Afocal refractor telescopes

Abstract

An afocal refractor telescope is formed by a fixed focus achromatic telephoto objective system (10) and a fixed focus eyepiece system (11) aligned on a common optical axis (12) and is arranged to provide an internal real image (13). The objective system (10) is formed by a primary lens element (D) and a secondary lens element (C) and the eyepiece system (11) has at least two lens elements (A, B). Each of the lens elements (A,B,C,D) of the telescope is made of a material having a useful spectral bandpass in the infrared wavelength region and has refractive surfaces (1,2,3,4,5,6,7,8) intercepting the optical axis (12), at least one of the refractive surfaces (7,8) of the primary objective lens element (D) being aspheric whereas the refractive surfaces (1,2,3,4,5,6) of the lens elements (A,B,C) are substantially spherical. The aspheric surface (7,8) possesses only a small degree of asphericity and is defined by an equation in which the third and higher order aspheric coefficients are zero. The secondary objective lens element (C) is negatively powered and has a refractive index equal to or lower than the refractive index of the primary objective lens element (D) which is positively powered and the arrangement is such that the telescope has an internal f-number in the airspace between the primary and secondary objective lens elements (D,C) which is less than 1.5.