Active optics
Encyclopedia
Technology
Technology is the making, usage, and knowledge of tools, machines, techniques, crafts, systems or methods of organization in order to solve a problem or perform a specific function. It can also refer to the collection of such tools, machinery, and procedures. The word technology comes ;...
used with reflecting telescope
Reflecting telescope
A reflecting telescope is an optical telescope which uses a single or combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century as an alternative to the refracting telescope which, at that time, was a design that suffered from...
s developed in the 1980s, which actively shapes a telescope's mirror
Mirror
A mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection...
s to prevent deformation due to external influences such as wind, temperature, mechanical stress. Without active optics, the construction of 8 metre class telescopes is not possible, nor would telescopes with segmented mirrors be feasible.
This method is used by, among others, the Nordic Optical Telescope
Nordic Optical Telescope
The Nordic Optical Telescope is an astronomical telescope located at Roque de los Muchachos Observatory, La Palma in the Canary Islands. First light came in 1988, with regular observing beginning in 1989. It is funded by Denmark, Sweden, Iceland, Norway and Finland...
, the New Technology Telescope
New Technology Telescope
The New Technology Telescope or NTT is an Alt-Az, 3.58-metre Richey-Chretien telescope part of the European Southern Observatory and began operations in 1989. It is located in Chile at the La Silla Observatory and was an early pioneer on the use of active optics...
, the Telescopio Nazionale Galileo
Telescopio Nazionale Galileo
The Telescopio Nazionale Galileo, or TNG, is a 3.58m Italian telescope located on the island of San Miguel de La Palma , in the Canary Islands archipelago. It is one of the largest telescopes hosted by the Roque de los Muchachos Observatory, a very important observing site in the northern hemisphere...
and the Keck telescopes, as well as all of the largest telescopes built in the last decade.
Active optics is not to be confused with adaptive optics
Adaptive optics
Adaptive optics is a technology used to improve the performance of optical systems by reducing the effect of wavefront distortions. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, and in retinal imaging systems to reduce the...
, which operates at a shorter timescale and corrects different distortions.
In astronomy
Most modern telescopes are reflectors, with the primary elementPrimary mirror
A primary mirror is the principal light-gathering surface of a reflecting telescope.-Description:The primary mirror of a reflecting telescope is a spherical or parabolic shaped disks of polished reflective metal , or in later telescopes, glass or other material coated with a reflective layer...
being a very large mirror
Mirror
A mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection...
. Historically, the mirrors had to be very thick to hold its shape to the required accuracy as the telescope travelled across the sky. This limited their maximum diameter to 5 or 6 metres (200 or 230 inches), such as in the Palomar Observatory
Palomar Observatory
Palomar Observatory is a privately owned observatory located in San Diego County, California, southeast of Pasadena's Mount Wilson Observatory, in the Palomar Mountain Range. At approximately elevation, it is owned and operated by the California Institute of Technology...
's Hale telescope
Hale telescope
The Hale Telescope is a , 3.3 reflecting telescope at the Palomar Observatory in California, named after astronomer George Ellery Hale. With funding from the Rockefeller Foundation, he orchestrated the planning, design, and construction of the observatory, but did not live to see its commissioning...
.
As telescopes were getting larger, the mirrors were getting much thicker and heavier, and it was no longer possible to keep this up. New generation of telescopes built since the 1980s use very thin mirrors instead , which are too thin to keep themselves rigidly in the correct shape. Instead, an array of actuator
Actuator
An actuator is a type of motor for moving or controlling a mechanism or system. It is operated by a source of energy, usually in the form of an electric current, hydraulic fluid pressure or pneumatic pressure, and converts that energy into some kind of motion. An actuator is the mechanism by which...
s behind the mirror keeps it in an optimal shape. The telescope may also be segmented into many small mirrors, preventing most of the gravitational distortion that occurs in large, thick mirrors.
The combination of actuators, a quality-of-image detector, and a real-time computer program to move the actuators to obtain the best possible image is termed active optics.
The name active optics means that the system keeps a mirror (usually the primary) in its optimal shape against all environmental factors such as gravity (at different telescope inclinations), wind, temperature changes, telescope axis deformation, et cetera. Active optics correct all factors that may affect image quality at timescales of one second or more. The telescope is therefore actively still, in its optimal shape.
Comparison with adaptive optics
Active optics should not be confused with adaptive opticsAdaptive optics
Adaptive optics is a technology used to improve the performance of optical systems by reducing the effect of wavefront distortions. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, and in retinal imaging systems to reduce the...
, which operates on a much shorter timescale to compensate for atmospheric effects, rather than for mirror deformation. The influences that active optics compensate (temperature, gravity) are intrinsically slower (1 Hz) and have a larger amplitude in aberration. Adaptive optics on the other hand corrects for atmospheric
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
distortions that affect the image at 100–1000 Hz (the Greenwood frequency
Greenwood frequency
In adaptive optics, the Greenwood frequency is the frequency or bandwidth required for optimal correction with an adaptive optics system. It depends on the transverse windspeed and the turbulence strength in the atmosphere...
, depending on wavelength and weather conditions). These corrections need to be much faster, but also have smaller amplitude. Because of this, adaptive optics uses smaller corrective mirrors
Deformable mirror
Deformable mirror represents the most convenient tool for wavefront control and correction of optical aberrations. Deformable mirrors are used in combination with wavefront sensors and real-time control systems in adaptive optics...
. This used to be a separate mirror not integrated in the telescope's light path, but nowadays this can be the second, third or fourth mirror in a telescope.
Other applications
Complicated laser set-ups and interferometers can also be actively stabilized.A small part of the beam leaks through beam steering mirrors and a four-quadrant-diode is used to measure the position of a laser beam and another in the focal plane behind a lens is used to measure the direction. The system can be sped up or made more noise-immune by using a PID controller
PID controller
A proportional–integral–derivative controller is a generic control loop feedback mechanism widely used in industrial control systems – a PID is the most commonly used feedback controller. A PID controller calculates an "error" value as the difference between a measured process variable and a...
. For pulsed lasers the controller should be locked to the repetition rate. A continuous (non-pulsed) pilot beam can be used to allow for up to 10 kHz bandwidth of stabilization (against vibrations, air turbulence, and acoustic noise) for low repetition rate lasers.
Sometimes Fabry–Pérot interferometers have to be adjusted in length to pass a given wavelength. Therefore the reflected light is extracted by means of a Faraday rotator
Faraday rotator
A Faraday rotator is an optical device that rotates the polarization of light due to the Faraday effect, which in turn is based on a magneto-optic effect....
and a polarizer
Polarizer
A polarizer is an optical filter that passes light of a specific polarization and blocks waves of other polarizations. It can convert a beam of light of undefined or mixed polarization into a beam with well-defined polarization. The common types of polarizers are linear polarizers and circular...
. Small changes of the incident wavelength generated by an acousto-optic modulator
Acousto-optic modulator
An acousto-optic modulator , also called a Bragg cell, uses the acousto-optic effect to diffract and shift the frequency of light using sound waves . They are used in lasers for Q-switching, telecommunications for signal modulation, and in spectroscopy for frequency control. A piezoelectric...
or interference with a fraction of the incoming radiation delivers the information whether the Fabry Perot is too long or to short.
Long optical cavities
Optical cavity
An optical cavity or optical resonator is an arrangement of mirrors that forms a standing wave cavity resonator for light waves. Optical cavities are a major component of lasers, surrounding the gain medium and providing feedback of the laser light. They are also used in optical parametric...
are very sensitive to the mirror alignment. A control circuit can be used to peak power. One possibility is to perform small rotations with one end mirror. If this rotation is about the optimum position, no power oscillation occurs. Any beam pointing oscillation can be removed using the beam steering mechanism mentioned above.
X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
active optics, using actively deformable grazing incidence mirrors, are also being investigated.
See also
- Adaptive opticsAdaptive opticsAdaptive optics is a technology used to improve the performance of optical systems by reducing the effect of wavefront distortions. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, and in retinal imaging systems to reduce the...
– faster technology for smaller aberrations. - TelescopeTelescopeA telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...
- Active surfaceActive surfaceAn active surface is a surface of a radio telescope that is under active computer control of its shape.Large radio telescopes always bend during operation, due to their enormous weight and the fact that even the strongest materials are not perfectly stiff...
– similar technology for radio telescopes. - List of telescope parts and construction
External links
- An introduction to active & adaptive optics (European Southern ObservatoryEuropean Southern ObservatoryThe European Southern Observatory is an intergovernmental research organisation for astronomy, supported by fifteen countries...
web-site) - Active optics on ESO's NTTNTTNTT may refer to:*Nippon Telegraph and Telephone, a telephone company that dominates the telecommunication market in Japan*Name That Tune, a now defunct game show*New Technology Telescope, a 3.6m telescope located at La Silla Observatory, Chile...
. - Active optics at the Gran Telescopio CanariasGran Telescopio CanariasThe Gran Telescopio Canarias , also known as GranTeCan or GTC, is a reflecting telescope undertaking commissioning observations at the Roque de los Muchachos Observatory on the island of La Palma, in the Canary Islands of Spain, as of July 2009.Construction of the telescope, sited on a volcanic...
.