From naked eye to telescope

1. For most of history the only instrument available was the naked eye, so astronomers recorded the positions of bright stars and planets and how those positions changed.
2. That method could reach only a few thousand objects, because the pupil of the eye collects very little light and cannot store it up.
3. The optical telescope changed that from the early $1600$s, first with lenses and later with curved mirrors.
4. A telescope gathers light over a much larger area than the eye and brings it to a focus, so far fainter and more distant objects become visible.
5. Mirrors can be built much larger than lenses, so telescope apertures grew from a few centimetres to the ten metre class instruments used today.
6. From the nineteenth century photography replaced the eye at the eyepiece, and a long exposure builds up light from a faint object over hours instead of an instant.
7. A photograph is also a permanent record that other astronomers can measure and check, which a personal sketch at the eyepiece never was.

Observing beyond visible light

1. Visible light is a narrow band of the electromagnetic spectrum, so an optical telescope reveals only a small part of what objects in space emit.
2. From the $1930$s radio telescopes began detecting radio waves from the Milky Way, which opened up objects that give out almost no visible light at all.
3. Detectors were then developed for infrared, ultraviolet, X-rays and gamma rays, so the whole spectrum could be used.
4. Each band reveals different physics, since cool dust clouds show up in the infrared while extremely hot gas and violent events show up in X-rays.
5. The cosmic microwave background could only be found once microwave receivers existed, which is a direct example of new instruments producing new evidence.
6. Modern telescopes use digital detectors rather than film, and computers combine, sharpen and store the images automatically.
7. Signals from several telescopes spread across the world can also be combined, giving detail that no single dish could produce on its own.

Why telescopes are put in space

1. The atmosphere absorbs most of the electromagnetic spectrum before it reaches the ground, so those wavelengths simply never arrive at a ground based telescope.
2. Gamma rays, X-rays and most ultraviolet are blocked completely, and much of the infrared is absorbed by water vapour in the air.
3. Visible light and radio waves pass through reasonably well, which is why those two types of telescope still work from the ground.
4. Moving air also bends light continuously, which blurs the image and makes stars appear to twinkle, so fine detail is lost even on a clear night.
5. Cloud, rain and light pollution from towns and cities cut observing time further and swamp faint objects.
6. A telescope placed in orbit sits above the whole atmosphere, so it receives every wavelength, produces far sharper images and can observe continuously.
7. The price is that a space telescope costs far more to build and launch, and repairing one is difficult or impossible once it is in orbit.
8. Ground based telescopes are therefore still built for visible and radio work, where the atmosphere causes least trouble and a much larger instrument can be afforded.