infrared afocal telescope, telescope type

摘要

An afocal telescope (20) comprises an objective system (21) and an eyepiece system (23) aligned on a common optical axis (19), the objective system (21) forming a real image (24) of radiation received from object space (17) and eyepiece system (23) being arranged to provide a magnified view of the object space scene at a pupil phi in image space (18). The eyepiece system (23) is formed by a triplet of lens elements A,B,C of which A and B are positively powered and C is negatively powered. Element C has a concave refractive surface (6) adjacent image (24) and a convex refractive surface (5) which is separated from the adjoining refractive surface (4) of element B by an air space which, in the axial direction, is substantially zero on the axis (19) and which progressively increases in magnitude as the distance off axis increases. The six refractive surfaces (1-6) of the eyepiece system (23) are free from aspherics and refractive surface (1 ) which is adjacent the pupil (5) is coated with an antireflection coating to avoid narcissus effect. Elements A, B and C may be made of germanium for use in the 8-14 micron waveband or of silicon for use in the 3-5 micron waveband.