How to Combine Ha data with an RGB image?

[PhotoGav]

Oh Oracle of SGL, I seek your help.

I am trying to combine Ha data with RGB data, typically to enhance star forming regions within a galaxy, but am not convinced by my efforts so far. Please tell me what is the best method for doing this?

My method to now in Photoshop has been to stretch the Ha stack pretty aggressively and to put it as a layer over the extracted R channel from my processed RGB stack. I then set the Ha layer blending mode to 'Lighten' and fiddle with some selective masking to bring through only the most obvious Ha regions. Is this a good technique or is there a better way?

Thank you.

[cfpendock]

A very good question, Gav.  My method is much as you describe, except I usually do quite a bit of processing in the Ha stack before combining.  I have also used the blend mode "overlay" for some nebula.  

My problem is that whereas the Ha image can look fantastic, the colour can in some cases diminish the impact of the image.  I suppose it depends on the make-up of the target.  But it will be interesting to hear how others do it.

Chris

[RobH]

That's the technique I use Gav. Works fine for me.

[frugal]

The other technique I have seen is to use the Ha data as a Luminance layer for the image over the top of the RGB (LRGB with the Ha as the L). I think which one you chose depends highly on the amount of data in each channel.

[PhotoGav]

Thank you all so far.

The other technique I have seen is to use the Ha data as a Luminance layer for the image over the top of the RGB (LRGB with the Ha as the L). I think which one you chose depends highly on the amount of data in each channel.

Absolutely, that is a technique that I use when there is lots of Ha in the image, for instance in a nebula. However, I'm talking about when there are small areas of Ha within an image, such as in the spiral arms of a galaxy. I have since done a bit more internet reading and an alternative technique is to put the red layer above the Ha and set the opacity of the red channel to about 20%. Yet to try that one...

[carastro]

I have used both methods described, and sometimes both in the same image.

Carole 

[ollypenrice]

The other technique I have seen is to use the Ha data as a Luminance layer for the image over the top of the RGB (LRGB with the Ha as the L). I think which one you chose depends highly on the amount of data in each channel.

Beware of Ha as Lum. What you are doing is lighting your green and blue as if they were deep red and they are not that. On galaxies I would never do this. On very strongly Ha dominated nebulae I yeild to temptation at very low opacities - sometimes.

Ha to red in blend mode lighten for me, but here's the thing: once you have your Ha on top of red in this mode you don't have to stick with the stretch you have prepared so far. It can be very helpful to play with the curve while the Ha is in situ over the red. If it isn't doing much, give it a bigger stretch. Lower the bottom of the curve. Black clip it if that helps. All the time you are looking to see when the star forming regions you want are beginning to appear above the red. Because these are the brightest parts of your Ha signal you have little to fear from noise. The fainter parts of the Ha which you may now have dragged up past their noise floor will not be applied in blend mode lighten. You can foget about them.

Olly

[RobH]

I have since done a bit more internet reading and an alternative technique is to put the red layer above the Ha and set the opacity of the red channel to about 20%. Yet to try that one...

The problem with this method is that you'll mess up the colour balance of your stars. Stars in Ha are much smaller than in red, and for a good star colour balance you need the red at 100%.

[PhotoGav]

Brilliant, thank you Olly, Rob & Carole. This gives me some great ideas to try out.

[lensman57]

Oh Oracle of SGL, I seek your help.

I am trying to combine Ha data with RGB data, typically to enhance star forming regions within a galaxy, but am not convinced by my efforts so far. Please tell me what is the best method for doing this?

My method to now in Photoshop has been to stretch the Ha stack pretty aggressively and to put it as a layer over the extracted R channel from my processed RGB stack. I then set the Ha layer blending mode to 'Lighten' and fiddle with some selective masking to bring through only the most obvious Ha regions. Is this a good technique or is there a better way?

Thank you.

That is the accepted way doing this but you may also paste a very small percentage of the Ha on the top of the RGB to enhance it even further but need to watch out for strange star colours.

Regards,

A.G