In the section on image processing, there was the presentation to treatment # 2 of the Color balance using levels. This method is completely automated and does not allow for more refined adjustments. The Color balance using curves offers more control over it. For example, the background colors of the sky are never uniformly the same in all sections of the image. In addition there may be a color cast in the photographed object that we want to remove (for example the red-salmon color caused by the H-Alpha luminance layer).

With curves, we can have 4 control points (instead of 2 using levels) for the color balance. We can, for example, distribute these control points as am:

  • A control point for the brightest part of the image (represented by a star most of the time)
  • Three control points for the sky background in different sections of the image

This treatment can therefore replace or supplement process # 2 when you want to have more control over the color balance. On the other hand, it requires more experiences and judgments on the part of the user. If using a luminance image, it is suggested to perform a color balance on the RGB image (after processing # 4 in the section on image processing) and then on the LRGB image.

  • Open the image to be processed in Photoshop
  • Make sure that the window of Layers is open (Window | Layers) (Window | Layers) as well as the window Information (Window | Information) (Window | Info)
  • In the window Layers, click on the layer which is above all the others
  • Merge all previous layers by pressing keys simultaneously Shift+Ctrl+Alt+N+E and rename this layer Color balance. To rename a layer double click on its name and type in the new name. The layer Color balance is therefore above all the others and it is the active layer (click on it to make it active otherwise)
  • Click on the icon Create a fill layer Create a fill layer (at the bottom of the window Layers) and select Curves (Levels). At this time the tool Curves appears on the screen
  • We will create 4 control points on the image. We will put a mark representing the lightest region of the image by clicking on the tool Pipette Popette Tool, then press the key Shift and click on the brightest part of the image (a star most of the time). A checkpoint bearing the #1 will appear. Make sure that the Select (Sample Size) of the mark is Medium 3 x 3 (top left of the screen). To find the brightest point of the image, use the window Information and move the mouse pointer over the brightest stars and look in the window Information values RGB. Choose the star that displays the highest value. If the star is small, it may be necessary to perform a Zoom (Ctrl and +) before selecting the star
  • We are going to create 3 other control points for 3 different regions of the image representing the sky background (the black sky) by proceeding in the same way as in the previous point. We will make sure for the mark # 1 of each RGB layer is close to the value 245 and for the control points # 2 # 3 and # 4 of the value 20 (or between 15 to 30 according to your preference)
  • In the Layers tool, select the Blue layer

  • Looking in the window Information, make the adjustments using the control points to approximate the reference values. Here is an example of corrections I made on the treatment of the nebula B33 Horse Head:

  • Proceeding in the same way, make the adjustments for the green and red layers.
  • Here is an example of the result that appears in the window Information on the 4 control points of the image of the galaxy NGC891:

Information on checkpoints is displayed before and after treatment. For example for the value # 1R, the information is 250/246; 250 is the value before processing and 246 after.

Watching control point # 1 the RGB values ​​are near the reference value 245. Control points # 2, # 3 and # 4 are also close to the reference value which is 20. The deviation is only 1 between the RGB values ​​for control points # 2 (23-22-23) and # 4 (20-19-20). For # 3 there is no gap (24-24-24). The maximum deviation with the reference value 20 is found on the control point # 3 which is 24. The deviation of 4 is minimal. Personally, I prefer a sky background closer to the value 20 (instead of 15) and if there is a gap between the control points, I prefer to create the gap above 20. So my preference is for a clearer sky background.

Here is the image of the galaxy NGC 891 which is the result of this processing:

In the image above, we can see the 4 control points that I have chosen to perform the color balance using the curves.

I also use this technique to adjust the colors of the deep sky object. For example ; remove a color cast due to light pollution from the observation site or correct the color salmon-red caused by using the Ha filter as a luminance image:

By putting a control point above the sky background (more than 20) and another below the lightest portions of the image (less than 245), this corrects the colors of the sky object. deep without affecting the sky background and the colors of the stars.

Richard Beauregard
Sky Astro - CCD

Revised 2021/01/16