I've got to solve this problem and I am having quite an hard time figuring out how... Sorry, but I think I need help: "An astronomer takes a spectrum of a distant quasar using a 4m telescope. With an exposure time of one hour, the optical spectrum contains about 10,000 photons. The source emits 1/10 as much power in the X-ray band as it emits in the optical band. The astronomer manages to get a 30 kilosecond observation with an X-ray observatory that has an effective area of 1000 cm2. Assuming that the effciency of the telescope and spectrograph combination is 30%, how many photons will the resulting X-ray spectrum contain?" As far as I understand, What I should do here is 1) to compare the two telescopes as if they had the same area and the same exposure time, so that I can infer how many optical photons the optical telescope would have gathered if it had the area and the exposure time of the X-Ray one. 2) Then, I would take 1/10th of this result (the source is emitting 1/10 of Xray photons as compared to the optical ones) which will tell me the theoretical number of X-Ray photons the X-Ray telescope would have gathered with perfect efficiency. 3) But knowing that the efficiency was only the 30%, I could have an estimate of the number of photons actually detected in the X-Ray spectrum. Problem is, I have no idea how to calculate step 1) Any suggestions? Thanks a lot.