First pictures with Dual Narrowband Filter - fixing the color fringe

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When taking my first pictures with the L-eXtrem dual narrowband filter I was quite pleased with the results I was able to get. This filter passes only the Wavelengths of the Ha and O III spectral lines that are very prominent in may nebulae. It turns out being a good tool to expand imaging time beyond just the moonless nights. 


Core of M42 Orion Nebula im HaOIII, 50x10sec
C9.25 on EQ6-R Pro, ASI294MC PRO, Reducer 1/6.3, L-eXtreme

IC405 in HaOIII, 30x300s, 
C9.25 on EQ6-R Pro, ASI294MC PRO, Reducer 1/6.3, L-eXtreme


IC433 in HaOIII, 40x120sec
C9.25 on EQ6-R Pro, Reducer 1/6.3, L-eXtreme Filter

Detail of NGC2246, Rosette Nebula in HaOIII, 30x90sec
C9.25 on EQ6-R Pro, ASI294MC PRO, Reducer 1/6.3, L-eXtreme


However, I noticed that particularly the pictures that were taken with both the reducer/corrector and the filter showed some light color fringe around the stars. I assume that something in the geometry of the imaging train wasn’t quite optimal. As I didn’t have to many options right away to change the geometry e.g. by putting the filter closer to the camera I was looking for other ways to deal with this issue.


Detail of star images with high color saturation 
a color fringe around the stars is visible 


Fortunately many astronomical image processing programs like e.g Fitsworks or Siril provide tools for re-aligning the color layers using the stars as a basis.


Aligning the color layers in Siril 



Star images after aligning the color layers
even with hight color saturation the fringe is gone. 
The "unnatural" star colors are normal for a filtered image.


With that it is possible to deal with the color fringes - at least until I find a more permanent solution. 













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