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  #21  
Old 03-09-2014, 11:24 AM
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RickS (Rick)
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Originally Posted by alistairsam View Post
so Shot noise just refers to the rate of incoming photons or variation in the rate, more like jitter (in networking) and has nothing to do with sky background?? why is it referred to as "noise"? sorry to detract with the basics..
To add to what Barry said, sky glow is not noise (it is unwanted signal) but it has shot noise associated with it just like the signal from the object does. That's why we hate sky glow... it adds shot noise to our subs.
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  #22  
Old 03-09-2014, 01:28 PM
clive milne
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This thread is not entirely unrelated to a conversation I had with Ray over the last day or so, the gist of which relates to optimising colour channel exposure times.

Consider this; conventional wisdom suggests that if a camera's QE in the blue is half that of the red, then the blue channel requires double the total accumulated exposure time of the red. However, even though this will ensure both blue and red channels have equivalent read-out and shot noise, there will be additional noise contributed by effectively doubling the dark current.
It then stands to reason that in situations where dark current is significant, conventional exposure ratios derived from estimates of relative spectral sensitivity are not an optimum use of exposure time. This is perhaps more relevant to Kodak chips and narrow band imaging. What it potentially translates to is that the results obtained with linear colour/QE scaling could be achieved x% more quickly accounting for dark current shot noise in areas of the spectrum where any given CCD is inherently less sensitive.

Anyway... just thought I would put it out there for public discussion and contemplation.

regards
c
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  #23  
Old 03-09-2014, 03:23 PM
SpaceNoob (Chris)
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Quote:
Originally Posted by clive milne View Post
This thread is not entirely unrelated to a conversation I had with Ray over the last day or so, the gist of which relates to optimising colour channel exposure times.

Consider this; conventional wisdom suggests that if a camera's QE in the blue is half that of the red, then the blue channel requires double the total accumulated exposure time of the red. However, even though this will ensure both blue and red channels have equivalent read-out and shot noise, there will be additional noise contributed by effectively doubling the dark current.
It then stands to reason that in situations where dark current is significant, conventional exposure ratios derived from estimates of relative spectral sensitivity are not an optimum use of exposure time. This is perhaps more relevant to Kodak chips and narrow band imaging. What it potentially translates to is that the results obtained with linear colour/QE scaling could be achieved x% more quickly accounting for dark current shot noise in areas of the spectrum where any given CCD is inherently less sensitive.

Anyway... just thought I would put it out there for public discussion and contemplation.

regards
c
Decent RGB filters can help with 1:1:1 exposures for subs. I know my Astrodon TRU Balance LRGB filters seem to do a decent job in this regard with the 8300 sensor.
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  #24  
Old 03-09-2014, 10:20 PM
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Originally Posted by strongmanmike View Post
Always interesting reading these threads

So to put it simply and apart from taking up less disc space and putting the tracking and unexpected bump/wind/plane/satellite problems etc (which are indeed significant considerations) aside...under what conditions and with what sort of equipment is doing a single 30min sub a significantly better option than doing 6 X 5min subs?

if you have high read noise and low QE, you need longer subs. eg, a system with an 11002 would maybe need 30 minute subs, but an equivalent system (aperture and sampling) with a 694 could possibly only need 5 minute subs, since the 694 has high QE and low read noise. In typical systems, an 11002 would be next to useless with 5 minute subs, but would work fine with 30 minute ones. The main driver is the read noise and the sub length scales with the square of the read noise (all else being equal). If doing narrowband, use subs as long as is practical, regardless of equipment.

Last edited by Shiraz; 03-09-2014 at 10:35 PM.
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  #25  
Old 04-09-2014, 11:07 AM
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Originally Posted by RickS View Post
A good example would be a narrowband sub with a sensor like the KAF-16803. The signal (including sky glow) will be relatively small and in a 5 minute sub the read noise from the camera will swamp the shot noise from the target (and you'll incur that read noise 6 times.) A 30 minute sub will have much better SNR than a stack of 6 x 5 mins.
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Originally Posted by Shiraz View Post
if you have high read noise and low QE, you need longer subs. eg, a system with an 11002 would maybe need 30 minute subs, but an equivalent system (aperture and sampling) with a 694 could possibly only need 5 minute subs, since the 694 has high QE and low read noise. In typical systems, an 11002 would be next to useless with 5 minute subs, but would work fine with 30 minute ones. The main driver is the read noise and the sub length scales with the square of the read noise (all else being equal). If doing narrowband, use subs as long as is practical, regardless of equipment.
Thanks Guys, just wanted to have it spelled out, there seems to be a lot of confusion over sub length and when one actually really needs to do very long subs. For me unless one is using a narrowband filter especially the very narrow models (3-6nm) the difference the very long subs make may well be unoticable..? unless you have a very noisey camera?

Mike
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