Active optics
Active optics is a technology used with reflecting telescopes developed in the 1980s[1], which actively shapes a telescope's mirrors 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[2], the New Technology Telescope, the Telescopio Nazionale Galileo 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, which operates at a shorter timescale and corrects different distortions.
In astronomy
Most modern telescopes are reflectors, with the primary element being a very large mirror. 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's Hale telescope.
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 actuators 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 optics, 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 distortions that affect the image at 100–1000 Hz (the Greenwood frequency[3], 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. This used to be a separate mirror not integrated in the telescope's light path, but nowadays this can be the second[4], 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. 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 and a polarizer. Small changes of the incident wavelength generated by an acousto-optic modulator 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 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 active optics, using actively deformable grazing incidence mirrors, are also being investigated[5].
See also
- Adaptive optics – faster technology for smaller aberrations.
- Telescope
- Active surface – similar technology for radio telescopes.
- List of telescope parts and construction
References
- ^ Hardy, John W. (1977). "Active optics: A new technology for the control of light". Proceedings of the IEEE: 110. Bibcode:1978IEEEP..66..651H.
{{cite journal}}: Cite journal requires|journal=(help); Unknown parameter|month=ignored (help) - ^ Andersen, T. (1992). Ulrich, Marie-Helene (ed.). "Active Optics on the Nordic Optical Telescope". ESO Conference and Workshop Proceedings. Progress in Telescope and Instrumentation Technologies: 311–314. Bibcode:1992ESOC...42..311A.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help); Unknown parameter|month=ignored (help) - ^ Greenwood, Darryl P. (1977). "Bandwidth specification for adaptive optics systems" (PDF). Journal of the Optical Society of America. 67 (3): 390–303. Bibcode:1977JOSA...67..390G. doi:10.1364/JOSA.67.000390.
{{cite journal}}: Unknown parameter|month=ignored (help) - ^ Riccardi, Armando (2003). "Adaptive secondary mirrors for the Large Binocular Telescope" (PDF). Proceedings of the SPIE. Adaptive Optical System Technologies II. 4839. SPIE: 721–732. Bibcode:2003SPIE.4839..721R. doi:10.1117/12.458961.
{{cite journal}}:|editor1-first=missing|editor1-last=(help); Unknown parameter|coauthors=ignored (|author=suggested) (help); Unknown parameter|month=ignored (help)CS1 maint: multiple names: editors list (link) - ^ "Research Partnership Advances X-ray Active Optics" (PDF). adaptiveoptics.org. 2005. Archived from the original on March 2005. Retrieved 2 June 2011.
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External links
- An introduction to active & adaptive optics (European Southern Observatory web-site)
- Active optics on ESO's NTT.
- Active optics at the Gran Telescopio Canarias.