I debated between this and an effective delta function (e.g. a Pixel with very small sides).  

Using the full pixel size seemed right to me though, since the noise at separations less than `pixel_scale` is fully correlated (since they are the same value in the original image), so the covariance = the variance.  Right?

But I've always been a bit iffy about all this stuff, so I'm happy to be told otherwise.