Bigger eyes
Telescopes
A telescope does two jobs: it collects more light than your eye, and it spreads a small patch of sky over a larger angle.
- 1608first patent asked
- 37 mmGalileo's lens
- 6.5 mWebb's mirror
- 39 mELT, coming
How the light goes
Galileo used a convex lens in front and a concave one at the eye: upright image, tiny field. Kepler swapped in a convex eyepiece: wider field, upside down. Newton replaced the front lens with a curved mirror, which brings every colour to the same focus. Every large telescope since 1900 has been a mirror.
20×
magnifies
49 mm
eyepiece
upright
image
- M: magnification: how many times bigger an angle looks
- f_o: focal length of the big lens or mirror (Galileo's best: about 980 mm)
- f_e: focal length of the eyepiece: shorter eyepiece, more power
Aperture is everything
Magnification alone makes a blur bigger. What sets the finest detail and the faintest star is the width of the lens or mirror, its aperture.
3.7″
sharpest
27.9
× eye light
one blur
pair 3.2″
- θ: the smallest angle that can be split, in radians (× 206 265 for arcseconds)
- λ: the wavelength of the light: 550 nanometres for green
- D: the aperture: double it and you see detail twice as fine
- 1.22: a number from the shape of the circular blur, the Airy disc
Light collected grows with the area of the aperture. Webb's 6.5 m mirror gathers about 860 000 times what a dark-adapted 7 mm pupil does.
Beyond visible light
Radio dishes, X-ray mirrors that only work at grazing angles, infrared telescopes kept colder than Pluto, and arrays linked across the planet all follow the same rule: wider aperture, sharper view. The Event Horizon Telescope used the whole Earth as one dish to resolve about 20 millionths of an arcsecond. See what it saw on Black Hole.
Timeline
2 Oct 1608
Hans Lipperhey, a spectacle maker in Middelburg, asks the Dutch States General for a patent on a device for seeing far. It is refused: others already make them.
1609
Galileo builds 3×, then 8×, then about 20× telescopes and turns them on the sky.
1611
Kepler's Dioptrice describes a telescope with two convex lenses: wider view, upside-down image.
1668
Newton builds a reflector with a metal mirror, beating the colour fringes of lenses.
13 Mar 1781
William Herschel discovers Uranus with a home-made 6.2-inch reflector.
1789
Herschel's 40-foot telescope, with a 1.26 m mirror, the largest in the world for 50 years.
1845
Lord Rosse's 1.83 m Leviathan in Ireland shows the spiral shape of M51.
1897
Yerkes 40-inch (1.02 m) refractor: the largest lens telescope ever used for science.
1917
Mount Wilson 100-inch Hooker telescope. Hubble uses it to show other galaxies exist (1924) and that the universe expands (1929).
1932
Karl Jansky hears radio waves from the Milky Way: the start of radio astronomy.
1948
Palomar 200-inch (5.08 m) Hale telescope.
24 Apr 1990
Hubble Space Telescope launched; its flawed mirror is fixed by astronauts in 1993.
1993
Keck I on Mauna Kea: a 10 m mirror built from 36 hexagons.
10 Apr 2019
The Event Horizon Telescope, an Earth-sized array of radio dishes, shows the shadow of the black hole in M87.
25 Dec 2021
James Webb Space Telescope launched: 6.5 m of gold-coated beryllium, cooled to about 40 K, seeing in infrared.
2025
Vera C. Rubin Observatory releases its first images, beginning a 10-year movie of the southern sky.
late 2020s
ESO's Extremely Large Telescope, 39 m across, due to see first light in Chile's Atacama.
The people behind the first ones: Galileo and the theory he tested, Copernicus.
Sources
- Albert Van Helden, The Invention of the Telescope (1977)
- Henry C. King, The History of the Telescope (1955)
- Museo Galileo, Florence: Galileo's telescopes
- NASA: Hubble Space Telescope and James Webb Space Telescope mission pages
- Event Horizon Telescope Collaboration, ApJL 875 L1 (2019)
- ESO: The Extremely Large Telescope