The art of astrophysical measurements: An elementary lecture on photon counting and S/N

Have you ever dared to understand how many photons are detected by your camera when you are pointing the telescope to the sky? Is the action of the spectrograph fully understood when analyzing those photons? Do typical image formation rules hold when studying the polarization of light? All these are a few examples of questions addressed by researchers when designing instruments, which are later constrained by technological capabilities. This talk will summarize them with the aim of triggering basic curiosity of scientists to get an insight on the means at their disposal to investigate the Universe.