What is an advantage of the infra-red observations in comparison to optical ones? Can optical CCD detectors be used to measure infrared photons? Q 5(b) [5] Can a silicon CCD (forbidden band gap is 2x10-19 J) be used for infra-red (=1m) observations? Assume that 100% of photon energy is given to electrons. Q 5(c) [5] Why cooling of the detector is of crucial importance to the infra-red observations? Q 5(d) [5] A detector has a background level of 2 photons per second. How long should one observe a source with a flux rate of 0.5 photon per second to get a signal to noise ratio equal to 10?