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Different Colors after processing?

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#1 Dane90

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Posted 17 August 2019 - 04:55 AM

Hi,

 

So I was at a campsite in a bortle 3 zone last week, and it got nice and dark when the moon had set.

While waiting for the moon to set I decided to shoot NGC7000 - the North America nebula with my unmodded Canon 800D.

 

I have about 30 minutes of integration time on the subject, consisting of 5 minute subs.

I did a quick stretch on a single sub and noticed nice blue and red colors in the image, and I was happy before moving on to the next target.

The first picture is a quick stretch of a single sub.

 

When I got home, I stacked my images, added my flats, darks, and bias frames and started editing.

After a while I realized I couldn't pull out the blue colors, as can be seen in the second picture.

 

After a little bit of investigation I noticed my flat frames had a slightly blue color to them.

I stacked my frames without these flats, and managed to get the blue color back, as can be seen in the third picture.

 

My question is, how can I know what colors are the most accurate?

If I look around for this nebula online I see a lot of red/orange images, and barely any images with blue in them.

 

Is the blue simply an artifact of poor white balancing?

Did I make a mistake while recording my flat frames?

It would really help me to get some feedback on this, and maybe a few tips which could help me with this in processing.

Attached Thumbnails

  • Differences_Small.jpg

Edited by Dane90, 17 August 2019 - 04:56 AM.


#2 Tapio

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Posted 17 August 2019 - 05:37 AM

How did you take your flats?

#3 Dane90

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Posted 17 August 2019 - 05:44 AM

I used a white T-shirt and some rubber bands to stick it to the end of my telescope without any folds.

The telescope was inside my tent, but it was light enough outside to capture the vignetting gradient.



#4 sharkmelley

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Posted 17 August 2019 - 06:11 AM

A colour cast in the flats is a common problem. Measure the RGB ratios in your bias-subtracted flat.  The white balance to neutralise this needs to be applied to your stacked image as well as the standard white balance for the 800D.

 

Don't be afraid of some blue/purple.  A lot of images of NGC700 you find on the web have a very strong H-alpha component (from a narrowband filter) and so appear very red because other emissions such as the blue of H-beta are suppressed.

 

Mark


Edited by sharkmelley, 17 August 2019 - 06:17 AM.


#5 Dane90

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Posted 17 August 2019 - 06:19 AM

The white balance to neutralise this needs to be applied to your stacked image as well as the standard white balance for the 800D.

I'm sorry, do you mean that the white balance to correct my flats needs to be applied to the standard white balance for the 800D? Or the white balance for the 800D needs to be applied to the stacked image in addition to the white balance correction from the flats?

English is not my native language, and I am not sure how to interpret that sentence, haha.



#6 sharkmelley

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Posted 18 August 2019 - 12:51 AM

I'm sorry, do you mean that the white balance to correct my flats needs to be applied to the standard white balance for the 800D? Or the white balance for the 800D needs to be applied to the stacked image in addition to the white balance correction from the flats?

English is not my native language, and I am not sure how to interpret that sentence, haha.

No worries!  I'll describe it in a different way.

 

Suppose the centre of your bias-subtracted master flat has the following values:

  • R   800
  • G 1200
  • B 1500

In this example, the red channel is weak so dividing by the flat will boost the red too much. So in your stacked image,  you will need to apply the following multipliers to each colour channel to remove the colour cast caused by of the flat. 

  • R    800/1500
  • G  1200/1500
  • B  1500/1500

However you may not need to do this because some software does this automatically when calibrating with a master flat.

 

 

You then apply the standard white balance multipliers for the 800D to your stacked image, I don't know what they are but they might be something like this:

  • R  2.1
  • G  1.0
  • B  1.75

Mark


Edited by sharkmelley, 18 August 2019 - 12:54 AM.


#7 Dane90

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Posted 19 August 2019 - 02:32 AM

No worries!  I'll describe it in a different way.

 

Suppose the centre of your bias-subtracted master flat has the following values:

  • R   800
  • G 1200
  • B 1500

In this example, the red channel is weak so dividing by the flat will boost the red too much. So in your stacked image,  you will need to apply the following multipliers to each colour channel to remove the colour cast caused by of the flat. 

  • R    800/1500
  • G  1200/1500
  • B  1500/1500

However you may not need to do this because some software does this automatically when calibrating with a master flat.

 

 

You then apply the standard white balance multipliers for the 800D to your stacked image, I don't know what they are but they might be something like this:

  • R  2.1
  • G  1.0
  • B  1.75

Mark

Thanks Mark, this makes everything very clear!

I'll have a go at this later this week, when I have a day off.




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