Telescopes of the Future - Caltech Astronomy

1 Telescopes of the FutureENCYCLOPEDIA OF Astronomy AND ASTROPHYSICSC opyright Nature Publishing Group 2001 Brunel Road, Houndmills Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK1 Telescopes of the FutureNew Telescopes , new detectors and new regions of theelectromagnetic spectrum have often revealed totallyunsuspected aspects of the universe. Consider the carefulspectroscopic observations of nebulae started around1912 by Vesto Slipher, using the modest 24 in (61 cm) telescope of Lowell observatory.

2 The recession velocitiesof thousands of kilometres per second that he found werecompletely unexpected and, only in 1917 did Willem deSitter begin to understand them as galaxies in anexpanding universe. Dark matter, a still unidentified formthat outweighs all normal matter in the universe andcontrols the condensation of matter after the big bang,was discovered by Fritz Zwicky and Vera Rubin duringspectroscopic observations to weigh galaxies. The rapidmotions reflected a gravitational pull too large to beaccounted for by ordinary matter in stars and interstellarmaterial.

3 The microwave background radiation, thecooled light of the big bang itself, was completelyunforeseen by its Nobel-prize-winning discoverers, ArnoPenzias and Robert Wilson (although not by others).What common threads may be found in these and themany other unexpected discoveries in Astronomy ? Veryoften the observers were pushing new equipment to itslimits to accomplish a highly focused goal unrelated tothe actual discovery. Slipher was hoping that the diffusenebulae might turn out to be nearby planetary systems information. Zwicky and Rubin were using the largesttelescopes to see fainter objects than ever before.

4 Penziasand Wilson were testing a highly sensitive new type ofradio telescope for generally, unforeseen discoveries are madewhen the range of observations is enlarged. This happenswhenever new parts of the electromagnetic spectrum areopened for observation. New vistas are also opened whenlarger Telescopes or better detectors allow us to reachfainter objects or bigger samples or to study brighter onesin more detail or with better time resolution. A major new development in Astronomy that is likelyto drive new instruments in the coming decades is theexploration of exoplanets, the planets of other stars.

5 Theexistence of other living worlds like our own has beenthe subject of speculation for centuries. However, Telescopes have not been powerful enough to discernextrasolar planets. In the solar system, the Earth alreadyoccupies the prime location and our neighbours appeartoo hot or too cold. Now at last, with clear evidence thatgiant planets exist around other stars, we have thetechnical capability and the incentive to build radical newtelescopes to study them. With them we should be able tofind planets even as small as Earth and to search theirspectra for the biochemical signs of life.

6 The potential forunforeseen discovery is enormous. The first discoverieshave already revealed completely unpredictedphenomena: planets of Earth s mass orbiting a pulsar andplanets with the mass of Jupiter orbiting closer to theirstars than does Mercury to the Sun, a place far too hot forthem to form. Ground Telescopes come of ageDuring the 40 yr after the Palomar 200 in (5 m) telescopecame into operation, the power of optical telescopesincreased enormously, even though no significantlybigger instrument was built. The gains were made byimproving detector sensitivity 100-fold to reach thefundamental limit set by photon noise, by extendingdetector sensitivity into the infrared and by multiplexingso that dozens or hundreds of objects could be analysedat once.

7 However, as these advances have reached theirlimits, the past decade has seen the construction of a newgeneration of much larger Telescopes . Starting with thetwo 10 m Keck Telescopes , there are now around a dozenof size 10 m in operation or under construction. Withsuch an increase in collecting area, some profoundunforeseen discoveries can be expected. This will beespecially true when these Telescopes are able to removeatmospheric blurring and are linked together asinterferometers. Until now, astronomers have had tochoose between small-aperture Telescopes in space thatare free from blurring (the Hubble Space telescope orHST) or large-aperture Telescopes on the ground withblurring.

8 Adaptive optics is a new technique that willgive both advantages at once. In fact, because the naturallimit to resolution set by diffraction improves inproportion to aperture, the bigger ground Telescopes willbe several times sharper than key element in adaptive optics is a mirror whoseshape can be altered rapidly in response to the measuredatmospheric distortion. By giving the mirror equal butopposite distortion, the original image sharpness isrestored. It has been difficult to make the measurementand correction fast enough to keep up with the constantlychanging turbulence, but today s detectors and computersmake it possible.

9 Correction of bright objects has alreadybeen accomplished and in a few years we should start tosee sharp images of even the faintest objects corrected atinfrared wavelengths. When a target itself is too faint toallow fast measurement of atmospheric distortion, anartificial star created by a laser searchlight may be usedas a surrogate. Experimental laser systems are inoperation, although the combination of exquisite tuningand high power needed to excite scattering very high inthe atmosphere is proving adaptive optics is in place, another technique toincrease the scope for discovery becomes possible.

10 Thisis interferometry, long used by radio astronomers, whichrelies on combining the waves from separate measuring the strengthening and weakening ofintensity as the crests and troughs reinforce or cancel out,images with greatly increased angular resolution areobtained. Interferometry can be extended to opticalTelescopes of the FutureENCYCLOPEDIA OF Astronomy AND ASTROPHYSICSC opyright Nature Publishing Group 2001 Brunel Road, Houndmills Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK2wavelengths for very high-resolution imaging, butsensitivity is poor unless all the waves from each dishadd together coherently.