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NGC 1232 raw image |
NGC 1232 processed image |
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NGC 7331 raw image |
NGC 7331 processed image |
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M74 raw image |
M74 processed image |
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NGC 1232 raw image |
NGC 1232 processed image |
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NGC 7331 raw image |
NGC 7331 processed image |
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M74 raw image |
M74 processed image |
NOTE: This technique is an adaptation of the AntiVignetting (AV) technique that Matt BenDaniel has documented on his website
Notice the images on the left contain undesirable background noise due in part to vignetting of the f/3.3 focal reducer and also in part from not properly flat fielding the shots. The images on the right are the result of complete processing, the most significant of which is the AV process.
Following in Matt's footsteps I chose to use Layers in Photoshop. The idea is to create a layer with an AV mask which when blended properly with the image removes the undesirable background noise without robbing too much of the subject's signal. I chose to use a blending mode of 'Soft Light' and set the opacity to 30% for starters. Once you have a good mask try adjusting the opacity up and down slightly. Sometimes you can fine tune this to an optimal setting. I usually use 2 or 3 such layers to get the job done.
The first (or bottom-most) layer I usually create using either a radial or lateral gradient. It takes some experimenting to find the proper foreground/background colors and where to start and stop the gradient. The reason I like to use gradients as the first pass is twofold. First, the noise usually follows a gradient pattern and much of the noise can be removed in this first step. Second, since the gradient is automatically generated by the software it is indiscriminating. The most challenging aspect of this technique is knowing when you are removing noise and not signal. If you study the images carefully you can see that some of the faintest detail is sometimes slightly compromised. This is particularly difficult with galaxies and nebulas which contain such faint details. Try not to 'throw the baby out with the bath water.'
For the second or third layer I paint the mask freehand. Try using a soft brush (say about 30%) and around 9 pixels. The key is to experiment and find the brush that works well with your image. I start by using total black and only paint in the faintest areas away from the subject. I slowly add more white to the mask while closing in on the subject. When I find I am starting to remove signal vs. noise I back out and apply a mild Gaussian Blur to the mask. If noise still remains, add another layer and go at it again.
Another tip is to turn the brightness up on your monitor. I usually find that I can see more noise when I do so. Removing the noise in this amplified mode will tend to make your image look cleaner when viewed with a more normal brightness setting. Plus, you can't always count on how others have set their monitor settings. I've seen some monitors with the brightness turned so high it would burn my retinas!
The key theme here is to experiment and don't be shy. One of the beauties of using Layers is that in the end if you are not happy, you can just delete the AV layers and start over. Your background image is still there patiently waiting to be processed. If you get tired, save your work and come back later. We'll still be here tomorrow waiting for your latest masterpiece!
The images above presented some interesting challenges. NGC 1232 was bathed in a particularly hot hotspot. I feared I could not remove the noise without losing the signal, but alas I prevailed! This stuff really works! NGC 7331 didn't have much initial noise, however, when I started playing with curves to bring out the faint spirals the noise became amplified with it. Three AV layers later and its all gone! M74 was a challenge in that this image like NGC 1232 is bathed in noise with a lot of faint detail. The image is a bit underexposed, but with good AV layers I was able to salvage most of the details of the spirals.