Noise in the subtracted image. The image on the left is the subtracted image normalized by the Poisson deviations of both the reference and the image for the small field presented in the previous figure. On the right we show the histogram of the pixels in this image. We superimposed on this histogram a Gaussian of variance 1 (dashed line). Note that the deviations due to the bright stars are no longer visible. The variable star at center clearly stands out at very significant level. The dark pixel in the image is a cosmic ray.