Struggling with getting neutral background. Need help

[monty]

Hi all, I am new to PixInsight, but have been practicing and going through various tutorials online. But I have been having a hard time getting a neutral background for one of my stacks of the Pleiades cluster. Here is what I have captured and used.

Skywatcher Esprit 100 Triplet APO
Full Spectrum modified Canon T5i
Astronomik CLS-CCD clip in filter
55x300sec Lights
30x300sec Dark
30 Flats using EL Panel
Master Bias and Super Bias created using 360 images.

I did the image calibration and integration manually and did not use the Batch Processing. I followed the various tutorials from IP4AP to Light Vortex. (These tutorials have been amazing). But after stacking I cannot seem to get a neutral background. The images seems to have a reddish glow/tint in the background outside of the main nebulosity of the cluster (when viewing using STF). There is a slight greenish color cast also, but when I use SCNR to remove that, it amplifies the red more. I have attached 3 images. The first one is the direct output after Drizzle Integration. Second one is after DBE, and third is with DBE, Background Neutralization and color calibration. I would really appreciate any advice you guys can give. Also based on the image stack, could the issue be not enough lights to increase my SNR ?

Image 1 - Drizzle Integration https://drive.google.com/file/d/0B5CVPfPexNw_RFlwU09PMWtuaEU/view?usp=sharing
Image 2 - DBE https://drive.google.com/file/d/0B5CVPfPexNw_empPWlBITE96OGc/view?usp=sharing
Image 3 - DBE - BN - CC https://drive.google.com/file/d/0B5CVPfPexNw_c3RnaVZXbnVIYms/view?usp=sharing

Monty

[lucchett]

Hi, i think you have a problem with DBE and the green channel.
if you check teh histogram you can see that the DBE Image is not right.
I have just jumped DBE,  and run BN & CC your image.
Last, I had to apply SCNR on green channel and, besides the gradients the image  calibrates.

the first thing I would do is to check how do you apply DBE.
the second thing is to go back to the pre-processing phase and ensure you prepare the data in the right way.
my feeling is that something could be improved, as the green channel (or teh red) is a bit problematic.

Hope it helps
Andrea

[monty]

Thanks for the input Andrea.
I tried doing various different settings in DBE, but got the same results. I will try again more drastic tests on different channels.

Now when you say check the pre-processing, are you talking about the integration phase ? I went through that few times to make sure I did everything correctly. Based on data I uploaded, is there any particular phase of the pre-processing that you could advice me to re-check ? I followed the tutorial on LightVortex to the word. I even tried a small setting on the Dark Optimization based on a methodology by Juan in one of his post. Three other tests I did during lights integration are

1) Did not use the new Local Normalization
2) Used local Normalization using the best SNR sub as a source
3) Used local Normalization by using the best SNR sub after doing a stretch on it as a source.

But even for each attempt I got similar results in the background.

Monty

[monty]

So after various effort I had to give up on the Pleiades cluster stack. To figure out if the issue was with my subs or camera or CLS CCD filter, I tried out the same stacking and DBE process on my M31 stack. The M31 stack is of 36x300" subs calibrated with 30+ flats, darks and superbias. This item around the background was much more neutral. It still had a slight red and green hue in the background, but that can be easily fixed with further processing.

But now I see another issue that popped up with both my stacks, is the large stars have red glow and small stars have turquois glow. Could this be an issue with CSL CCD filter or focus issue or something else ? I had much harder time trying to fix this in post. One thing I did notice when I did channel separation that the stars in the red channel were slightly bigger. What could cause this ?

Here is the stacked, DBE, BN and CC image of the M31

https://drive.google.com/file/d/1eiQ7KOK9dyd55L6FSjse8_SxtVNi2kdz/view?usp=sharing

Any help would be greatly appreciated.

Monty

[lucchett]

Hi Monty,
Sorry for the late reply.
I meant to check calibration.
Check histogram before and after calibration to understand if everything is ok.
Andrea

[Alejandro Tombolini]

Hi Monty, it seems that the red channel is stretched with respect to the green and blue. You should check the integration of the red channel. I have applied LinearFit process with green as reference to both channels and then BackgroundNeutralization and i looks fine.
Hope this helps
Saludos, Alejandro.

[monty]

Thanks for the tip Alejandro. I will try that out. Since I am using DSLR image, I don't think I can do separate integration of the red channel. (Unless I am missing something). Now when you do Linear Fit, what criteria should be used to determine which channel should be used as a reference ? Also what settings did you use above for Background Neutralization ?

Also any input on the cause of the red glow on the large stars ? Even on the image you modified with Linear Fit has the red glow.
Thanks again for all your help guys.

Monty

[Alejandro Tombolini]

Hi Monty,

Since I am using DSLR image, I don't think I can do separate integration of the red channel. (Unless I am missing something).

Interesting. If you separate and compare the channels in the image you have shared you will notice that the red is different. As a test, please check the same in an individual frame (debayer it without calibration and compare if the red is also different to green and blue

Now when you do Linear Fit, what criteria should be used to determine which channel should be used as a reference?

You can choose any channel and the rest will be linear fitted to the reference.

Also what settings did you use above for Background Neutralization ?

In this video you can see how I used to determinate the values

Also any input on the cause of the red glow on the large stars ?

I think that you may have a problem with the optic. As a reference in the last image of the 2017 Solar Eclipse we used two equipment, in one of them (Canon 5DIII and lens Sigma 50-500 used in 500 mm with 1.4 adapter we had a perfect focus in red, quiet bad in green and extremely bad in blue).

Saludos, Alejandro.

[monty]

So I looked at the individual frame before and after calibration. In the before calibration frame, the red is shifted a bit, but not much. But the shift is a lot higher after calibration. (I am assuming this due to the Astronomik CSL CCD filter).

Sub before calibration https://drive.google.com/file/d/1lSFHYWN-Z8lMWgFKNNqFQHqabmxKfTzw/view?usp=sharing
Sub after calibration https://drive.google.com/file/d/1-iCiU7vFVwv5wx34Z85WJdmrGaQxM1cq/view?usp=sharing

The optics I am using is the refractor Skywatcher Esprit 100 APO. Its suppose to be a very good triplet APO for astrophotography and includes a flatnner. My camera is a full spectrum modified Canon T5i with the Astronomik CSL CCD filter. I doubt it would be the APO. I have my doubts on the cls ccd filter, but then I have seen some really good results other people have gotten on same images with the same camera and filter setup.

Monty