Rob R said: <How did you determine that a stacked group of 20 minute exposures was the best for this image?? Is this something you know by experience w/ your system or interpolated from a reference set of exposure guides?? Or is this a value imposed by the G11 on autoguide??> Joe Bauman said: <Now that we're picking your well-stocked brain, can you talk a bit about stacking vs. long exposure? I understand that this beautiful view is a stack of several 20-minute shots. I assume a whole bunch of 20-minute pictures will still result in a final view that is not overexposed because what you're doing is eliminating noise. But 20 minutes still seems like a long exposure. I've had a galaxy overexpose in (I think) five or ten minutes. Is the nebula pretty dim? And just for discusison, what do you think a view of four hours, without stacking, would look like?> My response to these questions: - Exposure duration..... This object (Vdb142) is quite dim and doesn't have any really bright stars in the field. This combination allows longer exposure times, for the reasons outlined below. Also, remember that I was shooting with a narrow band filter that only allows a narrow range of light to pass. The Ha filter greatly reduces the total light reaching the chip. The exposure time for any object is best determined by looking at the histogram of an exposure to see if the stars are becoming saturated. If they are saturated (some pixels at maximum value), the image will have white stars instead of colored stars. The exposure can be shortened to the point where stars are nearing saturation, but not completely saturated. I intend to use this image as luminance data for an RGB image at a later date, so I didn't want to saturate the stars. I like star color in my images! Just to give you an idea of why I might use shorter exposures, here is a link to another image that I took the same night: http://tinyurl.com/qfdcv The M92 image is composed of 3 minute subs, because longer exposures would have overexposed the core area and washed out the star color. - The reason for multiple exposures.... Even though the noise from the chip in SBIG cameras is pretty low, it is clearly visible in any single frame, even a 20-minute exposure. Dark, bias, and flat frames can be used to remove much of the noise, but even after these are applied, noise is pretty apparent. However, when images are stacked and summed or averaged, the random noise tends to cancel itself out and the true signal remains. Images composited from several frames appear much smoother and subtle details emerge from the noise and become clear. Does that make sense? - What about a single long exposure... If I were to go four hours on a single exposure, several things would happen. First, all the brighter areas would become completely saturated, and any detail in those areas would be lost forever. It is impossible to regain detail in areas that are composed of a bunch of pixels that are maxed out. Also, the noise in a single image would be objectionable whereas the noise from 12, 20-minute images smoothes out beautifully! I hope this helps a little. Cheers, Tyler