Reference

The aperture ladder

Every telescope on this site on one scale — from a beginner’s refractor to a 39-metre mirror still under construction in Chile. The scale is logarithmic, because on a straight line everything you could actually own disappears into the first half-percent.

The second number on each bar is the one worth carrying away. Light grasp scales as the square of the diameter, so a telescope twice as wide collects four times as much. It is why the step from a 60 mm refractor to an 8-inch Dobsonian changes what you can see far more than the numbers 60 and 203 suggest.

  1. Sharpstar 61EDPH III Triplet APO61 mm · 0.1× the light
  2. Celestron Inspire 100AZ100 mm · 0.2× the light
  3. Celestron NexStar 6SE150 mm · 0.5× the light
  4. Apertura AD8 Dobsonian203 mm · 1.0× the light
  5. Apertura AD8 DobsonianYours203 mm
  8. Hubble Space Telescope2.40 m · 140× the light
  9. Nancy Grace Roman Space Telescope2.40 m · 140× the light
  10. Hale Telescope5.10 m · 631× the light
  11. James Webb Space Telescope6.50 m · 1,000× the light
  12. Very Large Telescope8.20 m · 1,600× the light
  13. Vera C. Rubin Observatory8.36 m · 1,100× the light

    Collects as a 6.67 m mirror would — the primary is a ring.

  14. W. M. Keck Observatory10 m · 2,400× the light
  15. Gran Telescopio Canarias10.40 m · 2,600× the light
  16. Extremely Large Telescope39 m · 36,900× the light
Off the scale

Numbers that don’t fit this line

Some of the instruments on this site cannot be placed above, and that is the most interesting thing about them. Putting them on the ladder would produce a number that is arithmetically fine and physically meaningless.

Chandra X-ray ObservatoryNo filled aperture

Chandra focuses X-rays by letting them graze off nested mirrors at a shallow angle, rather than reflecting them off a filled circular surface. It has no aperture diameter to place on this line, and its collecting area changes with the energy of the light — 800 square centimetres at soft energies, 100 at hard.

Arecibo Observatory305 m dish, radio

Arecibo collected radio waves, where wavelengths are centimetres rather than fractions of a micrometre. Its 305-metre dish is a real aperture, but placing it beside optical mirrors compares two different relationships between size and detail.

ALMA has no aperture at all. Its resolution comes from the distance between 66 antennas — up to 16 kilometres — while its light grasp is their summed area. Everywhere else on this ladder those two things travel together in one figure; here they come apart.

Read more about the telescopes that left Earth or the largest eyes on Earth.