By Christine McGourty BBC science correspondent, on Cerro Paranal, in the Atacama Desert, Chile

The UK has joined one of the super-clubs of world astronomy by formally entering the European Southern Observatory (Eso) organisation.

This is a fantastic step... a huge boon for UK astronomers

Dr Sara Ellison, British astronomer in Chile

Britain's membership was sealed at the annual Eso Council meeting, held in London on Monday.

The move represents a boost for the Observatory, which has ambitious and expensive plans for new astronomy facilities in Chile, a country that is proving to be an ideal location for studying the southern skies.

It also represents a turning point for astronomy in the UK. After four decades of ploughing its own furrow, the country will now have access to the world's biggest optical telescope facility, the aptly named Very Large Telescope (VLT), in the Atacama Desert.

The UK will also now have a major input into some of the most exciting astronomy projects of the early 21st Century. One of these is the Atacama Millimetre Array (Alma).

Alma will be the largest ground-based astronomy project ever

This joint US-European project involves building a series of 64 giant microwave receivers - also in northern Chile - by the end of this decade. Once constructed, the array will allow astronomers to probe more deeply than ever into the early Universe, when the first stars were flickering into existence.

But joining Eso does not come cheap. The initial membership fee is about �80m - and then there is an annual subscription of �12m.

There is serious concern among UK astronomers about the financial impact of Eso membership on other science projects.

Financial squeeze

Much of the Eso cost is being met by new government money - and part of the membership fee is being offset by the UK's offer to let its new partners use its new Vista telescope, sited alongside the VLT.

The VLT has unique capabilities and we expect to see many more British astronomers having access to it

Dr Roberto Gilmozzi, Paranal director

But this still leaves the Particle Physics and Astronomy Research Council (PParc) - the body that funds astronomy in the UK - having to find at least �5m annually by 2005 to maintain its Eso commitment.

Cuts and redundancies have already been announced at observatories in La Palma, Hawaii and Australia. And there is an anxious wait now for the outcome of the next government spending review.

If astronomy funding is tight, there could be a squeeze on other projects, too. Despite this, few dispute the benefits of joining Eso, which in the VLT has a ground-based telescope to match the best in the world.

British tradition

The VLT is sited at the Paranal Observatory. Its director, Dr Roberto Gilmozzi, told BBC News Online: "While European astronomy has been trailing American astronomy for most of the last century, now, with the VLT, we are least at the same level.

"The VLT has unique capabilities and we expect to see many more British astronomers having access to it.

"But it's important to say that Eso will benefit, too. Britain's entrance brings a new part of the community in, bringing new concepts and ideas.

"The UK also has a long tradition in building instruments for telescopes, which will certainly be put to very good use here."

'Huge boon'

Dr Sara Ellison, one of the few British scientists currently working at Eso in Chile, said joining Eso would have a major impact on UK astronomy.

"This is a fantastic step. The majority of time on the telescopes here is restricted to Eso members, so this will be a huge boon for UK astronomers."

As an Eso member, Britain will be contributing roughly 20% of the organisation's budget, just a fraction less than the main contributors France and Germany.

It puts Britain's contributions on a par with Italy's. The other members are The Netherlands, Switzerland, Belgium, Sweden, Denmark and Portugal. As the host country, Chile is also a member.