I have a good image of planet with shadow on it (at center just under bottom line) which I want to make darker. And also I have another image with big exposure where the moons is visible. I want to make it lighter and add to the first image.

How can I do it in GIMP?

Good image:

Good image

Overexposed image:

Overexposed image

There is photo that I made later. There you can see a moon at the left edge of Jupiter and shadow moved to the right.

Moon and shadow

  • Hi Welcome to GSDE. Nice images BTW!!
    – Billy Kerr
    Aug 26, 2022 at 11:01
  • @user287001 In "casual" astrophotography, 4 moons are usually visible, so the missing moon (only three visible) is either behind Jupiter (and so completely out of the picture) or in front of Jupiter and casting a (small) shadow on it.
    – xenoid
    Aug 26, 2022 at 15:59
  • @xenoid it completely out of picture. I didn't have enough FOV to catch it too
    – Robotex
    Aug 29, 2022 at 15:42

1 Answer 1


Open the good image in GIMP, then using File > Open as Layers, open the second image so that you have it on a layer above the good image.

Select the top layer do Colors > Curves, click and drag the diagonal line to make a curve like the one shown here, until you can see the moons.

enter image description here

Add a layer mask to the top layer

enter image description here

Click on the layer mask thumbnail in the layers panel to select it. Then paint on the layer mask in black, to hide the planet which is too bright, revealing the properly exposed image in the layer below. You will have to zoom in closer and use a soft edged brush around the moon to isolate it from the planet.

You can change the softness of a brush by adjusting the hardness parameter in the Brush Tool options

enter image description here

Here's the finished example

enter image description here

If you make a mistake during masking, don't worry. You can paint in white to reveal more of the top layer. Layer masking is nice because it's non-destructive. The mask can be edited repeatedly without affecting the original image.

  • And how to make the shadow of moon on planet darker? It is barely visible now Just paint black dot?
    – Robotex
    Aug 26, 2022 at 11:11
  • I can't really see a shadow of the moon on the planet. I suppose you could add one on a new layer, but wouldn't that be considered cheating? Personally I wouldn't add something to the image that isn't there.
    – Billy Kerr
    Aug 26, 2022 at 11:20
  • @Robotex there is a somewhat slightly darker area, but there are similar areas in the northern hemisphere. If it's a shadow is is spread out by the blurriness, if you make it darker it will be much too big to be the shadow of a satellite.
    – xenoid
    Aug 26, 2022 at 11:34
  • @xenoid I added third image where the shadow was moved ti right. There it visible better Can I decrease luminosity of photo just in that place where shadow is and left the other photo as is?
    – Robotex
    Aug 29, 2022 at 12:04
  • @Robotex - you should probably use that image instead of the first one. Then you could just use the Dodge and Burn tool - set it to "Burn", a small brush size about the same size as the moon, and click on it a few times to darken the image in that region only. see example
    – Billy Kerr
    Aug 29, 2022 at 18:05

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