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Old 03-10-2007, 06:18 PM
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bkm2304 (Richard Brown)
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Angry Just remind me one more time....

I now this has been answered a few (thousand) times but.... What is the difference/advantage/disadvantage between a single 300 second exposure of a star field, say, and 10 x 30 seconds stacked?

And where on earth can I sit down with someone - anyone - who'll show me why my Registax efforts are abyssmal???? :mad2 :
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Old 03-10-2007, 06:33 PM
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ballaratdragons (Ken)
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Probably a nice simple way to explain it (not the most scientific, but plain):

Because 1 single image will show signal noise. By stacking many images, the noise is dispersed between all the images and blended out.

And because there are many images, more detail can be gained in the different images. 1 image may see certain detail in a globule inside a nebula for example, but not see another object within it. Whereas, with many images, you are garanteed to get lots of different perspectives in the images, hence collecting it all (well, most of it) when stacked together.

Others may explain it in a more detailed way, but that's a quick simple explanation.
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Old 03-10-2007, 08:23 PM
Dennis
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I’ll just add my 2c worth to what young Ken has described. A single 300 second exposure would tend to have a lot of signal (good stuff) and a relatively small amount of noise (bad stuff).

Signal is what you want – lots of it. These are the photons of the galaxy that you are trying to image. With a good mount and auto guiding, these photons will arrive consistently and linearly at the same place on the CCD chip, building up the signal strength.

Noise is usually more random and does not build up consistently and as quickly as the galaxy photons being captured at the same pixel locations. Noise comes from thermal effects in the CCD, read out processes, electrical circuit effects, etc.

However, 300 seconds demands a good mount and good auto guiding to avoid trailed stars. In some cases, an airplane flying across the 300 sec frame can ruin the frame.

Now if we take say 10 x 30 second frames, we have the following:
  • Tracking and auto guiding errors are slightly less critical.
  • The airplane will only ruin a single 30 sec frame – you can do a median combine with the other (good) 9 frames and the airplane trail will be rejected.
  • However, with only 30 sec exposures, the signal is so much weaker and thus the noise is not so much “drowned out” by it. But, the signal will build up consistently from 30 sec frame to 30 sec frame so when you stack these, the signal re-enforces itself across all the frames. The noise in each frame is mostly random and so when to stack the frames, the noise is not consistently amplified by the stacking process, so it is averaged out, and thus less dominant than in a single 30 sec frame.
The single 300 sec frame will still have a better Signal to Noise ratio than the 10x30 sec frames stacked, and I have forgotten the formula that describes this.

Cheers

Dennis
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Old 03-10-2007, 09:46 PM
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Neither the best is to combine both. Long exposures bring out more signal, for example a very dim star maybe become more visible using longer exposure. However brighter stars will be over exposed. A short exposure will be good to get the bright stars and then you can add this using layers to the long exposure. Note this is only my opinion based on what I have read and seen and understood but its no based on any experience. A fine example as i pointed out in another thread is to consider Eric's latest image of the Orion Nebulae. By combing stacked short exposures with stacked longer exposures he got a balanced image of what is actually up there.

The problem is to take longer exposures requires better tracking precision.

It is not true to say that 10x10sec exposures = 1x100sec exposure. It also depends on the subject being photographed. There is a very good read on imaging in the General thread written by Phil (i think) which advises people to study your target and understadn what it is you capturing. The more you understand of the nature of what is up there the better you will become at capturing it.

Regards
Fahim
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Old 05-10-2007, 10:56 AM
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bkm2304 (Richard Brown)
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Thanks lads.

I would like to know if and when there will be a course/get together to look athe basics of stacking technique and such. If there isn't one or hase never been one in recent times, then maybe we should organise one! Any takers?
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Old 05-10-2007, 11:12 AM
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h0ughy (David)
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Quote:
Originally Posted by bkm2304 View Post
Thanks lads.

I would like to know if and when there will be a course/get together to look athe basics of stacking technique and such. If there isn't one or hase never been one in recent times, then maybe we should organise one! Any takers?
there are a few select imagers here in Newcastle that are going to start up a astrophoto group to get together and trade and swap info - let you know when -
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Old 05-10-2007, 12:32 PM
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As alluded to above, the longer exposure isn't necessarily better, due to the charge capacity of the individual pixels in the camera sensor. As you approach full charge capacity, or saturation, they start behaving non-linearly and you begin to lose data. If you care about the data in those regions, yet have a good mount that will track forever without trailing or PE creeping in, you need your exposure times set to around 70-80% of what will saturate, unless you are playing around with HDR.

Eric
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