Patent ID: 6049434
Filing Date: 2000-04-11
Classification: G02B

Abstract:
An afocal telescope capable of operating in the thermal infra-red waveband, the telescope comprising a three-element objective for receiving radiation from object space and a two-element eyepiece for delivering image-forming radiation to an exit pupil, the eyepiece and objective being aligned on a common optical axis and arranged such that an intermediate image is formed between the objective and the eyepiece, wherein the three-element objective comprises a positively-powered central lens element disposed between positively powered and negatively powered lateral lens elements, the negatively powered lateral lens element being located proximal to object space, the positively powered lateral lens element being located proximal to the inter-mediate image and incorporating a hybrid surface simultaneously to correct longitudinal and field colour, and wherein one of the positively powered lens element s of the objective is made of a material having a negative temperature refractive index co-efficient and has an optical power selected to render the telescope athermal.