1. Technical Background (based on a paper in ISAS (Japan) report SP No. 14, April 2000; updated and expanded).


Extremely Large ground based Telescopes (ELTs), with apertures between 25 and 100 m, are now being seriously studied by a number of groups. These proposals have several major technological barriers to overcome, but it seems probable that on the same timescale (~2010) as space missions such as NGST from NASA + ESA and SPICA (neé H2L2) from Japan  (ISAS) respectively, one or more such telescopes may be becoming operational. Their performance, with atmospheric seeing correction by multi-conjugate adaptive optics (MCAO; see sidebar on linked site), will make them dominant in sensitivity for both imaging and spectroscopy for 1 < lambda < 2.5 um and, in the larger apertures being considered, competitive  in sensitivity for spectroscopy with resolution R > 10,000 in the atmospheric windows at  3-4, 8-13 and 20 um wavelength. At all these wavelengths the ELTs would have angular resolution far superior to that offered by any alternative facilities yet proposed for these wavelengths.

1.1 Introduction: how the ELT idea developed.

A decade ago there were approximately ten 4-m class ground based telescopes in operation with one 10-m telescope coming into service. Today there are perhaps 12 telescopes in the 4-m class while, remarkably, no less than 16 telescopes in the  ~8-m (``VLT") class will soon be coming into operation. In space the now-veteran 2.4-m HST continues to generate a steady stream of dramatic results, while the proposers of its successor, the NGST, plan to hurdle the 4-m category completely and, at 8 metres aperture, to begin an era of space VLTs.

On the ground, meanwhile, the advent of the VLTs, and the prospect of NGST in space, have not  stopped the progress of ambition. Even in the early '90s consideration of possible 25-m class ground-based facilities was developing (c.f., for example, Ardeberg et al. 1993, Owner-Petersen et al. ,1994 and references therein) and technical studies progressing (e.g. Ardeberg et al. 1996). By 1996 also, serious exploration of alternative technologies (Bash et al. 1996) was producing cost estimates as low as $150M for some 25-m telescope designs, while current science drivers and technical options were being closely examined. Mountain (1996) set what is now a benchmark scientific objective: to be able to secure spectra of the faintest objects in the Hubble Deep Fields.  This requires a  50-m telescope working close to its diffraction limit. The possibility of 100-m telescopes was soon explored (Gilmozzi et al. 1998); such a facility could perform the same service for a large fraction of the objects expected to be seen in deep images with the NGST.

Even more recently, the critical technique of Multi-Conjugate Adaptive Optics (MCAO) , originally proposed by Beckers (1988, 1989)  has been demonstrated to work (Ragazzoni et al. 2000a). This promises (c.f. Ellerbroek & Rigaut 2000) to liberate Adaptive Optics (AO) from the limitations of classical single-guidestar techniques, which constrain fields of view (FOVs) to a few arcsec, across which the point-spread function (PSF) varies greatly. MCAO has the potential to provide, at least in the important NIR wavelength range 1 - 2.5  um, quite uniform, near-diffraction-limited, images over FOVs of order arcminutes.  The combination of MCAO with extremely large (>20 m) telescopes promises facilities of such power as to offer a discontinuous change in astronomical capability ``comparable to that of the invention of the telescope itself" (Gilmozzi et al. 1998, section 6).