Serif PhotoPlus tutorials - combing RGB / narrowband captures to make colour images

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Combining RGB and / or narrowband to create colour images in PhotoPlus x4

If you have a monochrome CCD and use filters to capture different colour bands and / or narrowband images you will need a method of combining them if you want to end up with a colour image.

Method 1 - Assigning and combining each colour using separate layers and layer groups

This method pretty much follows the procedure described by Ken Crawford in his Clipping Masks and Color Mapping video tutorial. Ken's tutorial uses PhotoShop CS3 but I really recommend that you watch his video through as it does a great job of describing the process and some of the theory behind the method.

This tutorial assumes that you have already:

1. stacked your subs so that you have a monochrome image from each filter
   
2. registered each stack so that they are the same size, orientation and the stars are in the same positions in each
   
3. stretched each stack to bring out the level of detail you want
   
4. decided which colour to map each stack to (for example, if you have captured data using red / green / blue filters you will probably want to map red to red, green to green, etc. - but you don't have to! The famous narrowband Hubble Palette maps SII data to red, Ha data to green, and OIII data to blue)
   

I am going to use captures of M27 - the Dumbell Nebula - taken with red, green and blue broadband filters and combine them, but the tecnique is exactly the same if you are using images captured using narrowband filters.

Firstly open all of the images you want to combine (File -> Open -> Ctrl+left mouse click to select images).

Change the colour mode of each image to RGB 16 Bits/Channel (Image -> Adjust -> Colour Mode).

Then choose one of the images to be the base image that you are going to work from. If in doubt, choose the image that you want to map to red. Make this the active image and create a copy of the Background layer by right clicking on the layer in the Layers tab in the bottom right of PhotoPlus and selecting Duplicate. In the dialogue box that appears, where it says "As:", type a meaningful name - in my case I'm calling it 'M27 - Red' since that's the data in this channel.

Next we are going to colourize the frame which we do by adding a Hue/Saturation/Lightness adjustment layer (Layers -> New Adjustment Layer - > Hue/Saturation/Lightness).

To colourized red - in the dialogue box that appears:

• check the Colourize tick box
  
• leave Hue at 0 (zero hue means red)
  
• set the Saturation to 100 (move the slider to the far right or I find it easier just to type 100 in the box)
  
• set the lightness to -50 (you have to typr the number first and then add the minus sign for some reason!)
  

So far the process is pretty much the same as that followed by Ken Crawford in his tutorial, but Ken then creates a Clipping Mask which is a feature that is not supported by PhotoPlus (as far as I can tell), so we are going to do things a bit differently... and use a series of Layer Groups.

Create a new Layer Group (Layers -> New Layer Group). In the dialogue box that appears, name the Layer Group 'Red', leave the other settings for now and click OK. While we are at it, let's go ahead and create Layer Groups for the Green and Blue channels too. Now your Layers tab (bottom right) should look exactly like this:

Now in the Layers tab click on the image that you duplicated from the Background and renamed (in my case M27 - Red) and drag it on top of the Red Layer Group. Do the same thing with the Hue/Saturation/Lightness adjustment layer and make sure that the adjustment layer is above the image, if necessary dragging the layers around, so it looks like this:

Next we want to copy the image that we'll use for our green channel into the Layer Group we've called Green. To do this, make the image you want to be the green channel active, copy it (Edit -> Copy or Ctrl+C), then make the image with your Layer Groups in it active, click on the Green Layer Group and Paste as New Layer (Edit -> Paste -> Paste As New Layer or Ctrl+L). Rename this image as something sensible by right clicking on the image thumbnail in the Layers tab, selecting Properties and typing in the Name box. Now colourize this by adding a Hue/Saturation/Lightness adjustment layer (Layers -> New Adjustment Layer - > Hue/Saturation/Lightness).

To colourized green - in the dialogue box that appears:

• check the Colourize tick box
  
• set the Hue to 120
  
• set the Saturation to 100 (move the slider to the far right or I find it easier just to type 100 in the box)
  
• set the lightness to -50.
  

Repeat exactly the same process for the blue, but this time to colourize blue:

• check the Colourize tick box
  
• set the Hue to -120
  
• set the Saturation to 100 (move the slider to the far right or I find it easier just to type 100 in the box)
  
• set the lightness to -50.
  

Now your stack of layers should look exactly like this:

But your image will be looking distinctly blue! So here is where the magic happens... Left click on the Layer Group called Blue (make sure you are working on the Layer Group rather than an individual image layer) and left click on the down arrow next to the box that says 'Normal'. This sets the blend mode and we want to change this from 'Normal' to 'Screen'. Do exactly the same thing for the Green Layer Group, again selecting 'Screen' as the blend mode. (You can try setting the blend mode on the Red Layer Group to 'Sceen' as well if you like but you will probably get better results leaving it set to 'Normal'.)

And that's it! You should now have a colour image.

You can set different transparencies for each Layer Group if you wish and also add new Adjustment Layers (for instance Curves or Colour Balance) to each of the images that make up the composite colour image.

If you save this project as a PhotoPlus Picture file (*.Spp) you can open the project next time you want to do an RGB combine and simply copy the R G B components of your new image over the old ones, so you don't need to go through the whole process every time.

Once you've done this, and you are ready to save your final version, flatten the image (Layers -> Merge -> Merge All) and export the image in whatever image format you like (I usually save as a TIF as it is a lossless format that keeps all of the detail, and a JPEG set to around 90% quality for posting on the web).

Hope this is useful - it may seem long and involved, but it is a piece of cake once you get used to it!

A couple more points:

1. You could, of course, colourize each image that you want to use for the relevant colour channel before and copy the result into your RGB image stack which means you won't need to create the Layer Groups, but the way I outline above is more flexible, allows you to go back and change the colouize process and to add other Adjustment Layers and effects to each channel if you want to.
   
2. I find that this method gives excellent flexibility and control over colour, but it does seem to carry through the noise of the individual images that make up the colour composite so it's best to use it with low noise captures.
   

If you have any comments or questions then please do not reply to this thread but post them to the general PhotoPlus thread instead.