Astronomy · Rung 6 of 12

Instruments

A telescope is not for magnifying, it is for collecting light. What matters is aperture: the diameter of the mirror or lens. Magnification is the telescope's focal length divided by the eyepiece's, and past a certain point it only enlarges a blurred image; the useful limit is around twice the aperture in millimetres. What really decides the night is seeing, the turbulence of the air, and light pollution. A pair of 10×50 binoculars on a tripod shows Jupiter's moons, the clusters and Andromeda, and costs what a dinner costs.

Where this sits

Comes after
The naked-eye sky
Opens up
Observing for real

Exercises from this rung

Generated by the app, from easiest to hardest

  1. In 10×50 binoculars, what does the 50 mean?

    • 50 mm front lens diameter
    • 50 mm in length
    • 50× magnification
    • 50 metres of field

    Why The first number is magnification and the second is aperture in millimetres. A 10×50 pair is the best first buy for the sky: it shows Jupiter's four moons, the clusters, Andromeda and lunar craters. Past 10× you want a tripod, because your pulse shows more than the optics do.

    easier

  2. Which optical problem affects lens telescopes and not mirror ones?

    • Light pollution
    • Air turbulence
    • The weight of the tube
    • Chromatic aberration

    Why A lens bends each colour slightly differently, so not all of them focus at the same point and coloured halos appear around bright objects. A mirror reflects every colour alike and has no such defect: that is why Newton invented the reflecting telescope in 1668, and why every large observatory uses mirrors.

    in between

  3. Why put telescopes in orbit when it costs a fortune?

    • There is no light pollution
    • They are closer to the stars
    • They weigh less
    • The atmosphere blurs and blocks light

    Why Two reasons. Air blurs the image, and without it you reach the instrument's theoretical limit. Above all, the atmosphere only lets through visible light, some infrared and radio: ultraviolet, X-rays and gamma rays are stopped. High-energy astronomy simply does not exist from the ground. Being “closer” counts for nothing: a few hundred kilometres is nothing.

    harder