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  #21  
Old 30-10-2013, 01:00 PM
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Quote:
Originally Posted by Shiraz View Post
2. this discussion is about exposures for subs, not the total overall exposure. clearly, longer overall exposure is always better - it is just longer subs that may be a waste of effort.
here is the point that I use for my decision to take longer subs. If you agree that longer integration will always be better then surely its better to take 20 long subs where the stacking isn't hit by diminishing returns too hard.
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Old 30-10-2013, 03:44 PM
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Also when you use these big chips the file sizes become an issue as well.
10 x 32mb files becomes slow to process.

Greg.
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  #23  
Old 30-10-2013, 05:42 PM
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Quote:
Originally Posted by multiweb View Post
See above from Placidus. Just a different scale.
Ken said Median, not Mean.

In my experience, Median does excellent noise reduction.

To quote directly from the CCD stack manual (with Median) "Data rejection is not necessary (though it is acceptable) because median is a de-facto data-rejection algorithm".
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Old 30-10-2013, 05:52 PM
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Quote:
Originally Posted by Peter.M View Post
here is the point that I use for my decision to take longer subs. If you agree that longer integration will always be better then surely its better to take 20 long subs where the stacking isn't hit by diminishing returns too hard.
The diminishing returns are related to the total exposure, not the number of subs (at least so long as your subs are long enough to make read noise insignificant).

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In my experience, Median does excellent noise reduction.

To quote directly from the CCD stack manual (with Median) "Data rejection is not necessary (though it is acceptable) because median is a de-facto data-rejection algorithm".
That's true but it comes at a cost in SNR - you lose around 20% using median instead of average. An average combination with a well chosen rejection algorithm and parameters will generally give you the best of both worlds. At least, that's what the maths says It seems to work in practice as well, at least the couple of times that I've done a comparison.

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Old 30-10-2013, 06:13 PM
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Originally Posted by RickS View Post

That's true but it comes at a cost in SNR - you lose around 20% using median instead of average. An average combination with a well chosen rejection algorithm and parameters will generally give you the best of both worlds. At least, that's what the maths says It seems to work in practice as well, at least the couple of times that I've done a comparison.

Cheers,
Rick.
Yes, median has worked well for me only when there was lots of data but the subs were swamped with artifacts, data rejection was way to severe and hard to set. Otherwise 2% or so reject and average combine was better.
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  #26  
Old 30-10-2013, 07:15 PM
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The diminishing returns are related to the total exposure, not the number of subs (at least so long as your subs are long enough to make read noise insignificant).
So you think that the 21st sub will give the same noise reduction as the third? Personally regardless of the maths, top photographers are getting better results from longer subs and that is enough for me.

The table here shows clearly how each subexposure gives you a lower and lower snr gain regardless of sub length.
http://starizona.com/acb/ccd/advimagingdetail.aspx
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  #27  
Old 30-10-2013, 07:34 PM
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if of any interest Peter, modelling suggests a sub length for your system under dark sky of about 20 minutes. How does that compare with your experience?

I am very wary of the concept of diminishing returns for 2 reasons:
1. you restrict your exposure options without any consideration of the system design - scary
2. planetary imagers happily use 10,000 frames and up - each frame helps, even though any individual one could be left out with no practical effect
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  #28  
Old 30-10-2013, 08:08 PM
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Quote:
Originally Posted by Shiraz View Post
if of any interest Peter, modelling suggests a sub length for your system under dark sky of about 20 minutes. How does that compare with your experience?

I am very wary of the concept of diminishing returns for 2 reasons:
1. you restrict your exposure options without any consideration of the system design - scary
2. planetary imagers happily use 10,000 frames and up - each frame helps, even though any individual one could be left out with no practical effect
20 minutes seems about right actually, I am yet to experiment too much at a dark site. I have to admit being able to expose out to 30 minutes now successfully has really helped with narrowband imaging.

I guess the thing about diminishing returns is you still get a return, planetary images usually use more frames out of convenience I think. If I could somehow capture 10000 20 minute subs at 50fps I would too .
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  #29  
Old 30-10-2013, 08:32 PM
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Originally Posted by Peter.M View Post
20 minutes seems about right actually, I am yet to experiment too much at a dark site. I have to admit being able to expose out to 30 minutes now successfully has really helped with narrowband imaging.

.
that's comforting "as long as possible" for NB seems to be right as well - your helix is a beaut
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  #30  
Old 30-10-2013, 09:05 PM
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Originally Posted by Peter.M View Post
So you think that the 21st sub will give the same noise reduction as the third? Personally regardless of the maths, top photographers are getting better results from longer subs and that is enough for me.
I believe what I said... sqrt(10 x 10) = sqrt(5 x 20). Once you can ignore read noise the SNR from a stack of 10 x 10 minutes subs is the same as a stack of 5 x 20 minute subs. This is not inconsistent with the Starizona table.

For narrowband filters the number of photons collected is much smaller and longer subs are needed to minimize the effects of read noise.

There's nothing wrong with long subs but there's a point where the gain is small and the risk of losing a whole sub due to a cloud or a tracking error becomes more significant.
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  #31  
Old 30-10-2013, 10:25 PM
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Here are two threads where you should be able to get the full res raw fit data of 16 minute subs in 3nm NII i.e. Stacked and only corrected for darks and flats. 9 frames, 21 frames and 33 frames.

http://www.iceinspace.com.au/forum/s...d.php?t=111601

http://www.iceinspace.com.au/forum/s...=112594&page=2

Here is an animated gif showing the improvement from nine frames to twenty one.

http://d1355990.i49.quadrahosting.co.../lmc_9vs21.gif

In my opinion more frames would improve the signal to noise even over the 33 frame stack. Unfortunately weather always beats greed!

By the way 16 minutes at f/3 collects the same number of target photons per pixel as 44 minutes at f/5. With about a third of the thermal noise.

This thread has been very interesting so far.
Bert
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