True star colours with LP filters

[Andy01]

Hi everyone,
Does anyone have a solution for keeping true star colours when using LP filters in the 'burbs?
I use an IDAS filter on my OSC CCD camera and it really messes with the colours. Quite a challenge to process as a result. Thoughts?

Cheers,
Andy

[cometcatcher]

Mmm, the IDAS filters are usually one of the least colour shifted brands.

The Baader Semi-Apo filter is pretty good with minimal shift as should be the Moon and Skyglow filter. The more narrow band the LP filter the more colour shift. Something has to give with big chunks of the spectrum taken out. Usually LP filters are for nebula with the stars and their colour being sacrificed to some extent.

[SimmoW]

Hey Andy, couldn't you just note what main stars were white and then just do a white balance adjustment against that star in PS later? Or is it harder than it sounds?!

[alistairsam]

I have an IDAS LPS-P2 and the colours are well balanced and I hardly need to make any adjustments.
I've tried it on a QHY8 OSC, QHY8L OSC and a dslr.
do you have your debayering settings right? what software do you use to process and can you post a raw non-debayered and debayered sub?

Cheers
Alistair

[alistairsam]

here are two images with a dslr or qhy8, can't remember, but it was an OSC.
60s raw without the IDAS and with the filter.
unprocessed.

[nebulosity.]

Gee that does a great job :thumbsup:

[SimmoW]

yes that's a great comparison! That'll have to be one of my first accessory purchases.

And strange, doesn't seem to wreck the star colours, not that this is a perfect example.

[alpal]

Interesting.

I use an Astronomik CLS-CCD 2" filter & seem to get quite good star colours
after balancing the 3 RGB humps in photoshop.
I use it in front of all 5 LRGBHa filters.

see here:
http://www.astronomik.com/en/photographic-filters/cls-ccd-filter.html

[cometcatcher]

Could you post an unprocessed example Andrew?

[Andy01]

Hi everyone,
Thanks for your considered replies, sorry to be getting back to you so late :)

I've attached a before and after post production image of Omega Centauri that I shot a few night ago. 30x5 min RGB from the SBIG 8300c with the IDAS filter in place. (Yes I just missed the focus, d'oh!)

I process with Nebulosity. The first image is basically out of the stack with levels and sharpening applied. The second has been tweaked, massaged leveled and colour balanced as best I can, minimizing the cyan and enhanceing the yellows stars.

It's never going to match the colour result as shooting under dark skies, but I am concerned about the oversaturation of cyan and desaturation of yellow and orange that result from the IDAS - it seems particularly harsh on yellow stars.

Has no-one else encountered this effect? :shrug:

Cheers

Andy

[alocky]

I've noticed exactly the same effect with both DSLR and mono CCD, it's probably because one of the key contributors to light pollution are yellow sodium vapour lamps, and the stars are a continuum source, so it gets hit too.
I've yet to be convinced that anyone can produce nice yellow stars with data from these filters, despite some heroic efforts on this forum.
Cheers,
Andrew

[alpal]

Using LAB mode in Photoshop I balanced & boosted the colours.
Plenty of yellow came out.

see link for Louie's video:
http://www.youtube.com/watch?v=fE2GS_87Ct8

I don't know Pixinsight but you need to work in the CIELAB colour space.

see pic which is modified from your first pic.

[Andy01]

Hey Alpal that's an interesting result. :)
Hmmm... what was your workflow in PS for that?

cheers

AC

[alpal]

I just added that info to my post

[alocky]

That's a significant improvement. I will try that on some of my older data. You can do the same thing in Pixinsight using the curves transformation tool very simply. I've been fixing my reds using this method ever since Alpal demonstrated it on an LMC image.
Thanks for sharing!

[alpal]

Thanks.
Yes - there is whole thread about it in this section of the forum:

http://www.iceinspace.com.au/forum/showthread.php?t=116632

[Andy01]

Cheers Alpal - gotta love PS - 1000 different ways to do stuff :) Andy

[alpal]

True -
make sure you thoroughly know all of Louie's videos here:

http://www.youtube.com/channel/UCJ5b6pFHBGe66vsuSaXb-0A?feature=watch

I consider that everyone of them is essential.
I always work in layers now.

cheers
Allan