The decade 2010-2020 will see the maturity of the current generation of telescopes (VLT, Keck, Gemini, Subaru, LBT, GTC, HET, SALT, Magellan, etc.) equipped with a second generation of instruments often performing at the diffraction limit through advanced Adaptive Optics (AO). Interferometry will have come out of its infancy to operate in the faint object regime (K~20) and to produce astrometric results in the micro-arcsecond range (1 ^as resolution resolves 2 mm details at the distance of the moon, although astrometric accuracy is not the same as imaging capability, which remains about 2 m at the moon for the VLT Interferometer, VLTI). ALMA will provide mm and sub-mm astronomers with a facility "equivalent" to

J.E. Beletic et al.(eds.), Scientific Detectors for Astronomy 2005, 3-12. © 2006 Springer. Printed in the Netherlands.

optical ones (both in terms of service offered to the community and of resolution and sensitivity). Space missions like the James Webb Space Telescope JWST, the X-ray satellite XEUS, the Terrestrial Planet Finder TPF/Darwin precursor missions and others will explore the heavens from above the atmosphere, exploiting the freedom from turbulence, sky absorption and gravity. And a new generation of ground-based optical/ near infrared 30 to 100-m telescopes may open a completely new window on the Universe and produce unprecedented results (with resolution of milli-arcseconds and sensitivity hundreds or even thousands of times beyond what is available today).

The new telescopes, collectively known as ELTs (Extremely Large Telescopes) all try to break one or both of the traditional laws of the art of telescope making: the cost law (^ D2'6) and the growth law (the next generation telescope is twice as large as the previous one). The rationale for breaking the growth law comes from the science cases. The motivation for beating the cost law is key to obtaining funding.

Figure 1. Brief history of the telescope. Stars: refractors, asterisks: speculum reflectors, circles: glass reflectors. A few telescopes are named.

Figure 1 shows the history of the telescope diameter, with a few future telescopes (the Thirty Meter Telescope TMT and the 100-m OWL) added for reference. There are two aspects that are immediately evident: (1) "local" scatter notwithstanding, the trend of diameter increase has remained substantially constant since Galileo (doubling every 50 years or so) and (2) the quantum jump between a 10-m and a 100-m telescope is similar to that between the night-adapted naked eye and the first telescope, which certainly bodes well for the potential for new discoveries. During the 20th century there has been some acceleration, with the doubling happening every 35 years: see e.g. the "California progression" with the Hooker [2.5 m, 1917], Hale [5 m, 1948], and Keck [10 m, 1992] telescopes.

Figure 2. Recent history of improvement in sensitivity of telescopes expressed in "equivalent diameter of a perfect telescope" = V (nD2), with n the telescope overall efficiency (the dashed line is an aid to the eye, not a fit).

One point that perhaps is not immediately evident, though, is that in the last 50 years there has been a larger increase in telescope sensitivity due to improvements in detectors than to increases in diameter (see Fig. 2). Now that detectors efficiencies approach 100%, large improvements can be obtained only through large increases in telescope diameter. For example, at the times of photographic plates, with efficiency of a few percent, even the 5-meter Hale telescope was only equivalent to a 1-meter "perfect" telescope (i.e. one with 100% efficiency).

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