and why not, it is almost sure that we wanted them all, refractors, reflectors…, but for many of us, the surprise was to learn that there were telescopes without any hole to look through. In fact, they did not look like conventional telescopes. They were similar to the parabolic antennae that some people had to watch several television channels. What was that? Was it possible to observe the Universe with those antennae?
These antennae are actually radio telescopes, and yes, the Universe can be observed with them. In fact, it is a must to observe the Universe with them.
Conventional telescopes, with a hole to look through, usually observe the Universe in the visible part of the spectrum. Of all the electromagnetic spectrum, they can only see the wavelengths corresponding to the visible light, which are the same as the ones we can see with our eyes. However, radio telescopes are capable of detecting other wavelengths, longer than those of the conventional telescopes. These wavelengths are in the radio part of the spectrum.
A radio telescope is, in general terms, a large parabolic surface (paraboloid of revolution) which acts as a radio waves collector. Having a parabolic form, the incoming waves are reflected by the surface and concentrated in point known as primary focus. In this point, two things can happen. One is that in this point there is a receptor in charge of transmitting the reflected radiation to the instruments that will measure it. The other one is that in the primary focus there is a sub reflector that reflects the radiation to a receptor located in the collector and, from there, transmit it to the instruments. Both options are feasible, but the second one permits accessing to the receptor when it is needed to perform maintenance tasks and it also allows the receptor to be heavier.
Radio telescopes are antennae that can be very large, reaching diameters of 100 m or even more like the 300 m of the Arecibo radio telescope. The size impacts on the resolution of the information gathered. The larger is the size, the larger the resolution. The main problem is that it is practically impossible to build antennae with several kilometres to get a large resolution. This does not mean that the smaller radio telescopes, 100 meters and below, are useless because of the lack of sufficient resolution. Several important discoveries have been made using these radio telescopes. But as any other scientist, astronomers and astrophysicists always want more, especially when for each answer new questions arise.
To give an answer to these new questions, not only larger radio telescopes are built, but several smaller radio telescopes are built and connected amongst themselves, either in a physical manner so that the radiation gathered by all of them are sent to the same analysis center at the moment of its reception or “virtually” so that each radio telescope gathers its own information and later sends it to other remote centers where it is analyzed together with the information gathered by other radio telescopes.
This is possible by using interferometry techniques. Interferometry consists in combining the radiation gathered by several sources (several radio telescopes) in a way that the resolution of the information being received is increased. Interferometry it is based on the fact that radiation is an electromagnetic wave. To understand what interferometry is, let’s talk about a classical experiment in the history of Physics: the double slit experiment.
When a source of light is located in front of a screen, and between them it is placed a plate which does not let the light go through it but with two thin slits drilled on it, the light when it goes through both slits it is diffracted and follows different paths. When the light impacts on the screen, the diffracted light coming from each slit interferes because it comes from different directions and at different moments. This interference makes that dark lines can be observed where the light has destructively interfered and bright lines where the light has constructively interfered. It can be also seen that when the source of light is a point (a small source) the contrast between bright and dark lines is bigger and when the light source is wide, the contrast is diffuse.
Interferometry using radio telescopes follows the same principle. Radio waves arrive to the radio telescopes that are separated a certain distance at different moments (the time difference is very small but noticeable with precise systems for the measure of time). This makes that the signal of the radio waves measured by all of the radio telescopes follows generates an interference pattern. By studying the pattern and the contrast between bright and dark signals measured, the form and characteristics of the source of radio waves can be reconstructed.
Using interferometry techniques we can increase the resolution of the image because, although we have small radio telescopes separated a few meters or kilometers away in the case they are physically connected (also known as long baseline interferometry) or several kilometers in the case they are “virtually” connected (also known as very long baseline interferometry), the final outcome will be as if we had a radio telescope of the size of the maximum separation of the smaller radio telescope. This technique can be used even with radio telescopes in orbit.
Radio telescopes are very useful to study different phenomena occurring in the Universe that we cannot observe with conventional telescopes as the birth of stars as well as the interstellar medium where stars are born. How radio telescopes are used and the physics behind the phenomena they observe will be explained in another post.
PARTNeR Project: http://partner.cab.inta-csic.es/