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!


  • 1
    $\begingroup$ Let's break it down: You know the surface area, the efficiency, the frequency. You need to find the time over which 80 photons are collected. What equations have you been given? $\endgroup$ – called2voyage May 9 '17 at 18:13
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    $\begingroup$ You have insufficient information to answer the question. Unless perchance you forgot to tell us that you were observing through a V band filter. Even then, the spectrum of the source matters if you want an exact answer. $\endgroup$ – Rob Jeffries May 9 '17 at 22:58
  • $\begingroup$ Yes. I am observing through the V band filter. I will try to add more information later today. I will give a small list with equations as well. $\endgroup$ – Adam Warlock May 10 '17 at 7:40
  • $\begingroup$ 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. $\endgroup$ – Rob Jeffries May 10 '17 at 10:23
  • $\begingroup$ "V-band" is a term for part of the microwave spectrum. We need an appropriate definition for the visible source here. I'm guessing Bessel V-band? $\endgroup$ – Carl Witthoft May 10 '17 at 14:01