# How long would it take to collect a certain amount of photons? [closed]

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.

• I'm not sure you'll get much more help, we are not here to do your homework. If you have a V-band magnitude you should be able to work out a flux (in W/m$^2$) in the V-band at the telescope and hence how many photons/s are detected by your particular setup. – Rob Jeffries May 10 '17 at 10:23