Using a telescope, you measure the light from the planet Mars and 
obtain 500 photons in one second.

(a) If there are no photons coming from the background, what is the 
    signal to noise of this measurement?
(b) If, in addition to the 500 photons from Mars, you also obtain
    500 photons from the sky background, what is the signal to noise?
(c) If, in addition to the 500 photons from Mars, you also obtain
    5000 photons from the sky background, what is the signal to noise?