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Old 14-09-2017, 05:46 AM
Imme (Jon)
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Exposures...when would you stop

So let's say in a theoretical fantasy land you could continue doing subs on a subject....night after night.......ob time wasn't an issue and 'life' wasn't getting in the way.

At what point could you say "that's enough subs....I'm not going to get the final product any better by doing more"

.......interested in people's thoughts
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Old 14-09-2017, 07:03 AM
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ZeroID (Brent)
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That depends a lot on the target. And the astronomer.
Eg Skyviking did 120 hours on one particular target, the results were spectacular to say the least. But would more time have made any difference ?
I guess there has to be a limit on the achievable resolution by the technology as well.
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Old 14-09-2017, 07:11 AM
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rcheshire (Rowland)
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Practically speaking, when noise is reduced to a point where additional subs provide no observable increase in SNR or more precisely; from Wikipedia...

"Signal averaging is a signal processing technique applied in the time domain, intended to increase the strength of a signal relative to noise that is obscuring it. By averaging a set of replicate measurements, the signal-to-noise ratio, S/N, will be increased, ideally in proportion to the square root of the number of measurements."

On the other hand acquiring faint signal may require many more subs. Loosely speaking...
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Old 14-09-2017, 07:33 AM
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RB (Andrew)
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Quote:
Originally Posted by Imme View Post
At what point could you say "that's enough subs....I'm not going to get the final product any better by doing more"

.......interested in people's thoughts
When the Port runs out......

RB
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Old 14-09-2017, 07:37 AM
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multiweb (Marc)
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When the Port runs out......

RB
Errr.... it never runs out for me.
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Old 14-09-2017, 07:50 AM
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RB (Andrew)
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Errr.... it never runs out for me.
That's why your images are far better than mine Marc.

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Old 15-09-2017, 08:52 AM
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personally, I don't see a limit exists. unless you've over exposed the shot to white, I like to chase the noise down with subs rather than processing and i rarely get a chance to reach my desired exposure in camera so getting those extra subs helps me boost signal in post, it also depends on what you are trying to achieve.

I sort of feel people are doing the same fad thing that happened when digital photography took off, in that people are going Filter Happy and overprocessing to exaggerate rather than produce a good and accurate photo. especially tone mapping (what the plebs wrongly call hdr)
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Old 15-09-2017, 09:00 AM
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When you see seriously deep images like the one of M42 (Was that Skyviking's image, 120 hours sounds familiar) where the very faint stuff was estimated to have as few as one photon hitting the sensor per 20 minute (Or longer) exposure you can see the point of gathering more and more data.

There does have to be a point of diminishing returns where you move on though, but only the person capturing the image can decide where that is. If you get to the point where each successive frame is no longer producing a measurable improvement in SNR you are probably pretty close.
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Old 16-09-2017, 08:15 AM
Imme (Jon)
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Thanks guys, you've got some good points there.......looks like the consensus is "until you're happy with it"....makes sense I guess
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Old 16-09-2017, 08:24 AM
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Atmos (Colin)
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That's pretty much in Jon! For me it is usually:
Image one object on a clear night for as long as possible, process the data as it's cloudy for the next week, get a finished result that I am reasonably happy with and keep telling myself I'll add more data to but never do
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Old 16-09-2017, 01:53 PM
Imme (Jon)
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With my new setup, obs/pier/goto/phd2/sgp I'm really hoping the ease of setup (open roof, uncover scope and click) will make data collection a breeze so the adding more data thing won't be a stress.

.....finally finished pier today and it's all bolted down......just waiting for the paint to dry now.
Really pushing to try and get everything setup for darks skies next week....fingers crossed I'll get some imaging done
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Old 17-09-2017, 02:33 AM
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ZeroID (Brent)
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Quote:
Originally Posted by The_bluester View Post
When you see seriously deep images like the one of M42 (Was that Skyviking's image, 120 hours sounds familiar) where the very faint stuff was estimated to have as few as one photon hitting the sensor per 20 minute (Or longer) exposure you can see the point of gathering more and more data.

There does have to be a point of diminishing returns where you move on though, but only the person capturing the image can decide where that is. If you get to the point where each successive frame is no longer producing a measurable improvement in SNR you are probably pretty close.
No, not M42, Centaurus A.

http://www.rolfolsenastrophotography...ield/i-4h46Szm
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Old 17-09-2017, 02:38 AM
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ZeroID (Brent)
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Originally Posted by Imme View Post
With my new setup, obs/pier/goto/phd2/sgp I'm really hoping the ease of setup (open roof, uncover scope and click) will make data collection a breeze so the adding more data thing won't be a stress.

.....finally finished pier today and it's all bolted down......just waiting for the paint to dry now.
Really pushing to try and get everything setup for darks skies next week....fingers crossed I'll get some imaging done
It is always easier and quicker from a permanent setup. I would probably have given up if I had to setup each time in my backyard let alone somewhere dark and remote. I can be imaging within 20 minutes and much of the delay is waiting for PHD2 to go through it's routine.
I feel I get better value for my investment in hardware as well.
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Old 17-09-2017, 08:16 AM
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I can be imaging within 20 minutes and much of the delay is waiting for PHD2 to go through it's routine.
If you have a permanent setup why not just have PHD restore it's previous calibration? Then all you need to wait for is the camera to cool down.
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Old 17-09-2017, 08:29 AM
Imme (Jon)
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Amazing picture Brent....shows what exposure time can do for you
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