Adaptive Optics in Ground Based Telescopes
Published on Nov 23, 2015
Adaptive optics is a new technology which is being used now a days in ground based telescopes to remove atmospheric tremor and thus provide a clearer and brighter view of stars seen through ground based telescopes.
Without using this system, the images obtained through telescopes on earth are seen to be blurred, which is caused by the turbulent mixing of air at different temperatures.
Adaptive optics in effect removes this atmospheric tremor. It brings together the latest in computers, material science, electronic detectors, and digital control in a system that warps and bends a mirror in a telescope to counteract, in real time the atmospheric distortion.
The advance promises to let ground based telescopes reach their fundamental limits of resolution and sensitivity, out performing space based telescopes and ushering in a new era in optical astronomy.
Finally, with this technology, it will be possible to see gas-giant type planets in nearby solar systems in our Milky Way galaxy. Although about 100 such planets have been discovered in recent years, all were detected through indirect means, such as the gravitational effects on their parent stars, and none has actually been detected directly.
WHAT IS ADAPTIVE OPTICS ?
Adaptive optics refers to optical systems which adapt to compensate for optical effects introduced by the medium between the object and its image. In theory a telescope's resolving power is directly proportional to the diameter of its primary light gathering lens or mirror. But in practice , images from large telescopes are blurred to a resolution no better than would be seen through a 20 cm aperture with no atmospheric blurring. At scientifically important infrared wavelengths, atmospheric turbulence degrades resolution by at least a factor of 10.
Space telescopes avoid problems with the atmosphere, but they are enormously expensive and the limit on aperture size of telescopes is quite restrictive. The Hubble Space telescope, the world's largest telescope in orbit , has an aperture of only 2.4 metres, while terrestrial telescopes can have a diameter four times that size.
In order to avoid atmospheric aberration, one can turn to larger telescopes on the ground, which have been equipped with ADAPTIVE OPTICS system. With this setup, the image quality that can be recovered is close to that the telescope would deliver if it were in space. Images obtained from the adaptive optics system on the 6.5 m diameter telescope, called the MMT telescope illustrate the impact.
As light from a distant star reaches the earth, it is made up of plane waves that , in the last microseconds of their journey to the telescope, become badly distorted by atmospheric turbulence. An adaptive optics system reflattens the wave fronts by reflecting the light of a deformable mirror whose shape is changed in real time to introduce an equal but opposite distortion.
The information on how to distort the mirror comes from a wave front sensor, an instrument that measures optical aberration imposed by the atmosphere on light from a star. A fast computer converts the signals coming from the wave front sensor into drive signals for the deformable mirror. The whole cycle operates at a never ending cycle of measurement and correction, at typical speeds of 1000 updates per second.
After the light reflects of the deformable mirror, a beam splitter sends part of the light to a camera that will capture the high resolution image produced by the adaptive optics.
BASIC FUNCTIONAL DIAGRAM
As we continue to develop this program, further improvements in our instrumentation will allow us to see fainter objects. The next major step will be the completion of the Large Binocular telescope, combining two 8.4 m primary mirrors on a single mount , each equipped with its own secondary mirror. The corrected light from the two halves of the telescope will then be brought together in the centre in a new nulling interferometer which is being built.
Predictions of the instrument’s sensitivity show that we can expect direct detection of several planets already known to exist like Ursae Majoris, and v Andromedae. Many others are likely to be discovered for the first time because of the instrument’s ability to explore a much greater region of space around each star than is possible with today’s indirect detection methods
• Only infrared is currently practical.
• Small isoplanatic patch (area that needs research)
Because the isoplanatic patch for the atmosphere is so small, only a tiny fraction of the sky will be near suitably bright stars that can serve as reference. But this has now been overcome now by using lasers to excite sodium atoms in the atmosphere, producing an artificial star that can be placed near any target of interest.
There are many substantial technological challenges in AO. Among them are the development of fast, very low-noise detectors in order to be able to correct with fainter reference stars; high-power reliable & easy to operate sodium lasers; very fast processors exceeding 109 to 1010 operations per second; deformable mirrors with bandwidths of several kilohertz and with thousands of actuators, and large secondary adaptive mirrors. The latter are especially interesting at thermal wavelengths, where any additional mirror raises the already huge thermal background seen by the instruments.
There are AO systems in the Infrared routinely achieving near diffraction-limited images and spectroscopic data cubes on large telescopes up to the present generation of 8-10 m diameter. Significant corrections have been obtained in the visible in exceptionally good seeing conditions
Many recent astronomical discoveries can be directly attributed to new optical observation capabilities. With the new generation of Very Large Telescopes entering into operation, the role of AO systems is extremely important.
With this capability, their huge light-gathering along with the ability to resolve small details, both spatially and spectrally, has the potential to bring major advances in ground-based astronomy in the new decade. Further down the line, the giant optical telescopes of the future will rely on advanced AO systems for basically all their observations; they will have to be incorporated right at the start of the projects.
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