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Homework Help: Signal to Noise ratio and exposure time question

  1. Sep 24, 2012 #1
    You obtain an image of a 7 magnitude star in a 1 minute exposure. You found that the 7 magnitude star was detected at S/N = 10 in the image. The S/N was too low for your science and you decided to take another image. If you want to obtain a 1% photometric result on this star, what should be the exposure time? Assume that Poisson noise is the dominant noise source in this observation.


    My attempt:

    [itex] (\frac{S/N}{t})^2 = 0.01 [/itex]

    [itex]\frac{10^2}{t^2} = 0.01[/itex]

    [itex]\frac {100}{0.01} = t^2[/itex]

    [itex]\sqrt{10000} = t[/itex]

    [itex]t = 100.[/itex]

    If you want to obtain a 1% photometric result on this star, your exposure time would be 100 minutes.
     
    Last edited: Sep 24, 2012
  2. jcsd
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