www.schursastrophotography.com/solar.html 

Updated 12/4/11
Removing Deconvolution Ringing (Shadows)
from Solar Prominence Images
Image Processing Technique

 

      
Introduction

      
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.

      
The problem.

      

        

          
 
          
 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!  


            
 
          
        
      


      
Conclusions

      
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. 



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