Abstract: A method of determining the gain of an electron multiplier, especially in an Electron Multiplying Charge Coupled Device (EMCCD). The electron multiplier multiplies, in use, signals received from a device, typically a CCD, comprising photodetectors. A test signal is input to the electron multiplier from the device, and the gain of the electron multiplier is determined from the output of the electron multiplier. The test signal comprises, or is derived from, electrical charge generated by the photodetectors other than as a result of the incidence of an electromagnetic signal on the photodetectors. The test signal comprises, or is derived from, dark current, or clock induced charge.

Abstract: The invention concerns a system for achieving an optical image of an object (136), and/or for achieving optical collimation (137) of light from a light source (130), comprising a concave mirror (131: 134), the surface normal of which forms an angle with the incident light in a beam. A negative lens (132, 133) that co-operates with the concave mirror is arranged in the incident and/or exit beam paths such that an image with eliminated or reduced imaging aberrations is achieved in a focal plane (135), and/or that the exit light from a light sources (130) is collimated (137).