A big problem in infrared (IR) astronomy is that the night sky is not dark. Hydroxyl molecules in the atmosphere emit a range of bright, narrow IR lines, which is why astronomers have had to launch their telescopes into space – or at least seek out mountains that are high enough for the air above to be clear and dry.

Now, J Bland-Hawthorn of the University of Sydney in Australia and colleagues have found another way.

A sophisticated filter based on Bragg gratings made from fibres selectively reflects back the unwanted lines and acts as a notch filter to give a clear view in the rest of the infrared spectrum. This new technology could open up the possibility for vastly improved infrared astronomy using Earth-based telescopes.