I thought some of you might be interested in the reason planet images often
show color fringing even when the imaging optics has little or no chromatic
distortion. Also, how to correct it. The image of Jupiter I got on May 14
suffered from this, and I had to correct by separating the colors, aligning them
effect occurs when shooting a planet that is in a low altitude position. In the
case of the May 14 image I posted earlier today, the planet was approximately 25
degrees above the horizon. Light from the planet had to bend due to refraction
when it entered the atmosphere and that bending depends on wavelength just as it
does in glass. I calculated the magnitude of the effect for the May 14 image
using values of the index of refraction from the CRC Handbook of Chemistry and
Physics. I got a result for the average image shift due to refraction in the
middle of the visible spectrum of about 2' (124" to be precise) in line with
that shown on p29 of the 2005 RASC Observers Handbook. The change in angle over
the visible spectrum due to slight changes in the index of refraction was 3.25".
That means the red colors are shifted 3.25" from the deep blue. Since the planet
is only about 40" in diameter the shift is significant. The result was
noticeable blue fringing at the top of the planet and red at the bottom.
Separation the colors, aligning and recombining nearly eliminated the effect and
helped tease more information from the image as shown in the attached photo.
The effect is less when the planet is high in the sky and can be avoided
altogether by finding a place on the earth where the planet is directly
overhead. For most of us, correcting the problem in software is less costly and