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What do you all mean by "Image Stacking"

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#1 dml33

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Posted 05 August 2020 - 12:02 PM

Lets say I want to take a photo of Jupiter with my Canon 6D.

 

How does image stacking apply here? If I am taking 10 second exposures wouldn't jupiter move out of frame? How do I take the same exact 10 second photo multiple times to get images to stack if the subject is moving?



#2 awong101

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Posted 05 August 2020 - 12:06 PM

1) Stacking refers to compiling multiple images of the same exposure into an editing software so you have more data to noise ratio. The whole point is to minimize noise of the final image. Having said that, I'd imagine if all of your images have more noise than useful data, than there's no point in stacking. Because then you've just tipped the scale and you end up with more noise in the final image.

 

2) You would need a way to track the movement of sky objects, and compensate for Earth's rotation. Theoretically, a tracker would keep your Jupiter dead center throughout your 10 second exposure.

 

3) And you just program your camera to take repeated exposures with a tracker that will compensate for Earth's rotation, and rotate your camera accordingly while you snap pictures. 

 

It's why sometimes you see people talk about total exposure times in the hours range. I'm still learning myself and just gathering my equipment, but it looks like it's common for people to take 10-50 shots of long exposures, and spend the time to put the image together.


Edited by awong101, 05 August 2020 - 12:12 PM.

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#3 APshooter

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Posted 05 August 2020 - 12:12 PM

Are you trying to take a detailed picture of Jupiter?  Planetary imagers take many thousands of frames with high speed cameras like the ZWO 224MC or the 462MC.  They use a stacking programs like AS3! and Registax to align and stack the thousands of frame automatically.  A ten second picture of Jupiter will come out as a very bight overexposed ball of light.  Most planetary cams capture Jupiter one frame at a time at 6-9 milliseconds.



#4 idclimber

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Posted 05 August 2020 - 12:18 PM

As was stated the point of stacking is to increase the signal over the background noise. The signal in this example is Jupiter and all its details. With planetary objects they are taking thousands of short duration photos hoping a percentage of them are taken at a moment when the atmosphere is more stable. This is called lucky imaging. 

 

We can't do the short exposures for deep sky objects, but the goal is the same. More signal and less noise. 

 

This requires registering the photos. This means aligning and scaling each photo so when they are stacked they lined up correctly. This is done with specialized software. 



#5 bridgman

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Posted 05 August 2020 - 12:24 PM

I haven't tried this, but with a Canon 6D the preferred approach seems to involve capturing video rather than single frames, and using "5x live view" (which I'm not sure how to enable yet).



#6 Stelios

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Posted 05 August 2020 - 01:58 PM

I haven't tried this, but with a Canon 6D the preferred approach seems to involve capturing video rather than single frames, and using "5x live view" (which I'm not sure how to enable yet).

Video *is* a bunch of single frames, saved in an .AVI file

 

To the OP: You would not be taking 10" images of Jupiter. 10" images would totally burn out Jupiter even at F/30. You would be taking images around 30-50ms (depending on camera sensitivity, aperture and focal ratio). 

 

Stacking software is different for planetary (AutoStakkert!3 is the standard) and deep sky imaging (DSS for free, many paid ones). They all do a great job though. 

 

Although it's possible to take planetary images and stack them with a non-tracking mount, it's simply torture. Invest in a camera tracker at least.


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#7 BinoGuy

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Posted 05 August 2020 - 06:29 PM

Dylan posted something about this in November   https://www.youtube....h?v=EMdEhQD2WxY  With a nice comparison of three different 300 second total exposures (10@30 seconds, 5@60 seconds, and 1@300 seconds IIRC).



#8 APshooter

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Posted 05 August 2020 - 06:53 PM

For deep sky stuff he is correct.

#9 Alex McConahay

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Posted 05 August 2020 - 08:15 PM

>>>>>>I'd imagine if all of your images have more noise than useful data, than there's no point in stacking.

 

That sounds sensible. And it would be true if noise was a thing in itself. However, your analysis kinda misses what "noise" is. Noise is part of the signal, just as the recorded energy of the collected light photons are signal. If you could record precisely the energy of the photons hitting the sensor, you would have no noise. But you cannot. Too many things get in the way. Heat photons, cosmic rays, variations in the way sensors react, "rounding" (quantization) errors in converting photons to ADU electron units, and such...... So noise is the uncertainty in your signal.

 

Your analysis sounds like noise is something above and beyond signal, as if it were added to the signal. It is not. It can be. But it can also be stuff taken away from the signal. It is uncertainty in the signal.    

 

In fact, if you stack enough noisy data the noise starts to disappear. You see, noise is not "noise" in the sense we usually think of it. For astroimaging purposes it is best to think of it in terms of "uncertainty." And the purpose of stacking is not to "add" the signal together, but to "average" out the signal. 

 

Lets say you get the following reading from a given pixel. 

 

9, 10, 9, 11, 10, 11, 9, 10, 11, 10, 10, 11, 9,  etc......

 

What is the "true" value that the pixel should be reading? I'd eyeball it at 10. But relatively few subframes are actually reading at 10. The noise puts the signal near 10, but rarely right at it. 

 

What stacking does is allow one to average out the readings to get you to that "10."

 

But it is more powerful than that. 

 

Let us say that your readings were instead:

 

9, 10, 1, 17, 10, 11, 9, 10, 11, 10, 10, 11, 27,  etc......  (the same as before except this time, you had three weirdos for some reason). 

 

C'mon, that 1, 17, and 27 are goofy. Before averaging you should throw them out. And average out the rest. That would get you back to your "10." Getting rid of the 1, 17, and 27 reading is called "rejection" and your software has a "rejection algorithm" which helps decide which readings are spurious and should be ignored. 

 

So, remember, when properly handled, stacking noisy pictures together is not ADDING the noise, but instead AVERAGING OUT the noise. 

 

Alex



#10 Ken Sturrock

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Posted 05 August 2020 - 11:28 PM

There's a nice article about stacking for beginners written by Sean Walker in the September 2020 issue of Sky & Telescope.


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