D15 Adaptive Optics

Topic

Adaptive optics measures and corrects, in real time, the distortions imposed by atmospheric turbulence on the wavefront of light reaching the telescope: a sensor detects the distortion, a deformable mirror equipped with numerous actuators immediately corrects it, and a fast control loop maintains this correction, as the turbulence causing the distortion is constantly changing.

The atmosphere's characteristic coherence scale—the size of the wavefront region left reasonably intact by the turbulence—is much larger in the infrared than in the visible spectrum; it is precisely this difference that makes adaptive optics far more effective when correcting in the infrared than when attempting to correct in visible light.

Without any correction, a large telescope achieves no better resolution than a telescope the size of that atmospheric coherence scale, regardless of how much larger its actual aperture might be.

With strong correction, however, the telescope's resolution approaches its inherent diffraction limit—though it never reaches it perfectly—which is why the resulting sharpness is expressed as a fraction of the ideal scenario (i.e., one with no turbulence). Thus, spanning everything from the initial wavefront measurement to the final fraction of the ideal case—and taking into account the atmospheric coherence scale and the uncorrected limit it imposes—adaptive optics enables a ground-based telescope to approach the performance typically reserved for space-based telescopes by correcting atmospheric distortions in real time.

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