An astrolabe consisting of a telescope with a prism and artificial horizon attached at its objective end so that only objects near one specific angular elevation can be observed. The usual form of this instrument has a horizontal telescope and contains a 60° prism, with the face nearest the objective being perpendicular to the line of collimation of the telescope, and a small pool of mercury attached beneath the prism to give an artificial horizon. In observing, two images of a star are seen. One image has been reflected directly into the telescope by the lower face of the prism. The other has been reflected first by the mercury's surface and then into the telescope by the upper face of the prism. The two images move in opposite directions either to or from coincidence. At the instant of coincidence, the star is at the apparent angular elevation imposed by the prism's angle. Prismatic astrolabes may be made with the angle at other that 60°. An English instrument uses a prism with 45° angle and has accessories.