To expand a bit on construction: it&#x27;s possible to take a long tube, put a fresnel lens at front, pyroelectric sensor with rotating shutter at the other end. Scan by rotating it with reductors&#x2F;steppers as a whole. It&#x27;s also possible to cool tube with contents down to -70 with dry ice to cut on background heat radiation. Maybe it&#x27;ll lessen noise in pyroelectric sensor, maybe not (and it&#x27;s quite easy to buy dry ice in Moscow).