Scrolling down will be seen a ‘Repair Image’ sub-menu. Scrolling down will be seen a ‘Repair Image’ sub-menu.Ĭlicking on this will bring up its sub-menu. The light polluted image should be loaded into GIMPĪnd the layer duplicated (‘Duplicate Layer’ within the ‘Layer’ drop down menu) to give two layers which can be seen when the Layers window is opened: in the ‘Windows’ drop down menu click on ‘Dockable Dialogs’ then click on Layers.Īny actions will only apply to the top layer.Ĭlicking on ‘Filters’ will show the C’MIC tab at the bottom of the menu. These will then appear at the bottom of the filters drop down menu.
#GIMP GRADIENT LIGHTING INSTALL#
The first thing is to do is to download and install the set of ‘G’MIC’ filters found at ‘’. It sometimes works best if this is done in two passes first only removing 50% of the light pollution by setting the ‘opacity’ slider to 50% before flattening the two layers and then repeating the process with the opacity set to 100%. This has removed the light pollution from the image which can be stretched to bring up the fainter stars. There is now as good an image of the light pollution as can be made.įinally the layer blending mode is set to ‘Difference’ and the two layers flattened. To smooth this further, a Gaussian Blur of, say, 50 pixels can be applied. Using the clone tool, any bright regions such as, in this case, the brighter stars in the M35 and NGC 2158 clusters should be cloned out from nearby regions where the light pollution has a similar brightness. The majority of stars disappear leaving evidence of some of the brighter stars in the two star clusters. Any actions take place on the top layer.įirst the ‘Dust and Scratches’ filter is applied with a radius of ~30 pixels. They can be seen in the ‘Layers’ window opened within the ‘Window’ drop down menu. The base image is loaded into Photoshop and duplicated to provide two identical layers. It shows significant and uniform reddish light pollution but with vignetting towards the corners.
#GIMP GRADIENT LIGHTING FREE#
It must be born in mind that any areas of brightness within the object such as, say, the outer arms of the galaxy M81, that are less bright than that of the light pollution itself cannot be imaged no matter how long the exposure or how big the telescope.įor both programs I am going to use a result of stacking 56 raw images of M35 and NGC 2158 in the Window’s free program Sequator (this is described in detail elsewhere in the digest ). Light pollution in wide area images of the Milky Way cannot be easily removed and imaging under dark skies without significant light pollution is really needed. If the objects in the image are star clusters, planetary nebulae or small galaxies within the field the there is no problem, but large nebulae such as the North America Nebula or galaxies such as the Andromeda Galaxy which fill an large area of the field of view are more problematic. It will work under many, but not all, circumstances. This article shows how the process can be carried out in both programs.
However the ‘Dust and Scratches’ filter that I have been using in Photoshop as part of this technique is not present in GIMP, but I have become aware of a downloadable set of filters that provides an alternative. Now that GIMP version 2.10 can handle 16-bit data it can provide a direct (free!) replacement to Photoshop.
NGC 2158, to its lower right, is around 9,000 light years further away than M35 and believed to be 2 billion years old.įor some years I have been using a technique in Adobe Photoshop to remove the Light Pollution from an astronomical image. It lies at a distance of ~3,800 light years and is scattered over an area of the sky about the size of the Moon. Heavily light polluted image of M35 and NGC 2158 in Gemini.Ī crop of the region containing the two open clusters – M35 is the large cluster containing many blue giant stars along with two red giants.
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