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Are darks worth the effort? Solved?


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A couple of weeks ago on this site I asked the question ‘Are DARKS worth the effort?’ The question arose since I could not see any difference between the final images I was creating whether I used DARKS or not, but I could not explain why.  Since then, I been busy trolling the web and playing with DARK files (actual and simulated) and finally today, I think I have an explanation to enable me to answer the question!

 

The first ‘eureka’ moment came after I had read this excellent article on noise in astrophotography:

 

http://www.nightanddayastrophotography.com/PDFs/Noise&Exposure.pdf

 

From that article, I finally realised what DARKs are really for – the removal of the image sensor fixed pattern coherent noise.  And that’s all they do, they don’t get rid of any of the other sources of camera random noise (in fact, if an insufficient number of darks are stacked, the resulting DARK MASTER will increase the random noise).  So, do I need to use them?  For my particular circumstances, I think the answer is NO.  I don’t want to go into the maths here (unless someone really wants me to) so I’ll just try to explain my reasoning.

 

If I my mount was perfectly polar aligned, each individual star in a particular LIGHT would be centred about exactly the same pixel in every other LIGHT and DSS (or whatever stacking program is used) would not need to apply any shift to the LIGHTS in order to align the stars.  That would mean that every pixel in a particular LIGHT would be stacked with exactly the same pixel in all the other LIGHTS.  If the LIGHTS contain fixed pattern coherent noise then, without using darks, the final image would also contain the same fixed pattern coherent noise irrespective of how many LIGHTS are stacked.

 

However, if my mount was not perfectly aligned, the stars will drift from LIGHT to LIGHT (and importantly each LIGHT has different drift).  That means that each pixel in a particular LIGHT will be stacked with a different pixel in every other LIGHT and that means that the fixed pattern coherent noise effectively becomes random noise and will be reduced as the number of stacked LIGHTS is increased.

 

Therefore, providing the image random noise is not significantly less than the fixed pattern noise, then using a reasonable number of LIGHTS means that DARKS are not required if there is drift between the individual LIGHTS.  As an example, if my LIGHTS have a measured noise of 50 ADU (taken at dark site, not from my house!) and the matching DARK MASTER has a measured noise of 25 ADU (typical for my 180 sec exposure at ISO1600 DARK MASTER using my camera) then 40 LIGHTS stacked using the DARK MASTER would have a measured noise of 8 ADU whereas the same 40 LIGHTS stacked without using the DARK MASTER would have a measured noise of 9 ADU.  If the difference between the LIGHTS noise and the DARK MASTER noise becomes even greater (as is the case for my sky background), then the difference between the resulting images becomes insignificant even when using a lot less LIGHTS.

 

This may have been perfectly well understood by everyone except me, but so far, I’ve not come across the explanation anywhere else (perhaps because it’s wrong?).

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i know some imagers don't bother with darks, and others use the bias frames as darks, there seems to be many, many ways to skin a cat in this hobby.  

Do you Dither Clive?? Since using dithering I have not done any Darks (well over a year now!). I do however use Flats and have noticed the difference by doing different stacks to check with and without Flats. Not sure this answers your question but it's just an observation I made..

 

Ron

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Thats is effectively dithereing.

If you have amp glow though esp on long exposures (mine shows on the 30 minute subs) then I would say darks are needed.

 

i do have a few hot pixels which the cone stamp tool sorts out.

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31 minutes ago, Ron Clarke said:

Do you Dither Clive?? Since using dithering I have not done any Darks (well over a year now!). I do however use Flats and have noticed the difference by doing different stacks to check with and without Flats. Not sure this answers your question but it's just an observation I made..

 

Ron

I don't dither at the moment but it would have a similar effect as a drift error so I guess that supports my argument. I always use flats and bias's but they are 'stock' images since using a DSLR and a fixed lens I don't have to worry about alignment effects.

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12 minutes ago, Ibbo said:

Thats is effectively dithereing.

If you have amp glow though esp on long exposures (mine shows on the 30 minute subs) then I would say darks are needed.

 

i do have a few hot pixels which the cone stamp tool sorts out.

I wish I could get 30 min subs! With my local level of sky pollution, 30 min would fully saturate every pixel. My recent image of the horse head nebula used the longest subs I've ever used. At just 5 min, the sky background was at 25% of the full well capacity.

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I am sure I would be the same with my 1000D.

Mono CCD and narrowband are a different beast.

I have not used 1000D for ages but I think I got amp glow with it , more modern DSLR's might be a lot better.

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