In the application of the technique LRGB, it can be very interesting to create a synthetic green layer. To achieve it, we use the Red and Blue images. This will produce a color image as natural as when using a normal Green image. Most of the time, this technique is designated by the acronym LRVsB.

The first advantage of using a synthetic green is to save imaging time, because we will not need to take a series of photos with the green filter. Also, when photographing planets with a monochrome camera, which requires taking all LRGB images before rotating it, precious time will be saved by avoiding taking exposures for the green layer. For example, for the photograph of the planet Jupiter, we have the constraint to take all the images in a maximum time of 1,5 minutes before its rotation!

I will explain how to create a color image using a synthetic green (RGB) produced with the software Photoshop. The Red and Blue images have been assembled (composite). No subsequent processing was applied to these images.

  • Open your images Red.tif et Blue.tif composed in the software Photoshop. The two images have already been aligned with each other
  • Click on the picture Red.tif
  • Convert image Red.tif monochrome to an RGB image (Image | Fashion | RGB color)
  • Click on the picture Blue.tif to make it active
  • Selection | Select all and then Edition | To copy
  • Click on the picture Red.tif to make it active
  • Click on the tab Layer and activate only the layer Blue
  • Paste the image Blue.tif in this layer (Edition | To stick on)
  • Reactivate all color layers by clicking on RGB. At this moment we have a picture RRB
  • Click on the tab Layers
  • Click on the icon Create a fill layer Create a fill layer (at the bottom of the window Layers) and select Diaper mixer (ChannelMixer).
  • In Output layer, to select Vert 
  • To put Red at 50%, Vert at 0% and Blue at 50%. We now have a picture RGsB (For RedSynthetic green et Blue)
  • Simultaneously press the keys Shift+Ctrl+Alt+N+E and rename this layer RGsB.
  • Balance the image with the Black and White Point Method (see treatment # 2 in section Image processing)
  • If the color balance is not correct, click on the icon Create a fill layer Create a fill layer (at the bottom of the window Layers) and select Hue / Saturation. Move cursor Colour left or right to bring out the natural colors of the object. This treatment is necessary to balance the Red, Green and Blue layers between them. After this treatment, reapply the Black and White Point Method to balance the image.
  • Save the image by naming it Name of your RGB image
  • For all the details of the assembly of this image RGsB with the image of Luminance and all the subsequent treatments to be carried out, click on this link.

Here is the NGC6960 veil nebula which was made using a synthetic green layer:

Click on picture to enlarge

Richard Beauregard
Sky Astro - CCD

Revised 2021/01/16