focusable in this embodiment the body 100 is formed of two parts the first body portion 102 is a cylinder which retains a forward portion 104 which has in it an optical aperture 106 the forward portion 104 is sealably attached to the first body portion 102 by means of seal 108 a forward portion of the optical system 110 is attached to the first body portion 102 aligned with the optical aperture 106 defining an optical axis 112 as previously mentioned the aperture 106 is covered by a window 113 which could also be part of the optical system a rearward second body portion 114 is also in the form of a cylinder and retains the rear closure 116 sealed by means of the seal 118 the first body portion 102 is telescoped inside the second body portion 114 mounted inside the second body portion 114 is a rear portion 120 of the optical system the sensor converter 122 and the preprocessing electronics 124 are shown mounted in and are connected in the second body portion 114 in a manner similar to that hereinbefore described on the outer periphery of the cylindrical first body portion 102 is a thread 126 and on the inner periphery of the second body portion 114 is a thread 128 the threads 126 and 128 being interengageable so that relative rotation of the first body portion 102 and the second body portion 114 will cause relative movement of the forward portion 110 and the rearward portion 120 of the optical system to accomplish focusing the telescoped cylindrical parts of the body portions 102 and 114 should be sized for close but freely movable fit for sealing a groove 130 in the first body.