How to compensate for CLS filter

[sharptrack2]

G'day,

Please ignore the technical errors in the attached image. It was a quick grab of 7 frames (actually more, but only 7 were worth keeping). And I hope it uploads big enough to see at 200KB. If you can't tell it's Eta Carinae.

I've recently acquired an Astronomik 2" CLS filter to have a play with.

I'm reasonably happy with the output as far as getting longer exposures but am not happy with the blue/green tint that now has to be dealt with.

I normally set my white balance to daylight and get what I think are normal colours after processing. Now the whole image has this pale bluish tint that I cannot seem to compensate for. I did take a quick stab at one image in ACR and could negate it a bit with white balance adjustments, but then I would have to convert everything from RAW to TIFF, or FITS, to stack and I'm just not that ambitious at the moment.

So simple question is... can you compensate in camera or is the output always going to be missing a lot of red due to the filter response. The description suggests that it should be colour balanced so I didn't expect this.

Setup is Bresser Messier 127 f/5 on an HEQ5 Pro mount, no guiding. Imaging train is Nikon D7200 DSLR with 50mm extension, Baader fringe killer filter, and the CLS filter. Camera settings are WB daylight, ISO 800, 120 sec exposures. Focus was best that could be achieved using live view. Stacked with darks and bias only (thought... do I need flats to correct the colour?).

Thanks for taking time to read through!

[xelasnave]

Hi Kevin
I will leave it to others to answer your questions but I would like to take the opportunity to say that I think you have done very well and I like what you have produced.

I tried capturing it last night with and old canon with a cheap telephoto lens and got about 10 minutes which when processed looks black and white...but was happy to get even a little something done.
Alex

[redbeard]

“The description suggests that it should be colour balanced so I didn't expect this.“

I think that means that you will need to do the colour balance. I use auto colour balance in processing software to fix things like that.

Cheers,

Damien

[ChrisV]

Great question Kevin. I can't answer your question and I've always wondered about this.

The things that gets me is that the filter is used to reduce light pollution. Now if the camera is then white balanced isn't that effectively reversing the effect of the filter.

By the way I like your Eta Carina!!
And hope someone enlightens us all

[glend]

It is a good start Kevin. I would not worry to much about your camera color balance as it can be fixed in final processing. And no you don't need to worry about flats yet. That said, i used to run my Canon with a floro (4k) colour balance, but that was a modified ( LP1 filter removed) camera. If your stacking those 7 subs in DSS, don't mess with them first, get your final stacked RAW file and take it to your final processing software. I could tell you how to fix it in Photoshop but that won't help you. If you have an auto colour correction option try that, if you can manipulate the balance of RGB, just dial back the blue slighly. It should not affect the overall luminance. Some software has a box to tick to "preserve luminance" when making adjustments.
Good luck. Its amazing you could get what you did under that massive street light.
One other thing, you are stacking the fringe killer and the CLS, it is probably worth looking at the sprectum passed by each. The CLS seems to let through alot of Oiii (blue) and Ha (red) so your image is no surprise as looking like a narrowband image, over lay the fringe killer and have a look at what gets through. I think you might be ok with just the CLS, or at least try it to gauge the difference.

[skysurfer]

Nice picture.

The CLS filter blocks yellow color out completely, hence the bluish tinge.
But there are several ways to compensate it.

In PS you can make a color balance adjustment layer, with balancing cyan to red, magenta to green and yellow to blue.
And another option is using the hue/saturation adjustment layer with which you can tweak the hue, saturation and brightness of each color.
Do this in the final image, i.e. after stacking.

Or use the 'lonelyspeck' method with Photoshop. Some more work, but works very good, last week I did it with my Vela Supernovan Remnant image.

https://www.lonelyspeck.com/photographing-and-processing-the-constellation-orion-astrophotography-image-stacking-and-lrgb-processing/

[LewisM]

PixInsight did a pretty decent job at "fixing it", considering I had a tiny lossy JPEG to work with :)

Lots more hiding in that data as you can see!

I ran:
SCNR to remove the green, followed by BackgroundNeutralization to remove residual blue (though the stars remain a touch cyan), then a ScreenStretch loaded into HistogramTransformation to stretch it while maintaining the structure fairly well. Blew out the core a little, but you need to tweak the main files NOT to (a single JPEG is NOT the place to maintain structure well :lol:)

You CAN do the same in PS, but never really looks as good because of how PS handles the dynamic range IMHO.

[sharptrack2]

Thanks for everyone's input and comments!

So to elaborate a little more... I am using DSS 3.3.6 to stack and Photoshop CC to process.




That is similar to what I've done, would like to know a little more of how you would fix in PS. I used every tool I know how to use.:thanx:



I agree, experimentation might be required... for no other reason than it should be interesting and fun... as well as to find an answer.:thumbsup:

BTW, the Bresser is an Achromat, hence the fringe killer to knock back some of the chromatic aberration.




While not a PS guru by any stretch of the imagination, I did try some of your suggestion, which got me the image you see. I'll have read of the link you posted. :thanx:


Would anyone be interested in having a go at the original stack and give me some feedback on what they did?

[sharptrack2]

Thanks Lewis,

I figured there is more than what I could see... I'm just not that good at dragging it out yet.. plus, I'm a cheap old fart and have enough trouble working PS, let alone trying to learn a different app for Astro than what I use for my regular photos.:ashamed:

Happy to link up the original TIFF for anyone who wants a go at it. ;)

[glend]

I will give it a go Kevin.

[bigjoe]

Enjoyed your results Kevin..Re FRINGE KILLER...

Baader suggests that the Fringe-Killer combined with their Red filter is a good Hydrogen-Alpha filter.
This combination has a 95% transmission rate for the H-Alpha wavelength
, and yet costs very little...ALSO Baader FK blocks the infra red.

bigjoe.

[sharptrack2]

Thanks Joe, appreciate the comment and information. Having been visual only up to now, it's a whole new world to start learning about... should be "a gas" :P:lol:



Hi Glen, here you go... and anyone else who might have some time to spare... https://www.dropbox.com/sh/urktkap98fbgqvb/AAAN0hDIGfbHbpuBJyEn0QeDa?dl=0

[glend]

Kevin, I have sent back your re-processed file via We Transfer. As Lewis indicated below, holding onto the core detail is difficult when your stretching that data (and that is not just a PS thing looking at his). Still I think it looks better than Lewis' version, ha ha.
Best I could do.

[sharptrack2]

Thanks Glen,

I've had a look and agree with everyone's assessments... kinda hard not to when you guys are the experts!:thumbsup::rofl:

I think dedicated apps targeted for a specific genre will have advantages but the end result will always be the "artist's perception". I'm absolutely positive we could have a very robust discussion and competition around what is "Best"

Now to keep practising until I can get something similar to what you have shown me! ;):thumbsup::D

I'm hoping tonight might clear soon enough to try again... maybe experiment with just the CLS filter and just the FK filter.