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I have statistics question. I have an (astronomical) image, and I want to sum up the pixels within a square, and also get the error of this sum. I use the program DS9 (in case somebody knows it), and it does give me a sum, and an error. But the strange thing is, the error is exactly the square root of the sum. And I have no idea why this is the case. I've never such an error computation.

Does anybody know more?

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  • $\begingroup$ What do these things you're summing consist of? Counts? measurements of some continuous quantity? $\endgroup$
    – Glen_b
    Jan 23, 2017 at 10:48
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    $\begingroup$ Yes, they are pixel counts. Imagine an image showing stars, and I want to sum up all pixels of the star. $\endgroup$
    – Pythoneer
    Jan 23, 2017 at 10:50
  • $\begingroup$ en.wikipedia.org/wiki/Shot_noise $\endgroup$
    – whuber ♦
    Jan 23, 2017 at 16:12

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