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Rainbow colour cast processing issue

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#1 NorthSideBound

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Posted 16 September 2020 - 06:51 PM

Brand new to this forum and looking for guidance! I recently purchased an STC Optics clip-in filter hoping to cut through the light pollution in my Bortle 7 backyard but in stretching my DSS stacked image, I've come across a rainbow colour cast that I don't know how to get rid of. So far have not found a solution online. It may be that the filter wasn't placed correctly.

 

Setup: Fuji Xt1 w/50-140mm f2.8 at 140mm f5.6 ISO1600, STC multispectra clip-in filter, Skyguider Pro.

Imaging: 67x70s Lights (57 minutes exposure), 66 Darks, 81 Flats, 81 Dark Flats, 71 Bias.

Target: North America nebula (NGC 7000)

 

 

 

pUfifG5.png?1


Edited by NorthSideBound, 17 September 2020 - 07:12 AM.


#2 rkinnett

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Posted 16 September 2020 - 08:17 PM

Try stretching your stacked flat and see if that has the rainbow pattern in it.  How are you capturing your flats?



#3 rkinnett

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Posted 16 September 2020 - 08:24 PM

Welcome to CN!


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#4 NorthSideBound

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Posted 17 September 2020 - 03:12 PM

I'm capturing my flats using the white cloth method pointed to a white tablet screen with the clip-in filter still in place with a normal distribution on the histogram.  

 

No apparent rainbow pattern as shown below:

zHSWhrG.png?1


Edited by NorthSideBound, 17 September 2020 - 03:17 PM.


#5 rkinnett

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Posted 17 September 2020 - 03:48 PM

I sometimes see rainbow patterns due to surface diffraction effects from glossy light panels, but that's not what's happening here, per your description.

 

What do your other stacked cal frames look like stretched?



#6 NorthSideBound

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Posted 18 September 2020 - 09:45 AM

I've stretched the darks and bias and all I get is noise and hot/dead/cool pixels. I'm almost 100% sure its the filter as stretching some raw lights started showing hints of the rainbow cast

 

I'll be imaging tonight, hopefully its just a matter of properly securing the clip-in filter in place. 



#7 NorthSideBound

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Posted 19 September 2020 - 07:32 PM

Update: I went for a widefield shot and made sure the clip-in filter is secure this time and the stretched image produced a more expected/normal colour cast vignette smile.gif *phew*. I'll be trying a closer focal length on my next image session and hoping for similar results without the rainbow!

 

 DvqpUs4.png?1

 

I did a gaussian blur layer subtraction to get rid of the cast and my final image after processing looks like this: https://i.imgur.com/HtLzXRN.jpg.

Any tips for improvement is welcome! smile.gif




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