Help adding Ha to RGB

[blinky]

Hi,

 

I got 4 hours on LDN1586 in Ha and really like what I have - so much so that I went back to get some RGB data for the stars with the intention of adding the 2 together.  Im a bit stumped though!  At this point in still in the Linear stage - I combined the RGB, did DBE and Colour Correction, then Blur and Noise X Terminator.  Then I extracted the Red channel, linear matched by Ha to the Red channel and did a combine with Pixel Maths but the result is rubbish!  Is there a better formula than 0.75XRed + 0.25XHa that I can use?  I think I want something like if the Ha is brighter than Red then take its value, else take the Red channel value.  Just noticed I have some colour casts in RGB but I think this is 'cos I'm stretching it so much to make it match the Ha data - wonder if I remove the stars from the Ha data, extract the red channel and remove everything but the stars from Red then add Ha and Red together then bring that back into the RGB image?

[wimvb]

I used a variation of the following worflow with good results some time ago.

 

[ollypenrice]

This has just come up in a recent and similar thread.  I explained how I do it in Photoshop using blend modes Lighten or Screen and another member provided the equivalent formulae for use in pixelmath.

To follow up on Olly's post above, pixelmath expressions for those two blend modes are

max(img1,img2)

~(~img1*~img2)

The thread is here: 

I've never done this in PI but perhaps it will help.

Olly

[The Lazy Astronomer]

On 19/01/2023 at 09:11, blinky said:

I think I want something like if the Ha is brighter than Red then take its value, else take the Red channel value.

As above, this could be done fairly simply with the max operator in pixelmath:

max(red,ha)

You can experiment with different degrees of stretch and masks to get the effect you're after, if you're not happy with the initial result. 

My actual preferred way to add Ha to RGB is with a pixelmath formula detailed in the book Mastering Pixinsight, but it's far too complicated for my tiny mind to attempt to recite it from memory!! 😁

[wimvb]

On 19/01/2023 at 10:11, blinky said:

I think I want something like if the Ha is brighter than Red then take its value, else take the Red channel value.

Be careful with this, it rarely is this simple. First you need to make sure that the Ha and Red background are similar. Next you need to have approximately the same noise level, or the max function will make the image noisier. If the Ha is noisier and generally weaker/darker than the red, the max function will only return the red plus the noise from Ha in the dark areas.

[The Lazy Astronomer]

Well, I was going to post the pixelmath, but I am not allowed without the express written permission of the author of the book.