Adaptive optics is a technology that improves the spatial resolution of ground-based astronomical telescopes in order to observe distant objects at or near the diffraction limit. It accomplishes this by correcting for the distortion of light caused by the Earth's atmosphere by removing optical aberrations.
The essential components of an adaptive optics system include a:
- Wavefront sensor
- Deformable mirror
- High-speed computer that receives input from the wavefront sensor and communicates with the deformable mirror
The following processes are undertaken in order to observe a faint object, such as a galaxy, by means of adaptive optics:
- A relatively bright star in the galaxy’s proximity is located. This is known as a “guide star”.
- Light from both the guide star and the galaxy pass through the telescope's optics.
- The guide star's light is sent to the wavefront sensor, which detects optical aberrations and measures the distortion.
- The distortion data is sent to the computer, which calculates the shape to apply the deformable mirror.
- Light from the galaxy is reflected off the deformable mirror, which subsequently cancels out the distortions measured by the wavefront sensor.
Distortions in an adaptive optics system result from atmospheric turbulence causing changes to the local index of refraction.
Air pockets of variable temperature introduce spatial and temporal variations in the optical path length along the line of sight, thus causing wavefront perturbations.
Shack-Hartmann wavefront sensors are often employed in adaptive optics systems for astronomy. They incorporate a series of lenslets across the aperture of a CCD camera, producing an array of spots that correspond to the wavefront. A plane wavefront incident on the wavefront sensor results in regularly spaced spots. The spots become irregularly spaced and move around rapidly as atmospheric turbulence distorts the wavefront. The computer reconstructs the shape of the incident wavefront by measuring the exact position of all the spots on the CCD camera and communicates this information to the deformable mirror.
The objective of the deformable mirror is to remove the optical path differences introduced by atmospheric turbulence.
Astronomical observatories typically use deformable mirrors that are made from very large glass substrates with aperture sizes from tens to hundreds of centimetres in diameter. Piezoelectric actuators are used to adjust the shape of the substrate.
Deformable mirrors used in educational and research laboratories can be on the order of millimetres in diameter, much smaller than those in real observatories. They would normally be mounted on a test bench for simulating large observatory deformable mirrors.
Laser guide star (LGS)
Wavefront corrections are based on a nearby reference – the guide star. Infrared and visible observations typically require a guide star within about 30 arcseconds and 10 arcseconds of the astronomical target, respectively. One can use a natural guide star (NGS) or laser guide star (LGS). Where a bright NGS is not available a laser guide star (LGS) may be employed as an alternative.
Two LGS mechanisms include:
i) A green laser that is focussed at an altitude of around 30 km. Rayleigh scattering occurs in this dense region of atmosphere which reflects the signal back to the WFS.
A 589 nm laser that excites sodium atoms concentrated in a layer at around 90 km altitude. The subsequent fluorescence can penetrate the atmosphere and returns to the WFS.