5 replies to this topic

[berniefe]

I am just learning to use Gimp software, and am trying to figure out the basics of maximizing the numbers of stars visible in my single images of the eclipse without blowing out the properly exposed moon.  Most of you may use other post editing tool(s) for this, but I would venture a guess that the basic process would be attainable with Gimp?

Thanks in advance for direction/link to other posts on this : )

[scanner97]

With Gimp, I imagine you'd use a mask and layers.  I expect Starnet would work as well, although I normally use it to separate DSOs from the star field.

[EJN]

I use Photoshop, created a duplicate layer then used a layer mask to stretch the background without stretching the moon.

 

 

[Messierthanwhat]

I use Photoshop, created a duplicate layer then used a layer mask to stretch the background without stretching the moon.

 

 

This is done essentially the same way in GIMP. It's a 2-layer operation, easier to do than to describe.

 

Open the image as your background layer and duplicate it as a second layer, then right-click on the top (duplicate) layer to bring up the "Edit Layer Attributes" menu. "Add Layer Mask" will be a bit below the middle of that menu. Clicking it will bring up the "Add Layer Mask" menu. Choose Black (fully transparent). A black rectangle with a white border will then appear as a thumbnail to right of the image thumbnail of the top layer in the pallet. At this point, the mask is making your top layer fully transparent, and what you're seeing is the bottom layer showing through it. The white outline indicates that the mask is active, so that any change on that layer will change the mask, rather than the image. 

 

With the mask still active (white outlined on the thumbnail in the layers pallet) but not showing, select the moon on the top image layer, and Use "Edit> Fill" to fill it with white. You'll see the white disk appear on the mask thumbnail to the right, but won't see it change the image. At this point, however, the moon you're now seeing is on the top layer, but the star field behind it is from your bottom layer. Now you can switch to the bottom layer, and use adjustments of your choice (curves, most likely) to bring out the stars without affecting the moon on the top layer. 

 

If you're very careful, you could do this on a single layer without a mask, just by selecting the lunar disk on an image layer, and using "Select>Invert," which will select everything in the image except what you originally selected. Using Curves or other adjustments will then affect only the selected area, and you would probably want to feather the selection a bit to avoid an abrupt transition. This method is a bit less versatile in terms of the adjustments, but it will work. 

Edited by Messierthanwhat, 13 June 2025 - 09:17 AM.

[Messierthanwhat]

OP, I got your PM, but CN refuses to let me respond. The Edit menu show options to fill with FG and BG colors, meaning foreground and background, respectively. That refers to the colors shown in the small overlapping squares below the tools.The current colors are also shown in small squares to the left of each of those items on the Edit menu. By default, GIMP sets black as foreground and white as background when it opens, so if you haven't changed the defaults, filling with white is "Fill with BG color." If you have changed the default colors, of course, you'll need to change at least one of colors to white, and  proceed accordingly.

 

[berniefe]

 ]Messierthanwhat, here is the screen shot I am talking about...

I'm not getting a white disc to cover the moon, but instead, it fills the whole mask thumbnail to the right...