30 July 2020 - APP 1.083-beta1 has been released introducing Comet processing! This 1st beta has comet registration. The stable release will also include special comet integration modes.
9 July 2020 - New and updated video tutorial using APP 1.081: Complete LRGB Tutorial of NGC292, The Small Magellanic Cloud by Christian Sasse (iTelescope.net) and Mabula Haverkamp
2019 September: Astro Pixel Processor and iTelescope.net celebrate a new Partnership!
Correcting a strong Blue/Violet Cast
Hi Mabula/APP users,
I wonder if anyone can offer some advice on how best to correct a strong blue/magenta cast that I am getting while processing an HaLRGB image of Crescent Nebula.
Attached are screenshots of my APP work in progress a) after Light Pollution Correction and b) Star Color correction.
After LPC I think the image is looking promising although it clearly needs some further work to improve the color.
During CSC however the image has taken on a strong Blue/Volet cast although the star colors look about right to my eyes and also according to tbe color balance charts as I understand them.
My problem is that with APP's HSL selective color module I am unable to correct the background colors to get anywhere near an acceptable result.
My questions therefore are:
a) Is it possible to use HSL module to reasonably correct the color cast and if so how should the problem be approached?
b) If HSL is not able to make a sufficient correction what should I be looking to achieve at the (?) RGB combine stage in order to get a better starting point for subsequent adjustment?
I am reprocessing this image after acquiring some extra data so I know with effort a resonable is result is achievable although even with the below image I had to resorted to Photoshop for final substantial color balancing.
I am happy to provide my FITS files if anyone wishes to have a try.
That might be a good idea (to share the files), the star colour graph looks really weird and there seems to be some over representation of green in the data.
Yes, I see the issue with the data. My first guess would be that the HaLRGB composite actually is the problem. If I look at the histogram in the Remove Ligh Pollution stage. Green versus Red+Blue is rather unbalanced and this most likely leading to the issue in star color calibration.
The star color calibration Black Body models need broadband data to work well, but you have supplied broadband with R,G,B data. So I suspect the issue is to be found in how the Ha data is actually added to the composite.
Regarding HSL selective color: it's needed for final small tweaks mostly. We first should get a better result at the star color calibration stage I feel... before continuing with processing.
I would be happy to have a look at the data, please send the integrations per channel to me. I will then have a look to find a possible issue and otherwise suggest a better workflow 😉
You can send the data to firstname.lastname@example.org using wetransfer.com or dropbox 😉
And I agree, the data itself looks promising, the nebulosity looks rich in signal.
Hi Mike @mestutters ,
Thank you for sending me the data.
When I load the R,G,B channels into the RGB Combine tool, I immediately get an odd result I think.
This is just with R,G,B multipliers at 1.
The magenta cast comes from Red and Blue dominating the green channel. The nebulosity is magenta everywhere which is odd.
If we look at the Red, Green and Blue channels:
We can see why, the nebulosity and Cresent is visible in Red AND Blue, not in Green.
I would almost think that somehow red and blue data was mixed in the final red and blue integrations.
The brightest stars in the red channel are also the brightest stars in the blue channel it seems and there is a big difference in quality as well.
Perhaps to investigate further, can you share some individual red, green and blue frames ? Maybe that will give a better understanding of what is happening here 😉
Thank you for taking a look at my earlier attempts at Crescent nebula image and for your suggestion as to what might be wrong. I took a close look at my all my subs and found that amongst my Blue frames I had several (c30% of total) that looked as if they had actually been recorded through Green filter rather than Blue. Not sure how that happened but an operator error I guess!
Anyway I managed to capture a few extra Blue subs the other evening to replace the faulty ones and have had another shot at processing. The result now looks rather better but on the B-G vs G-R chart the star profiles seem to have a negative slope. I think that this cannot be right although on other images I have done I generally have to set the slope value to near the lowest settings available.
I hope I have not made another silly mistake but I would much appreciate your view as to where I might still be going wrong.
I will very shortly forward my latest integration frames and examples of my HaLRGB frames.
PS : I've just enjoyed viewing your AstroImaging Channel talk and am looking forward to seeing the further improvements you have planned for APP. Hope you very much enjoy your holiday too.
Please consider this problem closed. I managed to capture a few more subs on this target and was then able to carefully compare what I had with the new results. After weeding out a few more older subs that looked suspisiously as if they too were incorrectly labelled I am now getting a result in CSC that looks about right.
Thanks for your time and patience
Was just about trying to be of assistance. 🙂 Glad it worked out ok!