A big telescope mirror never gives a perfect image. Atmospheric fluctuations, and the distortion of the mirror under its own weight as the telescope tracks its stellar target, degrade its focusing. The answer is adaptive optics. An array of fast-acting piezoelectric elements on the back of the mirror can distort it in just the right way to correct the aberrations as they arise. A computer control system, looking at a specific star in the field or a laser beam traversing the optics, drives the elements so as to tweak the mirror continuously into optimum adjustment.

Daedalus is now taking this idea to extremes. A really adaptive mirror would need no optical figuring at all; any random piece of silvered glass could in principle be bent into the perfect shape for the task in hand. Sadly, glass or ceramic cannot be bent very far. But a liquid mirror could be made into any shape whatever, by imposing the right forces on it.

The obvious liquid is mercury. Imagine, says Daedalus, a large dish of mercury with an array of electrodes distributed through it, and a set of coils by which a complex magnetic field can be imposed on it. A current in a magnetic field produces a force, which will appear as hydrostatic pressure in the mercury. So instead of having a flat surface imposed purely by gravity, it will develop some subtle shape determined by the interaction of the field with the currents. Great mathematical cunning would be needed to deform the mercury into a focusing surface, and even greater cunning to superimpose whatever second-order deviations were needed to cancel the effects of atmospheric ‘twinkling’ but the thing seems feasible. Such a telescope would need no tilting or steering. Sufficiently strong currents could tilt and shape its mercury surface into just the right form to focus light from any given target onto a fixed CCD retina; and could change its form continuously so as to track that target across the sky.

But this is an ultimate design. Daedalus's pilot scheme lets the mirror gaze vertically upwards, and steers its scrutiny around the sky by means of a big plane mirror or prism above it. These, of course, can be very cheap and crude; the infinitely adaptive optics will correct for their imperfections. Only the chromatic aberration of the prism will defeat it — but even this will be useful. Every object seen by the telescope will be drawn out into a perfect line spectrum.