by Chris Peterson » Tue Dec 25, 2012 5:31 pm
BMAONE23 wrote:I might suggest taking a series of images that are no longer than say 1/10th second and using a stacker program to combine the images into one for depth. If you image from a Fixed Tripod, a 30S(econd) exposure will give you depth but it will also show trailing since the stars will move over the time the iris is open. Even 30-1Second exposures stacked will give the same depth as a single 30second exposure. Though having the iris open for 1 full second will still slightly elongate the stars. The other solution is to get a motorized tripod that will automatically track the stellar motion so No trails will occur.
Unfortunately, a stack of 30 1-second exposures will give nowhere near the depth of a single 30-second exposure. Even stacking 30-second exposures will produce largely unsatisfactory results for all but the very brightest deep sky objects.
The quality of an image is determined by the signal to noise ratio, and while stacking images does linearly increase the signal, short exposures are dominated by readout noise, which sums up with each new subexposure stacked. It's the primary source of noise in stacked, short exposure images, and there's no way to eliminate it. That's why nearly all high quality astroimages utilized the longest practical subexposures- 10-20 minutes per image is common (although the high dark current noise of DSLRs usually limits the maximum exposure to about 5 minutes).
It's possible to take simple wide angle sky images (often incorporating landscape as well) with short enough exposures that trailing is minimal. But anything else requires some sort of tracking mount.
[quote="BMAONE23"]I might suggest taking a series of images that are no longer than say 1/10th second and using a stacker program to combine the images into one for depth. If you image from a Fixed Tripod, a 30S(econd) exposure will give you depth but it will also show trailing since the stars will move over the time the iris is open. Even 30-1Second exposures stacked will give the same depth as a single 30second exposure. Though having the iris open for 1 full second will still slightly elongate the stars. The other solution is to get a motorized tripod that will automatically track the stellar motion so No trails will occur.[/quote]
Unfortunately, a stack of 30 1-second exposures will give nowhere near the depth of a single 30-second exposure. Even stacking 30-second exposures will produce largely unsatisfactory results for all but the very brightest deep sky objects.
The quality of an image is determined by the signal to noise ratio, and while stacking images does linearly increase the signal, short exposures are dominated by readout noise, which sums up with each new subexposure stacked. It's the primary source of noise in stacked, short exposure images, and there's no way to eliminate it. That's why nearly all high quality astroimages utilized the longest practical subexposures- 10-20 minutes per image is common (although the high dark current noise of DSLRs usually limits the maximum exposure to about 5 minutes).
It's possible to take simple wide angle sky images (often incorporating landscape as well) with short enough exposures that trailing is minimal. But anything else requires some sort of tracking mount.