Processing: Color Control

Color is a very subjective area of processing. There are a number of issues with color that you may want to address even in simple processing: Color noise, color balance, background color, and color saturation.

Color Noise

The noise reduction methods described earlier deal all reduce color noise, but Astronomy Tools Color Blotch Reduction can be helpful to use.

Color Balance

If you think your image is suffering from a common flaw--green channel overabundance--you may find the HLVG plugin to be particularly helpful. Use the medium setting first on your open image to see what it does.

HLVG effects are mainly visible in the bright and midtone portions of your image.

If you really want to nail star colors you will have to look for a spectral type G2V star in your image and use the method given in the Background Color section immediately following this one. (Instead of putting a color sample mark on the background, you'll put it on the calibration star.)

Photoshop provides other ways of adjusting the color, and you can find some of them in the in the color toolbox linked to from the Processing home page.

Background Color

Here's how you can adjust the background color to a neutral gray or a specific tint (some people like a pale shade of red, others blue.)

  1. Open the Info Palette using Window > Info. You should see yet another dockable box become visible.
  2. Click the tiny eyedroppers to select Actual Color mode. Older versions of Photoshop may set this using a different method, so be ready to hunt around. You should see RGB next to the eyedropper as a result.
  3. Right click the eyedropper tool on the toolbar and select Color Sampler Tool. Options for the tool will appear on the horizontal bar above the work area. Select a Sample size between 5x5 and 11x11.
  4. Add a sampling point to the image by clicking on a representative dark area that you want to color balance. The RGB values for these points will appear in the Info palette.
  5. Open the Levels tool and move it to a portion of the screen where it doesn't block your view of the info palette.
  6. Let's say that you would like a uniform gray background with an RGB brightness of (20, 20, 20). Choose the levels channel and set the black point to the value of the darkest pixel in the histogram. (This will typically be the bottom of the background peak.) Adjust the midpoint up or down while watching the R value of your sample point until it reaches 20.
  7. Repeat the last step for the G and B channels.
  8. When all three channels equal 20 you're done!

You may be tempted to set your background to black, but remember what was said about clipping, which is exactly what you'd be doing by forcing the background to black.

Color Saturation

Pale colors are usually associated with imaging that incorporates a luminance channel, whether acquired or synthetic. Here we aren't doing that, but it's quite possible you'd like to boost the colors in your image.

Overall color intensity can be increased using the Block method:

  1. Open your Image
  2. Create two copy layers
  3. Change the top layer's mode to Luminosity using the drop down menu and the layer beneath it to Soft Light.
  4. Merge the three layers.
  5. Still not vibrant enough for you? Repeat steps 2-4, but use Screen mode in place of Luminosity. You can repeat those three steps alternating Luminosity and Screen modes for as long as you want, but you might want to stop short of Andy Warhol colors.
  6. Although this is a great way to increase saturation without introducing a lot of noise, it's probably a good idea at this point to do a little noise reduction. This can take the form of Deep Space noise Reduction combined with Less Crunchy More Fuzzy, or whatever method you prefer.
  7. One last Levels check is also a good idea before saving, and you can also use Levels to tune the brightness and contrast. (Be mindful of maintaining the non-black background you established in earlier processing.)

Sometimes all you want to do is increase star colors. There are many ways of doing this, but for now let's apply a method described by Jerry Lodriguss:

  1. Open your image
  2. Get the Magic Wand tool, set its tolerance to 10, Anti-Alias checked on, and Contiguous unchecked and Sample All Layers unchecked.
  3. Click on an area of blank sky background with the Magic Wand tool. This will select the background but not the stars or your target.
  4. Invert the selection using Shft-Ctrl-I to select just the stars and your target.
  5. Supposedly at this point you're happy with the saturation of your target, so we need to deselect it.Change to the Lasso tool. Set the feather to 0 pixels and check on anti-alias
  6. Hold down the Alt key, left-click and carefully drag the lasso tool around the target to form a closed loop. (It's possible in earlier versions of Photoshop you need to right-click for this to work.) The selection line around the target should disappear when you release the mouse button.
  7. Copy stars and paste as new layer: Ctrl-C followed by Ctrl-V
  8. If the star layer isn't selected, click on it.
  9. Do Image > Adjustments > Hue / Saturation.., and increase saturation to +95. Don't worry if this looks really garish, you can always go back and try a lower value.
  10. Blur the Star Color Layer: Filter > Blur > Gaussian Blur... with Radius equal to 1 or 2 pixels.
  11. Change the blending mode of the star color layer to "Color"
  12. Flatten the Layer: Layer > Flatten Image. colors a little uneven or garish? Use Edit > Step Backward to go back to step 9 and use a lower saturation value


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