Signal to Noise ratio and exposure time question

1. Sep 24, 2012

sikrut

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:

$(\frac{S/N}{t})^2 = 0.01$

$\frac{10^2}{t^2} = 0.01$

$\frac {100}{0.01} = t^2$

$\sqrt{10000} = t$

$t = 100.$

If you want to obtain a 1% photometric result on this star, your exposure time would be 100 minutes.

Last edited: Sep 24, 2012