Astrophotography Stacking Affinity Photo 2 and GBRG Green Cast on Images

[Ruyton]

No matter what I select in the RAW processing options for astro stacking, the stacked images have a strong green cast. These GBRG FITS images are from the Dwarf II smart telescope.

I also use Siril and Pixinsight and both of these programmes have an 'unlink' option that removes the green cast. At present, Affinity Photo 2 is unusable for me.

Maybe I'm missing something here?

[NathanC]

Hi @Ruyton,

Could we take a look at the FITS files you're using for the stack? We can then look into what options may be available. Since the files are likely to be too large to upload to the forums I've included a private upload link below.

https://www.dropbox.com/request/ZacuMSd7FHR9bmIRL4cP

Many thanks

[Ruyton]

Hi there, thanks for getting back to me.

I've uploaded a zip file to the dropbox link containing all the FITS files for the session plus the json file that shows in a text editor the shooting details. The stacked images produced internally by the Dwarf 2 are also in the zip file.

[NathanC]

Hi @Ruyton,

Thanks for sending over the files, I'm in the process of discussing your problem with one of our product experts internally, but we've found that when stacking the existing 115 FITS we're not getting a good result due to the number of hot pixels which Photo is interpreting as stars, resulting in much of the image content being lost during the alignment process. This is resulting in a drastically different final image to what's shown in your original screenshot.

Are there any calibration frames (Dark, Bias etc.) that you may have missed in the .ZIP file you provided to us?

Thanks

 

[Ruyton]

Hi there, I didn't include any darks as my issue was more with the green hue rather than the actual stacking in Affinity Photo 2.

However, I've uploaded a Darks_Arcive.zip file to the Dropbox link you posted above. I think the Fits file I sent were shot at 2x2 binning, but I've included the 1x1 binning darks in the zip file too.

Please note that the Dwarf shoots GBRG and it seems to be the final G that's causing the issue. No matter what RAW setting I choose, the GBRG option doesn't seem to have any effect.

Stacking those files in Siril and Pixinsight does work well, bearing in mind the limitation of the Dwarf 2 images - they are never going to be high definition

Thanks again for looking into this.

[James Ritson]

Hi @Ruyton, thank you for providing the files so I could experiment with the full stacked result.

From a bit of reading on various forums (PixInsight, Cloudy Nights etc), there are generally a few things to try in order to correct the green cast including an unlinked screen transfer function, photometric colour calibration, background neutralisation, SCNR etc.

I have cobbled together a workflow that does the trick, although it does involve using my astrophotography macros (free download, link here). I've attached a screen recording to this post, and here are the steps:

• Delete the Curves and Levels adjustments (these are providing rudimentary tone stretching, but we are going to use a colour preserving stretch instead)
• Run the SCNR Green Max/Additive macro. This will mostly neutralise the green colour cast
• Run the Colour Preserving Tone Stretch macro. This will colour balance and produce a decent stretched output
• Now use Filters>Astrophotography>Remove Background and sample tones until you neutralise the background. In your data there is a gradient so this requires slightly more work

Without the macros, you could achieve a similar result with a bit of manual work. I would use Remove Background on the Stacked Image 1 layer (sampling multiple points to remove the gradient), tweak the Curves adjustment to avoid blowing out the core of the nebula, then use a White Balance adjustment above Stacked Image 1 but below the other adjustments. Moving the Tint slider towards a magenta bias rather than green will help eliminate the green colour cast.

Hope the above helps! Here is the screen recording:

 

 

[Ruyton]

@James Ritson thank you so much for that, that does work very well indeed. I did stumble across your website and excellent macro package the other day, but I haven't really got to grips with it yet so your video is a great help.

I've had a quick look at your website macro video, but now I need to take a look at it in slow time so that I can get maximum use of the macros.

Thank you again, the learning curve continues and my processed images slowly get better

 

[jkmozzz]

Hello,

I've been testing Affinity the last couple of days with my Dwarf II. I too, have been getting the green screen. I very much appreciate your hard work to put together the macros to help process our images better. I am planning on giving it a try in the next couple of days. I'm hoping that Affinity will be my go-to versus Siril.

Any feedback you could give me on the comparison between the two softwares would be appreciated.

Thank you, John