telescope afocale to refraction 2 micrographs

摘要

An afocal dual magnification refractor telescope is formed by a fixed focus achromatic telephoto objective system (21) formed by a primary objective lens element (H) and a secondary objective lens element (G) aligned on a common optical axis (19) with a fixed focus collimation system (22) and interchangeable high and low magnification lens systems (23,24) each of which is arranged to provide an internal real image (25,26). The high magnification lens system (23) is formed by two lens elements (B,C) and the low magnification lens system (24) is formed by three lens elements (D,E,F). The eight lens elements (A,B,C,D,E,F,G and H) are made of materials which have a useful spectral bandpass in the infrared wavelength region and the refractive surfaces of these lens elements which intercept the optical axis (19) are substantially except for one or both refractive surfaces (15,16) of the primary objective lens element (H). Each aspheric surface (15,16) possesses only a small degree of asphericity, lens element (H) is positively powered and secondary objective lens element (G) is negatively powered with a refractive index equal to or lower than that of the primary lens element (H) and the arrangement is such that for the high magnification mode the telescope has an internal f-number in the airspace between the primary and secondary objective lens elements (H,G) of less than 1.5.