On 21 June 2013 17:10, Douglas Theobald <dtheob...@brandeis.edu> wrote:
> Yes there is. The only way you can get a negative estimate is to make > unphysical assumptions. Namely, the estimate Ispot-Iback=Iobs assumes that > both the true value of I and the background noise come from a Gaussian > distribution that is allowed to have negative values. Both of those > assumptions are unphysical. > Actually that's not correct: Ispot and Iback are both assumed to come from a _Poisson_ distribution which by definition is zero for negative values of its argument (you can't have a negative number of photons), so are _not_ allowed to have negative values. For large values of the argument (in fact the approximation is pretty good even for x ~ 10) a Poisson approximates to a Gaussian, and then of course the difference Ispot-Iback is also approximately Gaussian. But I think that doesn't affect your argument. Cheers -- Ian
