United Kingdom industry and universities will be major contributors to the construction of these powerful instruments, and the overall director of the Gemini project, based at a headquarters in Tucson, Arizona, is Dr. Matt Mountain, a graduate of Imperial College London, who has been a member of the scientific staff at the Royal Observatory, Edinburgh, since 1987.

International Cooperation
With a forecast cost of some £120 million, the Gemini project is big science with a capital B. For such huge concepts, international cooperation is essential. Each telescope will feature an eight-metre primary mirror capable of outstanding resolution at both visible and infra-red wavelengths: images approaching 0.1 arc seconds in size will be achieved near 2.2 micrometres, with near-diffraction limited imaging at longer wavelengths, optical imaging of less than 0.3 arc seconds (well suited to correction by adaptive optics) being anticipated.

As a contribution to optical steadiness the primary mirror will be located 20 metres above ground level, substantially above the potentially turbulent boundary layer; dome ventilation will further reduce the adverse effect of the wind and provide optimum flushing, with careful thermal matching of the enclosing dome the telescope itself, and the optics to minimise the effect of thermal instability.

Especially important is the multi-object spectrograph being developed at Durham University in northern England, the Royal Observatory in Edinburgh, and the Dominion Astrophysical Observatory in Canada, which will offer as many as 600 spectrographic slits formed in custom-made masks that are deployed in the focal plane.

Velocity Fields
Gemini offers a huge potential for such tasks as studying the velocity fields of galaxies imaged by the Hubble space telescope or determining the ionisation source in active galaxies.

With resolving powers of up to 10,000 and a capability for measuring radial velocities to an accuracy of 1-2 km/s from one line with hourly wavelength calibration, the Gemini multi-object spectrograph could, for example, characterise velocity distributions in a dwarf spheroidal galaxy belonging to the local group in the equivalent of a single night's viewing.

On the 4200 metre summit of Mauna Kea, in Hawaii, the concrete foundations of the northern Gemini instrument have already been laid, and first light is expected in 1999. The southern site, 2700 metres up at Cerro Pachon in central Chile, is expected to commence operation about a year later.

Chosen for the clarity of their weather and excellent atmospheric stability, the two Gemini sites offer outstanding observation facilities for astronomers, well above the water vapour that hinders most ground-based observing.

Professor Ian Robson, the British director of the Joint Astronomy Center in Hawaii that will operate the Mauna Kea instrument, has no doubt of the important contribution that Gemini is going to make to international astronomy.

Best Deal
"It's very important for UK astronomy," he says. "I was a strong supporter of it from the start. Gemini was the best deal going at the time for the UK to have access to the next generation of telescopes, I am very positive about the project: it will be great for Britain and it will keep UK astronomers at the forefront of world astronomical research."

One of the unsolved questions that Gemini will address is the mystery of how stars are formed. How do stars condense from clouds of dust, and what do protostars look like? Newborn stars are only going to be observable in infra-red emissions that can penetrate the swirling dust cutting off visible light. Gemini's infra-red performance will make this area of investigation a priority objective.

Mature stars initially burn hydrogen and then progressively more complex elements, until eventually their nuclear furnaces generate heavy metals. These each have their own distinctive signatures in isotopic abundance patterns; spectroscopic studies with the Gemini telescopes will reveal the history of heavy element production not only in our own galaxy, the Milky Way, but also in its two neighbours, the Magellanic Clouds.

Distant Light
Gravitational lensing will also come under scrutiny by the two Gemini instruments. Light from far distant objects such as galaxies and quasars (quasi-stellar radio sources, among the most distant and most luminous bodies known) is sometimes bent by the lensing effect of gravitating masses between the observer and its point of origin. The result is the manifestation of rings or multiple images.

Detailed studies of this phenomenon require the Gemini combination of wave length coverage, angular resolution, and sensitivity to probe matter distributions and distinguish the signatures of microlensing and variable substructures within lensed sources.

And at the furthest extent of the universe that has so far been probed by telescopes, Gemini will be looking at the most remote - and therefore the youngest - galaxies of all. They can tell us how galaxies are born and how they evolve. The infra-red response of Gemini will enable astronomer s to explore their structures and stellar content and provide the key to understanding the relation between stellar populations, chemical enrichment history, and the life cycle of present galaxies.

Gemini, the aptly named heavenly twins, will usher in a new era of astronomy and ensure British scientists continue to play a leading role in the exploration of the heavens.