A glittering prize

GETTING a telescope into orbit is challenging and costly. Astronomers want instruments with the biggest mirrors possible. Big mirrors gather more light, so can see fainter objects. They also have higher magnification, so can resolve finer details. There is a limit, though, to the size and weight of mirror that can be lifted into space on a rocket. Size can be dealt with by clever design--for example, making a mirror out of hexagonal petals that fold up on top of one another for launch. This is t...