It's a gas! Images from thin air.

Carefully designed variations in the thickness of a piece of glass can act as a lens, focusing a light beam onto a small area. Likewise, changes in the density of a gas layer placed across an aperture can also focus light.

Scientists at the University of Natal in Durban, South Africa, have now used this principle to build a gas-lens telescope. (The device was proposed some 30 years ago as a way to concentrate laser beams without destroying glass lenses.) In their setup, room-temperature air passes through the center of a heated, spinning tube. This forces the gas molecules to rearrange so that cooler (denser) gas lies at the tube's center and hotter (thinner) gas lies at the edges. The resulting density pattern focuses an incoming light beam onto a lensless camera - much as the air above a hot road bends light to form a wavy mirage.

The researchers, led by M.M. Michaelis, have used their novel telescope to image craters on the moon and sunspots during maximum solar activity Though the current version of the instrument has limitations, including a small field of view, it offers several advantages over glass, they say: The ultralight lens can function well in the near-zero gravity of space, and it focuses wavelengths ranging from ultraviolet to infrared.

"It has been suggested that 'gas telescopy' will never make its mark in astronomy but we contend that gas lenses are in their infancy," the team writes in the Oct. 10 NATURE.