36 images the panorama through a vacuum window 38 and then an iris 40 towards a focal plane 41 this permits the iris 40 to be vacuum cooled a detector 42 for detecting photons is situated subsequent to the iris 40 although fig1 shows lens set 36 as being composed of seven lenses there are many combinations of lenses and other optical components that could be used to reimage the panorama onto detector 42 all of the components shown in fig1 and the other figures below are enclosed in a suitable housing or are otherwise securely mounted to each other the relationship between the incoming light rays 20 20 39 24 and 24 39 the iris 40 and the detector 42 is discussed more fully below in connection with fig2 various infrared transmitting materials may be used for window 38 such as sapphire or silicon window 38 isolates detector 42 from atmosphere allowing the detector to be vacuum cooled the detection of photons in the mid infrared portion of the spectrum nominally 3 5 microns generally requires active cryogenic cooling of the detection apparatus iris 40 on the other hand serves as an aperture stop thereby masking out undesirable light the iris 40 can be constructed out of metal or other opaque material and also serves in part as a cold shield for detector 42 a 480 u00d7 480 array of cooled indium antimonide insb pixels 43 functions well.