| article: Planetary Imaging - Atmospheric Dispersion Correction |
How does it look - where does it come from?
The enemy number one of high resolution planetary imaging is the atmosphere.
Not only the effects of seeing can be harmful by smearing fine image detail.
At huge distances from the zenith there is another pain: dispersion or wavelength dependant refraction.
This is causing the red and blue fringes at the edges of the planet's disk.
Jupiter was 5 degrees higher in the sky and the dispersion is considerable less.
If you want to know more about that effect check the article on atmospheric effects in astronomy.
Correction
The angle of refraction in the atmosphere is depending on the wavelength of the incoming light.
Hence we have to apply a different amount of correction for every single wavelength in the image.
I do not know any standard software which would be able to handle that.
Fortunately it is already a big improvement if we only correct for Red against Green and Blue against Green.
While the method I soon will describe is not perfect I think the improvement is significant and worth the effort.
The correction should be performed before any other enhancement - just after the combination process of the raw frames.
I will describe the method for AstroArt, but a lot of other image processing programs will do the job.
- Open your image in AstroArt. It should be cropped to just more than the planetary disk (as the images above)
- Call from the menu Color / Split RGB
- Three more images are created with r, g, b as first letters indicating red, green and blue
- Activate the blue image and call from menu Image / Align and choose your green image as a reference
- In the alignment dialog choose "planetary" as the method
- The planetary disk should now be aligned with the green one. Depending on the qualitiy of your image the precision of alignment might be high or lower
- Repeat the alignment with the red image against the green one
- As the final step call Color / Trichromy, RGB
- A new and dispersion corrected planetary image should now appear on your desk!
If you know about a faster or more accurate method: Please send me a comment.
If you want to know how to apply a Multiple Unsharp Masking click here.
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