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What you will learn in Space Telescopes

1. Why put a telescope in space
Two separate reasons, and only one of them is about sharpness. A ten-metre mirror on a mountain out-resolves Hubble on paper by a factor of four and loses to it by a factor of nineteen in practice.
2. Hubble
A 2.4-metre mirror ground to within two micrometres of the wrong shape, because the instrument that certified it had been assembled wrongly and the two tests that disagreed were set aside. Corrected in orbit in December 1993.
3. JWST and L2
A mirror 2.7 times Hubble's across that gathers 7.34 times the light and, at the wavelengths each was built for, produces the coarser picture. Then the other half of the design: why it is a million and a half kilometres away, and why it cannot sit still there.
4. How the pictures are made
A telescope camera records brightness, not colour. Every colour in a Hubble or JWST picture was assigned by a person choosing which grey picture to show as red, green and blue — and for JWST, none of the light in the picture is light an eye could see.
5. What comes next
Eight telescopes in one table, and the two axes it does not have. How to read a launch announcement without inheriting somebody's slipped schedule, and why the newest telescope of all is missing from the comparison.

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1. Why put a telescope in space
Two separate reasons, and only one of them is about sharpness. A ten-metre mirror on a mountain out-resolves Hubble on paper by a factor of four and loses to it by a factor of nineteen in practice.
2. Hubble
A 2.4-metre mirror ground to within two micrometres of the wrong shape, because the instrument that certified it had been assembled wrongly and the two tests that disagreed were set aside. Corrected in orbit in December 1993.
3. JWST and L2
A mirror 2.7 times Hubble's across that gathers 7.34 times the light and, at the wavelengths each was built for, produces the coarser picture. Then the other half of the design: why it is a million and a half kilometres away, and why it cannot sit still there.
4. How the pictures are made
A telescope camera records brightness, not colour. Every colour in a Hubble or JWST picture was assigned by a person choosing which grey picture to show as red, green and blue — and for JWST, none of the light in the picture is light an eye could see.
5. What comes next
Eight telescopes in one table, and the two axes it does not have. How to read a launch announcement without inheriting somebody's slipped schedule, and why the newest telescope of all is missing from the comparison.