ok the question is, Calculate the photons you expect to recieve per Angstrom at the focus of a Cassegrain telescope of aperture 30cm for a star of V magnitude 9.0, observed at a zenith distance of 60 degrees. Assume extinction coefficient of 0.15 magnitudes per airmass, reflectivity at each surface = 90%, wavelength = 5500 Angstrom. I'm overwhelmed with all the numbers given and cannot find any equations that are useful to this question. If anyone has an idea please give me a hint on where to start or how to approach this question. Thanks in advance.