Photon sieves make super-cheap space telescopes: Traditional telescopes use lenses or mirrors to focus light by refraction or reflection, but the photon sieve uses diffraction instead. The sieve is an ultra-thin plastic disc perforated by millions of microscopic holes, each of which bends light at different angles to create a focal point.
Less light reaches the focal point compared with traditional lenses or mirrors, making it hard to image dim objects, and the device can only take black-and-white pictures. But the sieve is cheap, lightweight and easy to manufacture at large sizes. It can also be tightly folded and unfurled without being damaged.