Why does Comet NEOWISE look green in my photos but mostly white to my eyes?
Asked 7/21/2020
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I photographed Comet NEOWISE with a Canon EOS 1100D in manual mode using two different lenses, and the comet looks greenish in all of the images, including the RAW files. To my eyes it looked mostly white, and many photos online show it as blue or orange instead. My settings were f/4, 30 seconds, ISO 100, and auto white balance. What causes the green color, and could white balance or camera settings be affecting it?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
5y ago
2 Answers
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There are multiple possible reasons. First and foremost: Neowise is greenish. That's because of some chemical reactions (mostly diatomic carbon that emits green light when hit by sunlight). So yes: the green might very well be real.
On why C/2020 F3 Neowise seems to be white to the naked eye is because at night, all the cats are gray. Meaning: Below a certain level of brightness, our vision turn monochromatic. That's why most stars seem white to our eyes but take a color photograph of the sky and you'll see a wide spectrum of colors in the stars.
Originally by user24226. Source · Licensed CC BY-SA 4.0
user24226
5y ago
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The green tint is likely real. Comets can appear green because gases released from the nucleus—especially diatomic carbon (C2)—emit green light when energized by sunlight.
It can still look mostly white to your eyes because human night vision is poor at seeing color in dim light. In low light, our vision becomes much less color-sensitive, so faint objects often appear grayish or white even when a camera records color.
Your camera settings may also influence how the color is rendered, especially if you use Auto White Balance. In very dark scenes, AWB may not make a reliable choice. But since you said the color is visible in the RAW files too, the green is not just a JPEG processing artifact.
For astrophotography, it’s best to shoot RAW and adjust white balance later during processing. A fixed white balance is often more consistent than Auto for night-sky subjects.
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