Adobe Photoshop has earned its credits amongst astro photographers all over the world. There are also excellent filters available (like GradientXterminator from RC-Astro) to process images almost automatically. Although being one of the best image processors, it is also one of the most expensive one’s. However, there is a free “open source” alternative called GIMP (GNU Image Manipulation Program) which can be downloaded at www.gimp.org/. It does the job as well. Don’t worry about the screenshots below. Although they are in Dutch, GIMP can be downloaded in almost any language.
Every astro photographer knows the effect of long exposures: the glare of city lights etcetera called light pollution. Taking additional darks will improve the end result but sometimes additional processing is needed. Although there are several filters on the market, like the CLS-filter that I use, but somehow this light comes through anywaysJ. This problem can be resolved with a cool technique called subtraction. Imagine, take all the image information out of a picture and leave the light pollution at the background, behind.
Eliminating light pollution
Lets get to work! Removing light pollution has to be done with care. We don’t want to remove the faint details that are in the background of the image. Please take a look at the image below. This is Messier 8; The Lagoon nebula. I shot this image in the Provence, France under almost perfect conditions. After staking the image with DeepskyStacker (also a free program found at deepskystacker.free.fr/) this image needs some final processing.
Open the image (File – Open As Layers) and the image will appear. Remark: It is important to open the file with “Open As Layers” to be able to do the trick. Look at the right side of you screen and you will see the layers panel en a miniature of the image. This “basic” layer has to be duplicated. There are two ways to get the job done: through the menu by clicking Layer – Duplicate Layer or by dragging the image to the duplicate icon just below in the panel. If all went well you will have two layers now.
Select the “Layer 1 Copy” layer. That’s the one we are going to process. To isolate the background details in the image have to be removed as much as possible. Choose the repair tool. It can be found in the Toolbox, at the left side of the screen. The icon for it looks like two band aids in a cross. Adjust the size of the tool with the bracket key on the keyboard; [-key (smaller) of de ]-key (larger). Now choose a fuzzy brush. Find a spot in the image with no or very little detail, next to the detail you want to remove. Ctrl-click on that spot. Move the cursor over the star you want to remove and click. The star will disappear (sometimes you have to click several times to remove the star). Repeat this until all the big stars are removed. Small stars do nog have to be removed. In the blur action later on the will disappear too.
The DeepSkyObject (DSO) in the image can be removed with the Clone tool. Choose the clone tool in toolbox on the left of your screen. Also with this tool, choose your brush size and select an empty area on your image (Ctrl-click). Now move your cursor over the DSO and click one or several times until the DSO disappears. Now the image should appear empty (disregarding the small stars) relatively smooth. In the next step this smooth layer has to be blurred. We will use a Gaussian blur. Choose Filters – Blur – Gaussian blur. Select a value of 500 for the amount of blurring. This is my default value and works fine in most cases. You should end up with a layer without any details, just the background glare. Are you ready for the big moment? It time to remove this pesky light pollution!
Look in the layer pallet and find the dropbox “Mode”, just below “Layers”. Now select the option “Substract”. You will notice that the top layer is being subtracted from the original image. In most cases this effect is too strong and some details in the image are removed too. The subtraction had to be adjusted. In general a subtraction level of 50% will give good results. By choosing Merge Downwards in the Layer Menu, the subtraction will be finalized.
The image should look like the image below.
Now the light pollution is removed, the image can be stretched and colour enhanced. Choose in the menu Colours – Curves and your screen should look like the one above. Now you are ready to stretch the image. Notice the line across the colourcurves panel. By clicking on this line you can drag the line up or down to adjust the enhancement. Sometimes a S-curve will work, sometimes a bulge – trail and error! By clicking the checkbox Example you can check the effect you make on the image. If you are a beginner, it is advisable to duplicate the layer before doing any stretching. If you lose your way and mess up, simple remove the layer.
The final step is colour enhancement. This is a tricky part. Colours in DSO’s are often exaggerated. This is where astrophotography crosses over from science to art. What is true and what isn’t? It is all the matter of taste. Just remember not to overdo it. Colour can be enhanced in the menu option Colour – Hue / Saturation. Toggle the slider until you get the result you want.
The final image could end up like the one below. What ever the result, I’m sure You will have tons of fun just getting the most out of your pictures.