Part of the Cats Paw Nebula (NGC6334) in Ha

[lazjen]

I did this the other night, grabbing 7 x 30 min exposures. The guiding struggled in the seeing conditions, so the brighter stars are egg-shaped.

Mostly processed in PixInsight, with a light finish up in PS.

170201

Astrobin: http://astrob.in/121889/0/

Q: What's a good way of colorising a single channel image? I tried assigning it to Red, but that ended up too...red. :)

[RickS]

That's an interesting area of the Paw, Chris.

Here's one idea for colourising a mono image in PI: Use LRGBCombination with the mono image as the R, G and B channels to build a white RGB image. Take a copy of the original mono image, black clip it and apply as a mask to the RGB image (this will prevent you colouring the dim background areas; another option would be to make a RangeSelection mask.) Now use CurvesTransformation to mess with the colours, e.g. boost the red.

Cheers,
Rick.

[lazjen]

Thanks for that Rick.

I took your advice, but added to it a bit. On the cloned image I did a star mask on it as well and wiped out the stars. Then when I used it as a mask on the RGB version, the red manipulation left the stars alone.

Here's my quick attempt this morning. To improve it, I think I'd have to get the star mask more refined (I just took defaults for this from memory).

170208

Astrobin: http://astrob.in/121889/B/

P.S. I like how the small dark bit in the centre looks like another horse head. :)

[RickS]

Nicely done, Chris!

A RangeSelection mask might be another way to select the nebulosity but not the stars. You'd probably want to multiply it by the original image to get a non-binarized version of the mask.

Cheers,
Rick.

[lazjen]

I tried the RangeSelection but it seemed to include the stars as well. They came in to the selection much quicker than the nebula.

I think I understand your second sentence - the multiply is the mask rendering mode, right?

In my attempt I used the mask on the clone to actually wipe out the stars before then using that resulting image as the mask to the rgb image. Did I really need to do that or do masks "chain" through images?

[RickS]

Usually the stars are brighter than the nebulosity so you can drop back the Upper Limit to exclude them and lift the Lower Limit to exclude the background.



RangeSelection gives you a binarized mask where every pixel is 0 or 1. If you want to mask the RangeSelection area but want the mask to vary depending on the luminance of the unprotected area you can just use a PixelMath expression like "range_mask*luminance_mask". The result is 0 in the areas where range_mask is 0 and it is the same as the luminance_mask in areas where range_mask is 1.



You can only apply a single mask to an image and you can't chain them together. I frequently build masks using PixelMath, e.g. a clipped luminance mask with stars removed is good for applying HDRMT and LHE without bloating stars. It would also be good for colourizing a Ha image :)

Cheers,
Rick.