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Active optics uses controlled support forces and alignment to correct slow changes in telescope figure, particularly gravity and thermal deformation. It permits thin large primary mirrors. Adaptive optics instead corrects rapid atmospheric turbulence.

The isoplanatic angle is the characteristic angular separation over which a single-direction atmospheric correction remains useful. It depends on the altitude distribution of atmospheric turbulence as well as on wavelength.

Three complementary approaches replace the simple thick solid disk:

“Operational” and “planned” here refer to the 2018 examination date. The next generation included the 39-metre Extremely Large Telescope with 798 segments, the thirty-metre Thirty Meter Telescope with 492 segments, and the Giant Magellan Telescope using seven 8.4-metre primary mirrors. These designs extend segmentation or lightweight casting with active control, rather than simply making a thicker slab. The specifications are documented by the 2017 ELT announcement, TMT optics design and GMT primary-mirror description. Adaptive optics is also needed to exploit the large apertures against atmospheric turbulence; it corrects faster disturbances than active optics.

Both methods reduce structured stellar residuals, rather than removing all noise. In angular differential imaging, changing atmospheric turbulence, imperfect adaptive optics, flexure and thermal drift change the point spread function between frames. The reference then fails to represent the instantaneous stellar field. Small sky rotation makes close companions contaminate their own reference, causing differential-imaging self-subtraction. Extended disks are especially susceptible; subtraction can alter shape as well as total flux. More images help independent photon shot noise, but do not necessarily average away correlated residuals.

Simultaneous spectral differential imaging avoids the time delay, but different channels have non-common-path wavefront errors. Chromatic optical aberrations, wavelength-dependent amplitude errors and out-of-pupil propagation prevent a perfect radial rescaling of speckle patterns. Filter throughput, detector calibration, image registration and atmospheric dispersion also leave subtraction residuals. Nearby bands align the stellar field better but give less positional diversity; wider separation gives more displacement but larger chromatic mismatch. A smooth-spectrum companion, or one too close for appreciable rescaled displacement, can undergo severe differential-imaging self-subtraction.

Artificial-companion injection through the complete processing pipeline and forward modelling can calibrate lost throughput and photometric or astrometric biases. They do not guarantee that every correlated residual is a real source. Independent epochs or spectral evidence remain valuable.

Atmospheric extinction changes the amplitude of the incoming light through absorption and scattering. Its wavelength dependence alters the measured optical spectrum and colors; clouds and aerosols introduce additional time dependence. Scattered moonlight and atmospheric emission increase sky brightness.

Atmospheric refraction changes the apparent position of the source. Its wavelength dependence, atmospheric dispersion, spreads a broadband image toward the zenith. The mean refractive index gradient therefore affects direction even in the absence of small-scale turbulence.

Atmospheric turbulence produces rapidly varying optical path lengths, distorting the phase and curvature of a nominally plane wavefront. Different pupil regions acquire different phase delays, producing astronomical seeing, image motion, and short-exposure speckle patterns. Different lines of sight sample different fluctuations, giving anisoplanatism.

Propagation through the fluctuating medium also changes the intensity through atmospheric scintillation, the familiar twinkling of stars. A phase-only adaptive optics correction can reduce atmospheric phase errors but does not remove absorption, all scintillation, or the atmospheric emission background.

Atmospheric scintillation is the fluctuation of received stellar intensity produced by propagation through atmospheric turbulence. A phase-only deformable mirror does not generally remove these amplitude fluctuations.