Optical and Infrared Telescopes
Optical and infrared telescopes collect and focus visible and near-to-mid infrared light, setting the light-gathering power, angular resolution, and field of view that govern what astronomers can observe.
Definition
An optical or infrared telescope is an instrument that uses lenses or mirrors to gather electromagnetic radiation in roughly the 0.3 to 30 micron band and bring it to a focus where it can be imaged, dispersed, or photometrically measured.
Scope
This area covers the optical configurations of reflecting and refracting telescopes, the manufacture and support of large primary mirrors, the special demands of infrared observation including thermal background and detector cooling, and the mechanical mounts and drives that point and track telescopes against Earth's rotation.
Sub-topics
Core questions
- What determines a telescope's light-gathering power and angular resolution?
- How are large, accurate mirrors fabricated and held in shape against gravity and thermal change?
- What makes infrared observation distinct from visible-light observation?
- How are telescopes pointed and tracked precisely across the sky?
Key theories
- Aperture, resolution, and the diffraction limit
- Collecting area scales with the square of aperture diameter while the diffraction-limited angular resolution scales inversely with diameter, so larger telescopes see both fainter and finer detail.
- Reflecting telescope optical configurations
- Designs such as the Cassegrain, Ritchey-Chretien, and Gregorian arrange primary and secondary mirrors to control aberrations like coma and astigmatism over a usable field of view.
- Active support of large mirrors
- Large modern primaries are thin or segmented and are kept in figure by computer-controlled actuators that compensate for gravitational and thermal deformation as the telescope moves.
Clinical relevance
Optical and infrared telescopes underpin nearly every branch of observational astronomy, from surveys of distant galaxies to characterization of exoplanets; advances in mirror technology and infrared instrumentation directly expand the faintness and wavelength range accessible to research.
History
Galileo's refractor opened telescopic astronomy in 1609, and Newton's reflector solved chromatic aberration. The twentieth century brought ever-larger glass mirrors, culminating in the 5-metre Hale telescope, after which segmented and thin-meniscus mirror technology and active support enabled the current generation of 8-to-10-metre telescopes and the extremely large telescopes now under construction.
Key figures
- Isaac Newton
- George Willis Ritchey
- Henri Chretien
- Roger Angel
Related topics
Seminal works
- kitchin2013
- schroeder2000
- bely2003
Frequently asked questions
- Why are nearly all large modern telescopes reflectors rather than refractors?
- Large lenses sag under their own weight, suffer chromatic aberration, and can only be supported at their edges, whereas mirrors can be supported across their back and reflect all wavelengths equally. These practical limits effectively cap refractors at about a metre, so all large telescopes use mirrors.
- Why do many infrared telescopes sit on high, dry mountains or fly above the atmosphere?
- Water vapour and the warm atmosphere absorb and emit strongly in the infrared, swamping faint signals. High dry sites, airborne platforms, and space telescopes reduce this background, and infrared instruments are also cooled so the telescope's own heat does not overwhelm the observation.