2014 General Assembly in Victoria

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Atmosphere dispersion in planet images

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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 and recombining.

The 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 easier.

John McDonald

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Last updated: December 12, 2013

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