BBC News Online asked the English Astronomer Royal, Professor Sir Martin Rees, and the BBC Sky AT Night presenter, Sir Patrick Moore, for their views on Eso membership.

How will the UK benefit?

Sir Martin Rees: I personally think it's a very important day for UK Science that we are joining formally this European consortium which is going to be able to fully match the United States in scientific endeavours which will be among the highlights of science in the coming decade.

Which telescopes will UK astronomers now be able to use

Sir Patrick Moore: I have been to both the main installations at La Silla, the main visual observatory there, and of course the VLT or Very Large Telescope on Cerro Paranal in the Atacama Desert and that is an amazing thing. Four 8-metre mirrors working together, by far the most powerful telescope ever constructed. And now we'll have access to that.

The VLT works as an interferometer - what does that mean?

MR: The VLT is actually four telescopes - not just the one. Having four telescopes rather than one helps in two ways. One obvious way is that you collect more light from four big dishes than from just one. But also by combining them together, you can do a sort of triangulation technique and get sharper images; and so by combining them together in a technique called interferometry it's possible to get sharper images than you could from any one singly.

The difficulty of detecting a planet around a star for instance is that the star outshines the planet by a factor of as much as a billion and in the sky these are very close together indeed. So, the only way of doing this is to combine together the light from the different telescopes of the VLT and use a delay line in such a way that you can cancel out the light from the bright star itself and reveal a much fainter pale blue dot which could be an Earth-like planet orbiting around it.

European astronomers are planning a new facility called Alma. What is it?

MR: This is a large array of dishes which are going to study not visible lights, but much longer wave length radiation to study very cool places in the Universe where stars are forming, and where other phenomena are going on which we can't study in other ways.

The VLT units can be used together as an interferometer

And the project called Owl. What is that?

MR: Owl is a rather hubristic acronym, which stands for overwhelmingly large telescope, and this is appropriate because it is in fact a hundred times larger in area than any single dish so far built.

This instrument, which perhaps will develop into a world instrument rather than just European, is going to be the next important step beyond the VLT in looking at visible light. This will certainly enable us to get very sharp images of the Universe at all eras from the present back to the time when the first stars and galaxies formed.

The membership fees for Eso are very expensive. Are they worth it?

PM: To support our membership of the Eso, the UK Government is giving extra funds, �10m per year and at PParc an extra �5m - and believe me that's going to be money very well spent.

There are some people who say 'why spend money on astronomy'. All I say is these people simply have not done their homework. In past ages, they would have objected to the evolution of the wheel. After all, astronomy is the basis of all time keeping and navigation. You cannot now separate astronomy from any other science: physics, mathematics, biology, medicine. It's all linked up.