THE charge-coupled device (CCD) has become the detector of choice in optical astronomy, CCDs provide a very linear response to detected photons, are very efficient at some wavelengths, and can now provide coverage of a relatively wide field of view(1-3). But they become quite inefficient with decreasing wavelength, and they lack intrinsic wavelength and time resolution, The only way to select specific wavelengths is to place filters in front of the detector, which makes the total system less efficient, Time resolution can be achieved only,vith short exposures, which are possible only with very bright sources. Here we report a superconducting device that can overcome these limitations, and which has performance characteristics far superior to existing photon counting systems(4-7). Our superconducting tunnel junction can detect individual photons at rates up to 2.5 kHz in the wavelength range 200-500 nm, with an intrinsic spectral resolution of 45 mn and a quantum efficiency estimated to be about 50 per cent, The theoretical resolution of the present device is similar to 20 nm, but use of superconductors with lower transition temperature could improve that to 8 nm.