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Quote:Just another though, as I sit here, playing with some test images, taken last night to try out my new setup.... I'm looking at some 4 min, versus 10 minute m81 pics,. In the 4 minute exposures, I can use the histogram to bring the core to a pinpoint. However, with the 10 minute, the core is saturated and at best maybe 20-30x the size. Granted the setup did handle the 10 minute exposure fine, the stars don't show more trailing in the 10 minute, there is better detail in the arms etc... But the minimum resolution of bright objects, just isn't there.. Of course, you could always take both long and short exposures and blend them to get certain detail, which may end up more efficient than taking lots of short exposures, but as mentioned, it depends,
Quote:There is theory . . . and there is what YOU can do in YOUR world with YOUR equipment under YOUR skies!I quite accidentally discovered that I could get some nice results with (hold your breath) ONE MINUTE NB exposures!
Quote:Do you happen to have links to full-resolution versions of these shots, ideally without noise reduction? It's really hard to judge whether the 1 minute exposure versions are noisier at the image scale you posted.
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Quote: Also, narrowband signals are inherently weak, requiring long exposures of 20 – 40 minutes.
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Quote: There is zero gain from going beyond 16.
Quote:Noise is something you deal with after you have signal.
Quote:Anyways though, still in practice, you may well be better off taking longer exposures wot the low e- cameras, but as mentioned, I'm sure there a lot of factors to consider, even without considering camera noise models.
Quote:Thanks for the link Mike. I definitely wish I knew more about this stuff. I know as far as sampling goes, random noise should end up as 1/sqrt(n) where n is the number of exposures. So at 4 exposures, you'd have half the noise of 1 exposure. To get to half the noise of 4, you'd need 16. But, then to start really seeing an improvement, it does drift off, to cut that remaining noise in half, you'd then need to go to 64, and to cut that in half, you'd need to go to 256 etc... It looks like the example is for dark noise, but the same applies to any unbiased normally distributed random noise, so, read noise etc... in theory anyways... It looks like he uses "8" as a start value, I'm not sure what units, but the scale will be larger with a larger starting noise. I think in practice, there are lots of factors, such as sky fog, dynamic range of computer monitors, pixel size, tracking, seeing etc... that will limit real-world appreciation of gains after some point. I know though, you can see a gain after 16, as I've added integration to images, taken previous night for example, and watched the images improve. It does definitely take more and more to see an improvement as you go, and if the image is good enough to begin with, the improvement certainly may not be perceivable.
Quote:I was told directly by John Smith at AIC that indeed you do need to go longer at a dark site, especially with narrowband.
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Since most stacking programs "average" the images you might see an improvement in image quality as the new images may be of higher quality than the previous.
Quote:I have everything working against me. I have a slow-ish system (f/7.5) with narrow bandpass filters (3nm) and I image from dark to really dark skies (mag 5.4 at home, 6.5+ in the field). With a measured read noise of 9 electrons my camera isn't overly noisy either. I've done the calculations using test exposures for my own setup and from my magnitude 5.4 backyard at home I'd have to shoot upwards of 5 hour subexposures in order to bury the read noise in the shot noise.