So, I've received this question to solve, it's not a very advanced one, though advanced enough to cause headaches.
The situation is as follows:
Say, you are observing with a telescope with an effective area of 44000 $cm^2$. Assume that the total efficiency through the atmosphere, the telescope, optics and the filter is 50%. How long would it take to collect 80 photons from a source with V=20?
I have little to no idea how to tackle this problem. Can someone help? I've had lectures about spectral flux density, and I'd assume you'll have to use it here.
Thanks in advance!