Flattening the flats

The art of making true optical flats — glass discs with surfaces polished so accurately that any unevenness is small compared with the wavelength of light —was revealed in the nineteenth century. (Until then, people had merely ground the weakest possible mirrors.) The trick is to grind three pieces of material against each other: A against B, B against C, and C against A. All three then become true flats — or at least as flat the distortion of the Earth's gravity permits. Lens telescopes have an advantage over mirrors in that a small sag in the lens makes essentially no difference to its optical performance. Daedalus has been musing on these facts because of the high value of true flats to astronomy. The accurate distance of many stars could be deduced if a space telescope, equipped with flats, could be used as an optical interferometer.

So Daedalus now shows how make true flats. The best Earthbound flats would be made by skilled opticians, and would be flown on the space shuttle. In microgravity it should be easy to complete the ultrafine grinding of the flats against each other, giving three flats of amazing performance.

Ideally, two of these flats should be attached to a space telescope, one in its light path and the other many kilometres away. The art of stabilizing an object in space has been perfected over many decades, and Daedalus hopes that the distant flat can be turned and held so as to shine the light of a distant star unwaveringly upon its fellow flat at the space telescope. The result should be an amazingly accurate interferometric measurement of the distance of the distant star.

Daedalus has no interest in the distance of stars per se. He wants to measure the distance of stars in nearby galaxies, and thus determine the Hubble constant with accuracy. It is rather shocking that this important constant seems to vary from 60 to 85 — and that astronomers can choose whichever value they like. They can also measure it in km s⁻¹ Mpc⁻¹, instead of s⁻¹ as the rest of us would have to, but that's another matter.

An accurate Hubble constant would allow cosmologists to say clearly if the Universe were 'open' (doomed to expand for ever) or 'closed' (due to fall back on itself as the galaxies do not quite have mutual escape velocity). Daedalus likes the idea of a closed Universe, in which the galaxies only just slow before falling back. But he recognizes that a Hubble constant accurately related to the time of the Big Bang will be needed to make it work.