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Bias or dark frames?

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11 replies to this topic

#1 Whereisclearsky

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Posted 28 November 2021 - 02:12 AM

Hi,

 

For now I am taking my lights with dark frames, flats and dark flats. 
Now I have seen someone saying that you should take darks for amp glow,

and bias for noise and hot pixels. 
 

Is that true?

 

My camera produces a lot of hot pixels without darks.

Are bias enough to fight that?

 

Thanks



#2 sharkmelley

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Posted 28 November 2021 - 02:23 AM

If you are taking lights & darks and you are taking flats & dark-flats then that's all you need assuming your sensor temperature is the same for the lights and darks.  The darks will fix both the amp glow and the pattern noise arising from DCNU (dark current non-uniformity).

 

In practice, your sensor temperature will not be the same for the lights and darks so you will want your stacking software to use dark scaling (if it's available) which uses mathematical optimisation to scale the dark so it removes the DCNU from a sensor whose temperature is different.  For this you will need bias frames in addition to your lights & darks.  But if there is any amp glow that is not fully characterised by bias then dark scaling will not be able to properly deal with the amp glow.

 

Mark


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#3 Whereisclearsky

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Posted 28 November 2021 - 02:28 AM

If you are taking lights & darks and you are taking flats & dark-flats then that's all you need assuming your sensor temperature is the same for the lights and darks.  The darks will fix both the amp glow and the pattern noise arising from DCNU (dark current non-uniformity).

 

In practice, your sensor temperature will not be the same for the lights and darks so you will want your stacking software to use dark scaling (if it's available) which uses mathematical optimisation to scale the dark so it removes the DCNU from a sensor whose temperature is different.  For this you will need bias frames in addition to your lights & darks.  But if there is any amp glow that is not fully characterised by bias then dark scaling will not be able to properly deal with the amp glow.

 

Mark

But if I have no amp glow, then if I take bias WITHOUT taking darks,

will it remove noise and hot pixels?

 

Darks are taking ages. 



#4 sharkmelley

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Posted 28 November 2021 - 02:51 AM

But if I have no amp glow, then if I take bias WITHOUT taking darks,

will it remove noise and hot pixels?

No a bias cannot remove the fixed pattern caused by DCNU.  You need darks to do that and it's the whole point of darks. 



#5 17.5Dob

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Posted 28 November 2021 - 03:07 AM

But if I have no amp glow, then if I take bias WITHOUT taking darks,
will it remove noise and hot pixels?

Darks are taking ages.


No,you still need darks....and they don't take any time at all, compared to shooting 6-12 hrs of lights....

I can shoot 20-5 minute darks in just over 1-1/2 hrs.

#6 TxStars

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Posted 28 November 2021 - 04:47 AM

When shooting your darks the temp is the only thing to worry about.

You dont have to be tracking/guiding, you can be at the grocery store or sleeping..  lol

 

Charles



#7 matt_astro_tx

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Posted 28 November 2021 - 07:19 AM

Yeah you need darks.  Just put the lens cap on at the end of the night and run em through.



#8 Whereisclearsky

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Posted 01 December 2021 - 11:04 PM

I was watching this

https://youtu.be/7aGmh6KsWUE?t=2743 (time stamp included)

 

And they said that for noise you should be taking bias, not darks.

This is what got me confused.



#9 sharkmelley

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Posted 02 December 2021 - 01:35 AM

I was watching this

https://youtu.be/7aGmh6KsWUE?t=2743 (time stamp included)

 

And they said that for noise you should be taking bias, not darks.

This is what got me confused.

Different contributors were saying different things in that live discussion, based on different cooled astro-cameras.  The only contributor who mentioned DSLR cameras said he always shoots bias, darks and flats. I think the main conclusion of that discussion was that on some cooled astro-cameras (without amp glow) you can get away without taking darks but you should first test it out on your own camera.



#10 galacticinsomnia

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Posted 02 December 2021 - 01:57 AM

It does get colorful for sure when it comes to processes. 
Using mirrorless or dslr, at least mine, and this specific target, I used no calibration frames in bortle 5-6 skies.

Your milage may vary, and I can't say you should always take them, or say you should never take them, but I do regularly opt for Flats, and Bias, and don't take darks unless I see it is really necessary to the image and the outcome I am looking for.  There is a lot of gray in interpreting what is good enough for you and what is good enough for others.

https://www.cloudyni...-edph-ii-again/

I like this one, and is about good enough for me. 

Good luck on your journey..

Clear Skies !!


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#11 Whereisclearsky

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Posted 04 December 2021 - 03:26 AM

In practice, your sensor temperature will not be the same for the lights and darks so you will want your stacking software to use dark scaling (if it's available) which uses mathematical optimisation to scale the dark so it removes the DCNU from a sensor whose temperature is different.  For this you will need bias frames in addition to your lights & darks.

Mark

So it's a good idea for me to capture bias as well?

 

I'm using Pixinsight.



#12 BQ Octantis

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Posted 05 December 2021 - 07:02 AM

Until you understand the limits of your processing skill and workflow, I would advise full calibration (including bias, which are the easiest of all calibration frames to take). Pixinsight can handle it.

 

Once you've actually experimented with removing calibration steps to evaluate the effect on your outcome, you can decide what steps are not worth the time or effort.

 

BQ


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