Adaptive optics (AO) is used to compensateatmospheric turbulenceOr other factors caused by wavefront distortion in the imaging process.
Institute of Optoelectronics Technology, Chinese Academy of SciencesRao Changhui's research team successfully developed the first set of ground layer adaptive optics in China,GLAO) test system, after docking with the 1m new vacuum solar telescope of Yunnan Observatory, obtained the large field of view high-resolution adaptive optics correction images of sunspots and solar grains for the first time in January 2016, marking another major breakthrough in China's solar adaptive optics technology.[1]
Chinese name
adaptive optics
Foreign name
Adaptive optics
Cause
Compensation caused by atmospheric turbulence or other factors
Principle
The most promising technology of medium wave front distortion
Adaptive optics (AO) is a technology that uses deformable mirrors to correct the distortion of light wavefront caused by atmospheric jitter, thereby improving the performance of optical systems.The concept and principle of adaptive optics were first introduced byHaier ObservatoryIt was proposed by Horace Babcock, but it was beyond the limit of the technology level at that time. Only the US military secretly developed this technology in the Star Wars program.After the end of the Cold War, in May 1991, the US military declassified the research materials of adaptive optics, and the computer and optical technology were developed enough, so the adaptive optics technology was widely used.The telescope equipped with adaptive optics system can overcome the impact of atmospheric jitter on imaging, and significantly improve the spatial resolution by about an order of magnitude, reaching or approaching its theoretical diffraction limit.The first large astronomical telescope equipped with adaptive optics system isEuropean Southern ObservatoryBuilt in Chile with a diameter of 3.6mNew technology telescope。More and more large-scale ground optics/Infrared telescopeThis system has been installed, such as the 8-meter caliber in Hawaii's Mauna Kea MountainsGemini telescope, the 3.6 meter Canadian French Hawaiian telescope, the 10 meter Keck telescope, the 8 meter Japanese Pleiades telescope, and so on.Adaptive optics has gradually become the technology widely used by major observatories, and has opened the way for the construction of the next generation of larger aperture telescopes.
sinceAstronomical telescopeSince its birth 400 years ago, it has developed from a small manual controlled optical instrument to a huge complex instrument controlled by a computer.In the meantime, there are two extremely important parameters: the aperture of the telescope (the ability to focus light) and the angular resolution (the clarity of the image).For a telescope with excellent performance used in space, the resolution is directly proportional to the reciprocal of the aperture.The plane wave front emitted from a distant planet will be converted into a perfect spherical wave front by the telescope for imaging.The angular resolution of an image is limited only by diffraction -- we can call it the diffraction limit.
In fact, the influence of the atmosphere and the quality of the telescope will distort the spherical wavefront, resulting in phase errors in the imaging process.Even at the best observation site, the angular resolution of the telescope in visible light band on the ground cannot exceed that of a telescope with a diameter of 10 to 20 cm, which is only due to atmospheric turbulence.For a telescope with an aperture of four meters, atmospheric turbulence reduces its spatial resolution by an order of magnitude (compared with the diffraction limit), and the definition of the star image center is reduced by more than 100 times.This is due to the instability of the wavefront in time and space caused by atmospheric turbulence - the main reason why humans send Hubble to space for observation - to avoid the impact of atmospheric turbulence.In addition, the image quality is also affected by industrial and technical problems and wave front distortion caused by mechanical, temperature and telescope optical effects[2]。
principle
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The purpose of adaptive optics is to repair atmospheric turbulence and other factorslight waveDistortion of wave front.Adaptive optics first detects the wavefront distortion, and then corrects the wavefront in real time through a small deformable mirror installed behind the focal plane of the telescope.The actuator is installed behind the deformable mirror.Adaptive optics is different from active optics. The latter adjusts the distortion of image quality caused by gravity deformation and other factors by changing the shape of the primary mirror. The former is used to compensate for the impact of atmospheric turbulence.The size of deformable mirror installed on the ground large optical astronomical telescope with an aperture of about 8 meters is 8 to 20 cm, and the number of actuators varies from hundreds to thousands. Each adjustment should be completed within 0.5 to 1 millisecond, otherwise the atmospheric jitter will cause changes in the wavefront distortion.
Adaptive optics needs to adjust the shape of the mirror at a very high frequency, so the deformable mirror is generally small in size and has high requirements for materials.The deformable mirror was once broken due to its inability to withstand high-frequency adjustment.In addition, the number of actuators is required to be sufficient, which will also lead to a series of problems such as increased cost and excessive calculation.Adaptive optics on astronomical telescopes are more used for infrared observation than visible observation.Adaptive optics in visible light band has been widely used in small aperture telescopes of reconnaissance satellites[3]。
application
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The most obvious application is the direct use of filter imaging.All adaptive optics systems provide this basic mode, but they are often equipped with additional scanning filters (circular variable filters) to obtain rich data (two-dimensional plane space and one-dimensional spectrum).Considering that atmospheric turbulence changes with time, it sounds very attractive to obtain abundant observation data in a short time.This can be done with the full field spectrograph (IFS).Observations of the CMOS system of the Canada France Hawaii Telescope (CFHT) in the visible band and SpainCalar Alto Observatory 3D observation in infrared band is a pioneer in this field.Similar equipment is also installed in8m telescope, especially the application of the GMOS system installed in Gemini telescope in the visible band and the installation ofVery large telescopeApplication of (VLT) SINFONI-SPIFFI system in infrared band[4]。
Adaptive optics system has great technical challenges.These include fast and low-noise sensors (in order to use a dim guide star for correction);High energy, reliable and easy to operate sodium laser;Ultra high speed processor, requiring 109 to 1010 operations per second;Deformable mirror with bandwidth of several thousand hertz and thousands of actuators;Large two-stage adaptive lens.The latter is particularly interesting in the thermal band. Any small additional mirror will increase the already large thermal background caused by the equipment.
The adaptive optics system based on the natural guide star is helping modern 8 to 10 meter telescopes to continuously obtain the imaging quality and spectral data close to the diffraction limit.The correction of visible light band has been quite ideal, but so far it still can not reach the diffraction limit.The artificial guide star adaptive optical system has been used in many observatories, and the number is increasing.However, the stable application of artificial guide stars under extremely high sky coverage is still not realized.MCAO technology is still in its infancy.
Many recent astronomical observations are based on new optical observation technologies.Especially whenVery large telescopeAfter (VLT) is put into use (interferometry brings clearer image quality), adaptive optics system becomes more important.Strong light gathering ability and minimal resolution (spatial and spectral) will bring the most important progress for future ground astronomical observation.More deeply, the giant in planning and discussionOptical telescope(For example, OWL) will rely on advanced adaptive optics technology to achieve all astronomical observations - telescopes will be integrated with adaptive optics systems at the initial stage of construction of these projects.
Types of optics
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Passive optics
Not long ago, the astronomical telescope was still a "passive" instrument.Because there is no built-in corrector to actively improve and adjust the image quality during observation, the time when manual adjustment can be made is during the day or at the beginning of the night.
Although it is generally acknowledged that the impact of atmospheric disturbances is inevitable, people have made the greatest mechanical improvements to correct the errors of the telescope itself:optical glassThe cold working and polishing technology have been improved;Solid structure and glass are used to eliminate lens deformation caused by gravity;Glass with low expansion coefficient is used to eliminate the influence of temperature;In order to eliminate the influence of local temperature, the heat dissipation of engine and electronic devices is minimized at night;At the same time, the dome used to protect the telescope from wind induced vibration is cooled during the day.For such reasonably designed and carefully used small and medium-sized telescopes, the image quality will still be affected by atmospheric turbulence.
Active optics
With the birth of the new concept in the 1980s (in order to enhance the telescope's light collecting ability, the aperture of the main mirror should be better than 4 meters), it is clear that the traditional methods of maintaining image quality and preventing the lens from deformation due to gravity described above are no longer applicable due to the price and structure weight constraints.In order to improve the image quality of large and medium-sized telescopes, active optics was born: the built-in optical correction components automatically adjust the image quality during observation, and these correction components work at relatively low frequencies.
adaptive optics
The work of the developers of adaptive optics system is daunting - the plane wave front penetrates the atmospheric turbulence layer of 20 kilometers and passes through large astronomical telescopes, resulting in a phase difference of several microns.The adaptive optics system must make new corrections in every millisecond by analyzing the limited data.Another complicated factor is the size of the field of view applicable to adaptive optics - the isocolo angle is quite small (only a few angular seconds in the visible wave band).
Considering the relatively wide wave band and extremely small sky coverage, adaptive optics uses a small deformable mirror with a diameter of 8 to 20 cm, which is placed behind the focus of the telescope. However, in recent years, the possibility of using a large deformable mirror is also growing.The number of actuators selected to cause deformation must take into account the correction, observation band, selection of reference stars (see below) and available budget.For example, an 8-meter telescope needs about 6400 actuators to make nearly perfect corrections in the visible light band (such as 0.6/265m), while in the same case, only 250 actuators are needed at the wavelength of 2/265m.
A large number of actuators means that the wavefront sensor (used to measure the condition of wavefront distortion) needs the same large number of image detectors (each image detector corresponds to a secondary lens), which means that if correction is required in the visible light band,Reference starThe brightness of should be about 25 times greater than that when corrected in the infrared band.Most modern astronomical observation systems are designed to provide star images near the infrared band (1 to 2/265m) that are close to the diffraction limit, while partially correcting the star images in the visible band.However, some satellite military systems of the United States also provide complete correction of the visible band (at least 1 meter aperture telescope).
relevant
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sensor
There are two kinds of sensors to detect the wavefront distortion: Shack Hartmann wavefront sensor, which detects the actual wavefront distortion through the reference star image generated by each attached image detector.The other is the curvature detection system, whose correction is completed through the dual piezoelectric chip adaptive lens, which is composed of two piezoelectric planes.For these two methods, the completion of wavefront detection is based on the guide star, or the observation object itself (when the observation object is bright enough, it can be regarded as a guide star itself).The wavefront distortion can be measured in the visible wave band and applied in the infrared wave band. If the reference star is very dark, it can be directly measured in the infrared wave band (1 to 2/265m).
The control system of adaptive optics is a special computer, which corrects the shape of the mirror by analyzing the data collected by the wavefront sensor.The analysis must be completed in a very short time (0.5 to 1 millisecond), otherwise the change of atmospheric conditions will cause errors in the system correction due to delay.
The corona angle has a great impact on the adaptive optics system. When the wavelength is 2/265 meters, the corona angle is about 20 ", but when the wavelength is 0.6/265 meters, the corona angle is only about 5". It is difficult to find a bright enough guide star in such a small range.The above situation is much better in the infrared band than in the visible band: first, atmospheric turbulence has less impact on the long wave, so the wavefront distortion is less, and finding a darker guide star can often meet the requirements;In addition, the isoplana angle in the infrared band is generally large, so the adaptive optical correction in the infrared band is much more ideal than that in the visible band.
However, even at the wavelength of 2.2 μ m, the sky coverage rate applicable to adaptive optics (equivalent to the probability of finding a guide star within the range of the halo angle around the target object) is only 0.5 to 1 percent.Therefore, adaptive optics is generally applicable to objects that exist near the field of view, such as planets or bright star clusters.
Many large and medium-sized telescopes use adaptive optical systems. For example, the first adaptive optical system - ADONIS is applied toEuropean Southern Observatory(ESO) 3.6m telescope;Hokupa'a adaptive optical system installed on the 8m Gemini telescope in the northern hemisphere;PUEO adaptive optical system applied to the 3.6m Canada France Hawaii Telescope (CFHT);It is the first time to realize the laser guide star (see below), which is installed in the ALFA adaptive optical system of the 3.5m telescope of the Calar Alto Observatory in Spain;Although only the natural guide star was used for adaptive optics correction, the laser guide star was soon used for the LLNL adaptive optics system of the (Lick) 3.5m Shane telescope at Rick Observatory;There is also the adaptive optics equipment (AO facility) of Keck II, which is used for the first time in the super large telescope Keck II.In addition, many telescopes are building adaptive optical systems, includingVery large telescope(VLT) NAOS and SINFONI adaptive optical systems.
Laser guide star
In order to overcome the limitation of guide star, the most effective method is to artificially create a guide star, which is also called laser guide star (LGS).Sodium atoms in the middle layer of the atmosphere or some other particles in the lower atmosphere can reflect the pulsating laser and cause narrow spots.The reflected light of the former is concentrated at a height of 90 km (nano resonance), while the latter is concentrated at about 10 to 20 km (Rayleigh diffusion).Such an artificial guide star can be infinitely close to the target star, and the wavefront sensor can correct the wavefront distortion of the beam from the target star by measuring the reflected laser.
Some laboratories with military contracts in the United States have announced that the artificial laser guide star has been successfully applied to the 60 cm telescope [Defense Advanced Research Projects Agency (DARPA), Maui Optical Station (AMOS)] and the U.S. Air Force Starfire Optical Range at the Maui Optical Station of the Advanced Research Project Office of the Ministry of Defense.They all achieved a resolution of about 0.15 arcsec and proved the possibility of laser detection.The Active Strategic Defense Organization (SDIO) and the U.S. Navy announced that the image resolution on a 1-meter telescope in San Diego has been improved by nearly 10 times.For some systems used for astronomy (non military), the United States has completed the astronomical observation of the artificial guide star for the first time, and the Chicago Adaptive Optical System (ChAOS) used for the 3.5m ARC telescope.
There are still many physical restrictions on laser guide stars.The first is the focus isohalo phenomenon, also known as the cone effect, which is quite obvious at the initial stage of development.Because the artificial guide star is generally located at a lower height, the scattered light is collected by the telescope to form a conical beam, but the path of such beam and the turbulence layer through which the star light from the remote observation object passes is not the same, which will lead to phase estimation error.The solution is to use multiple artificial guide stars around the observation object at the same time.The error can be reduced through sodium resonance technology, and the final effect is equivalent to one8m telescopeThe result obtained by using the guide star 10 "away from the observation target after correction is reasonable for the observation object with 2/265m wavelength 9.
More serious is the movement or tilt of the image.The center of the artificial star appears motionless in the sky, but the position of the observation object appears to move laterally (also known as tilt).The simplest solution is to add tilt correctors to the adaptive optics system, but this is limited to limited photon data.A more complex solution is to use two sets of adaptive optical systems, one for the observation object and the other for the artificial guide star.Photon data will be greatly increased with the application of the second adaptive optical system.
Through the second technology mentioned above, the requirement for the brightness of the natural guide star is reduced, so the probability of finding a natural guide star around the observation object increases, that is, the sky coverage rate increases (if an 8-meter telescope observes in the 1 to 2 micron band, the sky coverage rate is about 80%).Obviously, the larger the telescope aperture, the greater the sky coverage, because the increase in image resolution brought by the increase in aperture has been fully utilized.On the other hand, it implies great technical difficulties, because all components are required to be the same (deformable lens, wavefront sensor and artificial guide star, etc.).
The application of multi-color lasers is also a way to solve the star image tilt, but this is only applicable to the sodium resonance scattering at a height of 90 km.Multicolor lasers excite sodium atoms in different states and make use of the tiny differences in the refractive index of light at different wavelengths in the atmosphere to make corrections.The main disadvantage is the limited reflection caused by the ionospheric saturation.The polychromatic light guide star does not need any natural guide star, and the sky coverage reaches 100%, but the experimental situation is not very ideal.
