Adaptive Optics - Engineering Seminar Report

Adaptive optics
                        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.
                                           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.
                        Two images of a small region in the vicinity of the middle star in Orion’s sword- a cluster of young stars were taken. The images show a close grouping of four stars. In the conventional blurred image, its not possible to make out more than two stars. With the adaptive optics on the other hand, sharpness improves by a factor of 13, making it clear that the fainter star is ,in fact a binary – two stars close together –and a fourth fainter member of the group appears that was previously undetected.

                            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.

                                  Astronomers are mainly interested in the near and mid infrared region. A modern telescope consists of a large concave primary mirror, designed to capture a lot of light, and a smaller, secondary mirror that focuses the light onto a detector. In the infrared, the hardware of adaptive optics consists of an existing telescope, complete with its primary and secondary mirrors, and adds a separate box of optics, including the deformable mirror, to perform the atmospheric compensation.
                                   This approach has two main disadvantages. One is that each additional optical surface beam train absorbs some of the light from the target object in the sky, making the object appear fainter. Also it emits light by virtue of its warmth, introducing thermal noise, further degrading the astronomers, ability to detect faint objects.
                                    In order to avoid this, at the MMT, a separate secondary mirror has been built which does double duty; it acts as a normal secondary by focusing star light onto the high resolution imaging system, but it is also deformable, top act as the adaptive optical wavefront corrector.                          
                           Thus starlight coming from the telescope is already fully corrected and focuses down to a high resolution image, with greater intensity and thermal background an order of magnitude lower than what a telescope equipped with conventional adaptive optics could deliver.
                        But the scientific advantages of wave front correction at the telescope’s secondary faces enormous technical challenges, the most important of them being how to make a piece of glass whose surface could be precisely controlled and shaped to within a few nanometers a thousand times a second.                      
                        In order to overcome this, expertise of astronomers was called for. To make the adaptive secondary mirror, two pieces of glass with a very low coefficient of thermal expansion were first ground with matching spherical shapes. They were then bonded together with a 100 micrometre thick later of pitch, a liquid that is very viscous at room temperature.                                      
                        This arrangement holds the two pieces of glass like a single rigid   body as the convex surface is ground down to a membrane just 2 mm thick. The desired optical surface, a hyperboloid was then polished into the membrane with the same technique used for the large primary mirrors. To release the membrane, the whole assembly was baked to 120 degree celsius, melting the pitch and allowing the membrane to slide off the front convex surface of the membrane, coated with aluminium, becomes the deformable mirror.
                                     The second difficulty, controlling the shape of the membrane at high speed and with extremely precision, was later solved. The problem is that the membrane is very floppy, so that in trying to push it around to change its shape rapidly, it rings in hundreds of resonant modes. Unchecked, these resonances would make it possible to control the rapid changes in the shape of the mirror. but by placing the membrane just 40 micrometre away from a second, rigid piece of called the shape reference plate, it was discovered  that the thin layer of air between them become so viscous that all the resonances are damped out.
                        In the fully assembled mirror, the membrane’s shape is controlled by 336 voice-coil actuators, like miniature loud speakers. They couple to 336 rare-earth magnets glued to the back of the membrane. The separation between the copper coils and the magnets is 0.2 mm. A current through each coil generates a variable magnetic field, which exerts a force on the corresponding permanent magnet and moves the glass membrane.                      
                        Unique to this deformable mirror are capacitive position sensors that measure the mirror’s local position. The capacitors are chromium rings deposited on the front surface of the reference plate around each of the 336 actuators. The capacitance between each chromium ring and an aluminiium coating on the back of the deformable mirror across the 40 micrometre air gap is about 65 pF. A square wave voltage applied across the capacitors allows them to be read at 40 kHz, giving a measure of the local position of the membrane with respect to the rigid reference plate, accurate to 3 nm.                             
                        In the normal orientation when installed in the telescope, the flexible membrane is at the bottom. Above that is rigid reference plate, 50 nm thick, pierced by 336 holes through which poke the actuators. The coils of the actuators are mounted on the ends of 10 cm long aluminium fingers that conduct heat to an aluminium cold plate, two machined pieces glued and bolted together.
                        Cooling fluid circulates through grooves milled into the lower plates. The fluid is a mixture of distilled water and methanol. This solution won’t freeze even at cold temperatures. Above the cold plate are three electronic units containing 168 DSPs. Each DSP is responsible for controlling two actuators, reading the capacitive sensors and updating the drive currents in the coils to keep the mirror in right shape. This makes it resistive to vibrations, wind buffeting and changes in the direction of gravity. It has been shown that the MMT adaptive optics system holds its shape to an astonishing 10nm against winds at speeds of about 50 kmph.
                        Thus with the difficulties in building an adaptive secondary mirror overcome, we will be able to see in detail a mechanism by which stars of varying masses are distributed through the galaxy.
                        Canceling distortion in the MMT’s adaptive optics system, light from the primary mirror, distorted by  the atmosphere, reflects from the adaptive secondary mirror that is deformed to correct for the distortion. A beam splitter shunts some light from this mirror to a wavefront  sensor. The sensor’s output goes to an array of digital signal processors in a control computer, which calculates how much and where to deform the mirror to compensate for the atmospheric distortion. The corrected light passes through a lens that focuses it into a high-resolution image.

                        Perhaps the most exciting scientific program to benefit from the new approach to adaptive optics will be to look at Jupiter like planets orbiting other stars. Roughly 100 such stars have been found out through observations of the effects on the motion of their parent stars, but none has ever be seen by direct imaging. It happens because they are extraordinarily faint and to compound the problem, they are right next to something that is enormously brighter.
                        We can learn about the environments in which the planets form. Measurements of the planet’s brightness at different wavelengths will tell us about the planet’s temperature and chemical composition and whether the system has conditions to support life. Observations in the thermal infrared region of the spectrum will be particularly valuable because many simple organic molecules like methane emit strongly there. We can also exercise many of the observational techniques and new technologies required to eventually find and study earth-like planets.
                        The big challenge here is to distinguish a planet’s light from that of  its parent star. In the visible range, where planets shine by reflecting starlight, contrast ratios between a planet and its star can be extremely large.
                            Younger giant planets, less than a billion years old or so, still retain much of the heat created by their evolution out of the primeval matter from which their solar systems were formed, and radiate strongly in the thermal infrared. But most planetary systems, like our own are much older. They will have cooled and will no longer glow in the thermal infrared as they once did. 
                        A further complication occurs when trying to capture an image of the stellar system at the telescope. Regions of the image close to the star, where its planets may be found, are swamped by a halo of starlight scattered by earth’s atmosphere. The halo adds photon noise orders of magnitude greater than the tiny
planetary signal. To hope of finding the planet, we must rely on adaptive optics to suppress the halo as much as possible. The very low thermal background radiation coming from the MMT adaptive optics system provides a crucial advantage by reducing the photon noise against which the planet must be seen.                           
                        The stellar halo can also be suppressed still further through destructive interference, using a technique called nulling interferometry. In this procedure, the two images from two telescopes are overlapped exactly and in such a way that at the location of the star, the crests in the light waves from one telescope fall on the troughs of the waves from the other, canceling each other out. Thus a dark image is formed where, before, the bright stellar image was found.
                           The principle of conservation of energy requires that the starlight not be destroyed, and indeed it appears at a second output of the nulling interferometer. It is removed, though, in the crucial region closest to the star we would expect to look for planets. Planetary images will therefore be seen with greatly improved contrast.

                        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.

1. Science does not so far have any means of avoiding atmospheric turbulence. So it is the only method used to study faint distant objects.
2. It greatly improves resolution of images obtained from ground based telescopes.
3. Telescopes fitted with adaptive optics provide an image even better than that provided by space telescopes.
4. The aperture size of ground based telescopes can be about four times that of space telescopes.


                        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. 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.

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