Updated 12/4/11 Removing Deconvolution Ringing (Shadows) from Solar Prominence Images Image Processing Technique
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This is a step by step procedure of an extremely powerful image processing method to remove the dark wavelets "shadows" that are created around the bright solar rim and prominences when using the wavelets processing program of your choice. This is a normal artifact that is created by many deconvolution routines when a bright saturated region is next to another part of the image of normal brightness. When processing solar images, we typically over expose the solar disk to bring up the very faint prominences for later superimposition with a properly exposed disk shot to form a complete composite. Here I will show this artifact, and how to easily use Adobe Photoshop layers to remove it completely with no loss in sharpness or details.
Especially at times when the seeing was poor, we often need to apply the wavelets filter to our stacked solar images to bring up its sharpness to acceptable levels. But usually, as you increase the strength of the filter, and the details really start to emerge, the ugly wavelets shadows start to appear. By the time the proms are getting superbly detailed, the shadows ruin the image completely. And there is no easy way to remove them afterwards. On the panel on the left, I am showing you what these shadows look like on a prominence shot. The brilliant rim of the sun creates the dark shadow that extends over into the prom area, and puts in an unacceptable dark band all the way around the limb. The good news, is that there is an easy method to rid your prom shots of this evil artifact, and in fact, there is no longer any excuse for having them in your fine solar images.
A typical wavelets sequence on a prime focus solar prominence image.
This image is a crop from a full frame with an Imaging Source DMK camera of the solar limb in Halpha Light. This is the result of a raw stack of about 300 of the sharpest images out of 1200 selected automatically in Registax 6. No wavelets have been applied yet. Although the disk is necessarily overexposed, the proms and spicules are not.
Here are three processed images with wavelets. The radius of the sharpening is always 1 pixel, but the amount varies by a factor of three:
Although the first image is sharper, more detail is waiting. The second image is getting there, but NOW you can start to see the shadowing. And by the third image, its all over with, the wavelets have created a nice sharp prominence, but the shadowing is unacceptable.
This is the set you start with, three or more images, sharpened by different amounts evenly into Photoshop. We paste the first two images onto the third so form a three layers stack. Here is the layers pallet with the stack:
This is just the three images above. It doesn't matter what order they are in, as long as they are all from the same starting image. No registration is then required. Change the combine method in the drop down box above for the layers over the background to "LIGHTEN". Then flatten. This will combine all wavelets layers into one image, with the dark shadows from the more strongly filtered images filled in by the less filtered images in the same areas. But because of the fact that the more sharp an image is, the brighter it is, ALL THE DETAIL will be retained in the final stack. The image below is the final result of this stack.
We can now apply any noise filtering, colorizations or levels adjusting to suit. I left these images very light so you can see the dark shadows easier! Below is the final prominence limb image, combining six sub frames (18 wavelets images) for creating this background for our disk shot.
Here is the entire limb of the sun processed in the manner described. Sharp and crisp, when the seeing was very poor. And no dark limb artifacts. After the disk is added, the shot is complete!
From what I have seen, this shadowing artifact appears quite regularly in amateur moon, planet and solar images. You've seen moon craters that have been over deconvoleved with big dark rings around them, or Saturn's rings with artifact that looks exactly like the Encke gap, but isn't quite in the right spot. Now you can deal with this ugly artifact, and your solar prom shots will sparkle with a brilliance and sharpness like never before.