A photoelectric device for intensifying faint astronomical images. In these devices a photoemissive surface, called the photocathode, emits electrons through the photoelectric effect. In most tubes the photocathode is semitransparent and is deposited on the inside of a transparent window that is mounted on the end of an evacuated glass or ceramic cylinder. When light from a telescope or spectrograph is imaged on the photocathode, electrons are ejected into the vacuum inside the tube. Electric fields or electric and magnetic fields in combination then accelerate and direct the photoelectrons through the device. In an image intensifier tube the photoelectrons are ultimately reimaged on a phosphor-coated output window that converts them back into visible light. An external television camera or charge-coupled device (CCD) is used to record the image. Optical gain is provided by the phosphor, which may release as many as a thousand photons for each photoelectron which crashes into it. In other devices the electron image is projected onto a solid-state diode array or a grid of anodes that is built directly into the tube.