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# homeworkec - AST 3722C Spring 2008 Homework#10 Extra Credit...

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AST 3722C - Spring 2008 Homework #10 – Extra Credit You don’t have to do this one Due April 15 Instructions: Solve each problem below. I’ll post the solutions online on April 15, so there’s no possibility of turning it in later than that date. These problems involve CCD measurements and would help prepare you for the final. 1. Suppose you are told that the night-sky brightness atop Mauna Kea in Hawaii is 21.5 magnitudes per square arcsecond in V-band. You want to find how many photons per second per square centimeter per square arcsecond this provides. Here are the steps involved in getting there. (a) Use handout #17 to convert the magnitude within 1 square arcsecond into a flux density. Assume that flux density is measured per wavelength interval. Your answer will effectively have units of erg/s/cm 2 / ˚ A/arcsec 2 . (b) Calculate the power per collecting area provided by that 1 square arcsecond of sky. Assume that when you look in V-band you’re looking through a filter that is a step function centered on λ = λ 0 = 0 . 55 μ m and is Δ λ 0 = 0 . 089 μ m wide. Also assume that the sky’s flux density is approximately constant at all wavelengths across the V bandpass. This means that to figure out power per collecting area within 1 square arcsecond, you simply need to multiply by the bandwidth Δ λ 0 . Express your answer in erg/s/cm 2 /arcsec 2 . (c) Convert the power (i.e. photon energy per unit time) to a photon number per unit time by assuming that all photons in V band have approximately the same energy, i.e. the energy of a photon of wavelength λ 0 . Express your answer in photons/s/cm

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