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I've been imaging Thor's Helmet for the last couple of nights in Ha and O3. I notice that RGB data can be used to correct star colour. What exposure is recommended to capture the stars and how many should I take? It looks as if I've one more clear night for the time being.

Thanks

Anne

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I'd be interested to hear what others do here too.  I have tried it a few times and only take short colour exposures for RGB stars i.e. 150 binned x 2.  However I don't consistently have success in adding them to the NB image.  Sometimes it works and sometimes it doesn't.  I register the images and then select and feather the stars on the RGB and then paste them onto the NB image and then use layers.  This is when it only sometimes works, as I have used blend mode lighten when it has worked.  If I don't use a blend mode then it doesn't work very well as the stars look artificial.  

Carole 

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Here is how would I do it, but do take into account that I've never done it :D

You need a good star mask.

For recommended exposure - go for one that will not saturate stars, even brightest ones. For number of exposures - same thing holds as always when imaging - as much as you can afford within your imaging budget.

Stack everything into corresponding stacks, make sure it's properly aligned to same reference frame.

Do your NB composure / channel mixing and stretch. Do the same with RGB - compose image, stretch and do color balance. Don't worry about star bloat in RGB or if you stretched to far (visible noise next to stars is not important), only thing that you need to worry is star color at this point - make it properly balanced and "rich" (to your liking).

Next steps I can't explain in terms of PS steps, but I'll list them in general - each processing software will have its own way of doing it.

Make a copy of NB image and split it into LAB components. Save L component of split (as mono image), or just keep it open - that is what we are going to need.

Take RGB image and do the same - split into LAB components, but keep A and B, discard L from this one.

Recompose RGB image from LAB by using L component saved from NB image and AB components from your RGB image.

Use star mask to blend this recomposed RGB in with original NB image.

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Similar to what I do Vlaiv, but I don't know of any Lab component in PS, only lab anything I know is lab colour, but I am sure that si completely different.

Carole 

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Nope - tis the same thing... Photoshop can operate in LAB color space and split into Lightness, A (Magenta/Green), B (Blue/Yellow).

Should be under Edit > Convert to Profile > LAB Color (or something like that iirc - word of warning: do this on a finished flattened image, as it'll flatten it automatically as part of the conversion). Usually I do this on a copy of the final component images, to pick up the L layer, and then drop it on top layer back in my regular RGB composite and set layer blending to Luminance.

Edited by Marci
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Firstly I would only add star colour to a NB image which was aiming to approximate natural nebular colour in the first place, probably HOO.

Rather than try to 'fill in' the star colour on the HOO you could use Photoshop like this:

1) Process the RGB, the Ha and the OIII and align them to fit each other. In processing the RGB concentrate on the stars, keeping them small and colourful and don't worry about the nebula which will be coming from the NB layers.

2) Add Ha to red in blend mode lighten, making sure your Ha stars are smaller and fainter than your red stars. (Use multiple iterations of a star reduction routine like Noel's actions or whatever on the NB layers.) save three copies of this. Call this Ha to red.

3) Add OIII to green in blend mode lighten in one of the copies and call it OIII to green.

4) Add OIII to blue in blend mode lighten in another copy and call it OIII to blue.

5) Make a three-layer stack like this:

OIII to blue

OIII to green

Ha to red.

Now you can choose opacities for the top two layers which give you the most natural nebular colour. Your RGB star colour should be unaffected.  As you can see, this method does not work by replacing star colour so it gives a more natural look, yet the NB contribution to the nebulosity is the same.

Olly

 

 

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Thanks for all the replies, that will give me something to think about. Unfortunately it looks as if it won't be clear enough tonight, so I can't take the RGB subs! As it's Thor's Helmet, I'm running out of time to image it as it's so low here. My image is HOO so I can try Ollie's method eventually, it looks a lot easier than removing stars in PI.

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