franco Initial stack looks ok but as soon as you touch the histogram it goes blank.
@franco,
Notice that both images have about the same average ADU (within about 3 dB), but the noise of the bottom one is much lower, wth standard deviation of 17 instead of almost 900. Looks like there is a lot of shot noise in the first histogram, as evidenced by the very Poisson looking probabiity density function of the histogram.
The stacking is properly getting rid of that noise, and narrowing the histogram.
However, it looks like you are using the breakpoints of the first histogram to stretch the second image, i.e., you seem to be stretching the second image using the stretch parameters that is matched to the first image.
Did you try to click on the Auto button again to make it stretch based on the second histogram?
You can also manually bring one of the sliders down below the peak of the histogram. Right now, both sliders are above the peak of the histogarm, which of course will lead to an almost all dark image, which you are seeing. One of those sliders need to be to the left of the peak of the hostgram, and the other slider need to be on the right of the peak.
I recommend (1) clicking Auto button to get a rough stretching, then (2) click on + button which will expand the horizontal scale of the histogram (otherwise, the two sliders would be too close to one another to manipulate), and then (3) you can use the sliders to get the best manual setting.
You do this with pretty much any photo-editing program.
For a well exposed image, one slider need to left of the peak of the histogram and the other slider to the right of the peak. With some photoediting programs, you can reverse the transfer function by swapping the position of the two sliders to form what looks like a photo-negative of the image (try it on the Shroud of Turin, for example :-). Makes your images of a star field look like the old telescope plates :-).
Chen
