Telescopes
Telescopes collect and focus light, letting astronomers see fainter and finer details than the eye ever could, across the entire spectrum.
Light-gathering power
A telescope's most important property is the area of its main lens or mirror. Light-gathering power is proportional to area, so it rises with the square of diameter: an 8-metre telescope collects 4 times as much light as a 4-metre one. Larger telescopes see fainter objects.
Resolution
Resolution is the ability to see fine detail. Larger apertures give better resolution, and shorter wavelengths can be resolved more finely. But on the ground, turbulence in Earth's atmosphere blurs images (the twinkling of stars), usually limiting resolution regardless of telescope size.
Refractors and reflectors
Refracting telescopes use lenses; reflecting telescopes use curved mirrors, an idea pioneered by Isaac Newton. Nearly all large research telescopes are reflectors, because large mirrors can be supported from behind, do not suffer from colour fringing, and can be made thinner and lighter than giant lenses.
Adaptive optics and big telescopes
Adaptive optics uses deformable mirrors that change shape hundreds of times a second to cancel atmospheric blurring, often guided by laser 'stars'. The largest optical telescopes have mirrors about 10 metres across; the Extremely Large Telescope under construction in Chile will have a 39-metre mirror.
Detectors
Modern telescopes record light with electronic detectors called CCDs (charge-coupled devices) and similar sensors, which capture far more light than photographic film or the eye. Long exposures accumulate faint light, and spectrographs and cameras can be combined with the same telescope.
Radio telescopes and interferometry
Radio telescopes use large dishes to collect radio waves, revealing cold gas, pulsars, and the cosmic microwave background. Linking telescopes across great distances (interferometry) gives the resolution of a telescope as large as their separation. The Event Horizon Telescope used this technique to make the first image of a black hole's shadow in 2019.
Space telescopes
Telescopes in space avoid atmospheric blurring and absorption. The Hubble Space Telescope (launched 1990) transformed astronomy with sharp visible and ultraviolet images. The James Webb Space Telescope (launched December 2021), with a 6.5-metre mirror, observes in infrared, seeing the earliest galaxies and the atmospheres of exoplanets. X-ray telescopes such as Chandra study hot, violent objects.
Worked example
Compare a backyard telescope with a 0.2-metre mirror and a 10-metre research telescope. The ratio of diameters is 50, so the research telescope collects 50 squared = 2,500 times as much light. A star 2,500 times fainter would appear equally bright in the big telescope, allowing it to study vastly more distant objects.
Key takeaways
Light-gathering power rises with the square of diameter. Resolution improves with aperture but is limited by atmospheric turbulence. Large telescopes are reflectors. Adaptive optics corrects blurring; 10-metre telescopes exist and a 39-metre ELT is coming. CCDs record light efficiently. Radio interferometry gives huge resolution; the Event Horizon Telescope imaged a black hole in 2019. Hubble (1990) and JWST (2021) observe from space.
Flashcard
Proportional to area
Check your understanding
How much more light does a 6-metre telescope gather than a 2-metre telescope?