Darkframe scaling  

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(@minusman)
Red Giant Customer
Joined:1 year  ago
Posts: 73
May 24, 2018 10:31  

Hello Mabula, first big praise to the new version. The multichannel and session function succeeded. A question about the new feature Darkframe Scaling. Do the Darks then have to match the ISO and exposure time to the lights and only the temperature can differ?


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(@mabula-haverkamp-administrator)
Quasar Admin
Joined:1 year  ago
Posts: 1632
May 24, 2018 19:17  

Hi @minusman,

Thank you very much, it's my pleasure 😉

The dark frame scaling requires a masterbias(or bias frames which will be converted to a MasterBias) to be loaded besides a masterdark (or darks which will be converted to a MasterDark).

The scaling should correct for any deviation to the light frames, as a result of different:

  • exposure time
  • temperature
  • iso or gain

values. So a masterdark of iso 200 shot at room temperature with sub exposures of 5 minutes can be used on light frames, with iso 800, temperature below zero..., and exposures of 15 minutes for example.

For the actual calculation, the differences in ISO/gain values and exposure times are used, as a first scale estimate. So the calculation has a reasonable starting point and should then correct for the actual temperature difference as well.

Kind regards,

Mabula

Main developer of Astro Pixel Processor and owner of Aries Productions


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(@minusman)
Red Giant Customer
Joined:1 year  ago
Posts: 73
May 25, 2018 10:21  

Can theoretically, the Darks then take during the day. And not necessarily at night during the recording session. Am I right?


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(@mabula-haverkamp-administrator)
Quasar Admin
Joined:1 year  ago
Posts: 1632
May 25, 2018 20:25  

Hi @minusman, yes indeed 😉

That should perfectly be  possible. And you want to create a very good quality masterdark in that case. So make one consisting of a lot of darks so the masterdark will have very little noise.

A masterdark of 100 frames of 5minutes exposure time will be a lot better (less random noise) than a masterdark of only 10 frames of 5 minutes.

The random noise in the masterdark will always be injected in your light frames, so by using more darks, this unwanted noise injection will be reduced 😉

Kind regards,

 

Main developer of Astro Pixel Processor and owner of Aries Productions


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(@minusman)
Red Giant Customer
Joined:1 year  ago
Posts: 73
May 25, 2018 20:52  

Thanks for the info, Mabula. That sounds good. 😀 


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(@mabula-haverkamp-administrator)
Quasar Admin
Joined:1 year  ago
Posts: 1632
May 28, 2018 14:58  

@minusman, let me know your experiences with the dark scaling 😉

Mabula

Main developer of Astro Pixel Processor and owner of Aries Productions


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(@minusman)
Red Giant Customer
Joined:1 year  ago
Posts: 73
June 1, 2018 16:03  

Hello Mabula, I did a test without dark frames.

Screenshot (46)
Screenshot (38)
Screenshot (37)

And with dark frames. The SNR drops slightly but the noise also drops.

Screenshot (47)
Screenshot (40)
Screenshot (39)

What I've noticed, at LN Mad Winsor clip are cut off parts of the image and RejectionMap looks strange. The same thing happens with other records as well.

Screenshot (48)
Screenshot (44)
Screenshot (41)

 


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(@mabula-haverkamp-administrator)
Quasar Admin
Joined:1 year  ago
Posts: 1632
June 9, 2018 21:46  

@minusman,

Thanks a lot for these results.

Regarding the noise of the integration with/without darks. The lower noise result, also has lower scale/dispersion. These quintities are related. If you were to reduce dispersion with a factor 2, noise drops with the same factor. And the difference in dispersion could be due to APP having chosen another reference frame I think to which it will normalize the dispersion. The LNC has influence on the SNR as well. So it's hard to tell what the difference really is.

But I think, based on the SNR results, the integration with darks is very good. Possibly only a slight noise injection of 1-2%  assuming the Signal of 2 stacks would be equal. You did use the dark frame scaling in this case or not?

Yes, i see the problem in the rejection map, I need to fix that, I have seen it myself in some data.

Thanks again 😉

Mabula

 

Main developer of Astro Pixel Processor and owner of Aries Productions


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(@minusman)
Red Giant Customer
Joined:1 year  ago
Posts: 73
June 10, 2018 15:08  

Yes that's right, APP has determined the reference frame itself. I activated the Darkframe Scaling on the Darks. I'm currently testing to calibrate to remove the light pollution. Then the integration looks even better. Makes only a lot of work a lot of frames.


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(@minusman)
Red Giant Customer
Joined:1 year  ago
Posts: 73
June 10, 2018 15:10  

What does dispersion mean?


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(@mabula-haverkamp-administrator)
Quasar Admin
Joined:1 year  ago
Posts: 1632
June 16, 2018 20:56  
Posted by: minusman

What does dispersion mean?

@minusman,

Dispersion is a measure of scale of a distribution. See this wikipedia:

https://en.wikipedia.org/wiki/Statistical_dispersion

So it indicates (more or less) the width of the histogram peak in most of our images.

The median, is the the central value or peak of the histogram and the dipersion is how the data is spread around this central value.

Image from provided wikipedia URL

Cheers,

Mabula

Main developer of Astro Pixel Processor and owner of Aries Productions


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