Optical telescope

The Giant Magellan Telescope design combines seven 8.4-metre primary mirrors, including off-axis segments. It was a planned extremely large telescope in the 2018 examination context.

The Thirty Meter Telescope design uses a thirty-metre primary with 492 hexagonal segments. It was among the planned extremely large telescopes in the 2018 examination context.

The two Keck telescopes each use a ten-metre primary made from 36 hexagonal segments.

The Large Binocular Telescope has two 8.4-metre primary mirrors with lightweight honeycomb structures.

A segmented mirror assembles smaller mirrors into a large aperture. Sensors and actuators maintain position and figure; coherent imaging additionally requires mirror segment phasing.

Phasing adjusts relative piston and tilt so adjacent segments form a continuous optical wavefront. Near normal reflection a surface piston changes optical path length by twice that displacement.

A lightweight mirror uses a thin optical face backed by ribs or a honeycomb structure. The deep backing provides rigidity with less mass than a solid disk and facilitates thermal equilibration.

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.

A Schmidt camera combines a spherical primary mirror with a thin aspheric corrector at the mirror’s centre of curvature. The corrector compensates spherical aberration; the symmetric stop location gives a wide useful field. The native focal surface is curved and lies inside the instrument.

A reflecting telescope forms an image using curved mirrors. Its primary and secondary shapes, spacing and aperture stop determine its optical aberrations, access to the focus, and obstruction.

A Gregorian telescope has a concave parabolic primary and a concave ellipsoidal secondary beyond the primary focus. Light crosses the intermediate focus, reaches the secondary, and returns through the primary to the final focus. The ellipsoid’s two foci are the intermediate and final focus.

A Cassegrain arrangement places a secondary before the primary focus and returns light through a hole in the primary to a focus behind it. In the classical design the primary is parabolic and the secondary convex hyperbolic. The compact arrangement has a long effective focal length; its classical implementation retains off-axis coma.

A Ritchey–Chrétien telescope has hyperbolic primary and secondary mirrors chosen to cancel third-order spherical aberration and coma. It retains astigmatism and field curvature; wide-field instruments may require additional correction.

A classical Cassegrain reflector uses a concave parabolic primary and a convex hyperbolic secondary before the prime focus. The secondary returns the converging beam through a hole in the primary. It corrects on-axis spherical aberration but retains off-axis coma.

A Newtonian telescope uses a concave parabolic primary and a flat diagonal secondary to send the converging beam to a side focus. The primary avoids on-axis spherical aberration, but has off-axis coma.

The angular diffraction-limited resolution of a telescope scales as $\lambda /D$, with wavelength $\lambda$ and illuminated diameter $D$. Larger diameter resolves smaller angular structure, while longer wavelength makes atmospheric phase correction easier.