An outstanding feature in the progress made in astronomy and astrophysics is the increasing focus on clarity resulting from technical innovations. The beginning of the 20th century marked a strange sense of achievements. Photography of celestial objects was successful. Even some asteroids were identified in photographs. The solar corona, sunspots and prominence’s made for impressive pictures. Helium was discovered on the sun before it was found on the ground. The solar spectrum fascinated astronomers.
Stars were classified on the basis of their spectra. Astronomers delighted in popularizing their star categories. One astronomer is reported to have told a lady, “Oh, Be A Fine Girl, Kiss Me” ! The embarrassed visitor to his observatory later realized that the sky watcher had not gone romantic but was only trying to help her remember the various types of stars indicated by the first letters of his endearing words viz. O, B, A, F, G, K and M. They represented the so-called main sequences stars, ranging from the very massive, most luminous and hottest to the very small and the least luminous ones. Clarity in the imagery depends on the stability of the mirror and atmospheric turbulence.
The stability of the mirror depends on gravity, which does not spare anything. A 10 meter telescope, weighing 250 tonnes, would sag under gravity . The mirror will suffer deformation by a fraction of a millimeter, enough to distort the image. Hence a technique called active optics has been developed. The techniques activate pistons attached to the rear of the mirror to counter its deformation. The process is quite fast. As larger mirrors do not give better resolutions, segmented mirrors have been tried. The segments are realigned every week or so using computer controls.
One test of clarity is to produce images with angular separations as small as the instrument’s limit. In practice, atmospheric effects will limit the optical power of the instruments. The observation that is possible is known as diffraction-limited. It simply means that the observations is limited only by the optical power of the instrument used and not to by the atmosphere, which is known as the seeing limit. And the resolutions of a given instruments is inversely proportional to the wavelength of the light being observed. Turbulent atmosphere distorts the light going towards a ground telescope. Ground – based telescopes have lower resolutions than diffraction -limited because of the atmospheric turbulence . that is why even the largest optical telescopes (10m ) today, which have apertures 500 times that of the lens first used by Galileo, do not have the same order of improvements in angular resolution.
Adaptive Optics : A Major Innovation
Until 1990, the three largest telescopes in the world were in the 4- m range. In the next 15 years, as many as 14 telescopes were built with diameters from 8 m to 10 m . When astronomers found that ground – based telescopes have severe limitations in terms of resolutions, a revolutionary concept came to their rescue. Known as adaptive optics, it is perhaps the most significant technical advancement in ground – based telescopes in its 400 year history.
The twinkle of the starry sky may send poets to rapture but its a hurdle forastronomers, this is because it dims their focus while capturing sharp images of celestial objects. Newton described the blurring of the images seen in a telescopes as a “tremor”.
Astronomers had to put up with the “tremor” as inevitable, until the early 1990s. actually, the solutions began to emerge in military research. In the early 1970’s, the United States was concerned about Soviet satellites. The high-resolutions photograph from the ground were blurred . The US detectors urgently needed a technique to improve the clarity of the photographs.
Ground – based telescopes are severely affected by the distortion of the waveform in front of the aperture of telescopes. The distortion varies according to the size and speed of the eddy in the air. Hence research was done to see if the waterfront could be sensed at a rate faster than its duration in the sky.
The technique was suggested in 1953 by Horace Babcock. Essentially, he thought of countering the aberration suffered by the wavelength before it is processed further. But then there was simply no way of putting the idea into practice in his time.
Advances in computers, microelectronics and lasers enabled the US military to develop a technique called adaptive optics to correct the waveform distortion. The technique needs actuators that would nudge a telescope’s mirror a few millionths of a millimeter, thousands of a second to compensate for the air turbulence that distorts the image. It was soon found that an artificial reference star was needed for the technique to work well. It was eventually discovered that a laser beam from the ground would bounce off oxygen and nitrogen molecules in space and serve as a reference point.
The technique was not initially available for civilian use. But in November 1989, the Berlin Wall collapsed (as did the Soviet Union), ending the cold war. Declassification of the technique of adaptive optics became easy. And astronomers readily made use of it.
Adaptive optics has revolutionized the power of ground – based telescopes. The power of optical telescopes has increased during the last 40 years. The detector sensitivity has gone up hundredfold, and has reached the fundamental limit set by the photon devices. Since 1990, a new generation of ground – based telescopes has become operational with 6.5 m to 10 m diameter mirrors. In contrast, the Hubble Space Telescopes in orbit has an aperture of only 2.4 m. it costs about 20 times more to build and launch a Hubble Space Telescopes than a ground – based telescope like Keck which has 20 times the light – gathering area compared to Hubble.
Merely increasing the diameter of the mirror in a ground – based telescope would not give clearer images. Adaptive optics offered a way out of the blurring caused by the atmosphere. Astronomers can now reap a double advantage: the clarity of a space telescope and the width of a ground – based telescope.
Ideally, the resolution of an optical system is limited by the diffraction of light waves. a fully dilated human eye, for instance, should be able to separate objects as close as 0.3 arc minutes in visible light, but in reality, due to imperfections in the cornea and lens of the eye, the practical resolution is only about 1 arc minute . Adaptive optics provides a way of compensating for the blurring effects of atmosphere and get sharper images of faint objects, though it is difficult to reach the diffraction limit objects, though it is difficult to reach the diffraction limit attained by space- based telescopes such as the Hubble.
The Japanese telescope, Subaru, has an adaptive optics system (fitted in October 2006) to image the Trapezium region of Orion nebula. A laser guide corrects the effect of atmospheric turbulence in real time. As a result, the ‘eyesight’ of the telescope has increased by a factor of 10.
Subaru is the fourth system to be enhanced in the world for the 8-10-m telescopes. It has a unique solid state laser and a fiber optic, both developed in Japan, for the purpose.
Adaptive optics makes it possible to image a detailed structure of faint distant galaxies and stellar population, besides detailed imaging and spectroscopy of quasars and gamma ray bursts.
Once adaptive optics is in place, the technique of interferometry – long used by radio astronomers – can be extended to optical wavelengths. Basically, the waves from each of several dishes, physically located at a distance from one another, can be added coherently. Large at a telescopes are built with a view to incorporating interferometer and making them function as one large telescope. For example two or four telescopes , widely spaced , can together provide a long aperture (say 22 meters ) to acquire a solution of 10 mille arc seconds over a field of about one are second.
If very birth objects are imaged, hundreds of very short exposures, called speckle grams can be obtained and reconstructed later to get an unblurred image. The process is, however, lengthy.
Adaptive optics seeks two goals:
- Probing the highly red-shifted early Universe, where the natural sky is darkest.
- Scanning other planetary systems. It is believed that there are 500 Sun like stars at a distance of 50 light years.
The star’s electromagnetic waves are fully cancelled, retaining the planet’s image to be recorded. For this, beamsfrom different telescopes are brought together with an accuracy of a nanometer (billionth of a meter). The infrared reflected by the planets needs to be captured, rather than the starlight they reflect! In the infrared, the atmosphere is “bright” from heat and molecular emission. The ground telescope optics too needs cooling, as it emanates heat.
Adaptive optics was first tried in detecting infrared emission. At longer infrared wavelengths, the areas of atmospheric turbulence that cause the blurring become larger. Hence, at infrared wavelengths, an adaptive mirror would have to make fewer corrections per second. After the mid -1980 ‘s, infrared detectors improved. Lead sulphide detectors came into use. Later , combinations of mercury cadmium, indium antimony or doped silicon that are sensitive to different wavelengths of infrared light were used initially in spy satellites. Gradually, space astronomy also benefited.