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Last night, I took some pictures of the NEOWISE comet. To my surprise, the comet has a greenish colour on all of them, e.g. this one which I've enlarged a bit:


(click to show the original image)

When viewing with the naked eye, it appeared mostly white; I've also seen blueish and orangeish NEOWISE photos on the Internet but never green. The green color also appears on the raw images. I'm wondering what might be the cause of this?

My setup:

  • Canon EOS 1100D, manual mode
  • I used two lenses (but the greenish colour appears on both):
    • EF-S 18–55mm, I think I have the IS II version
    • ET-60 75-300mm, the photo above was shot with this one at 75mm
  • f/4
  • 30 seconds exposure
  • ISO 100
  • Automatic white balance
  • If any additional information is needed to investigate this, please let me know. – Glorfindel Jul 21 at 20:18
  • Cameras are more sensitive to green light. (Just like Humans) 50% of your subpixels are green: photo.stackexchange.com/questions/104893/…. – SurpriseDog Jul 22 at 3:29
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    Auto white balance… are you in/near a city with 'sodium' lamps? – Tetsujin Jul 22 at 6:16
  • @SurpriseDog thanks, that's interesting information. But that would mean others should have this problem, too ... – Glorfindel Jul 22 at 7:34
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    This is probably an issue of auto white balance. – Jonas Jul 22 at 10:00
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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.

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  • +1 Good points. FWIW, this looks relevant: syfy.com/syfywire/… – Kahovius Jul 23 at 12:37
  • There are no actual green stars out there, only stars that appear green due to white balance issues. en.m.wikipedia.org/wiki/Green_star_(astronomy) – SurpriseDog Jul 23 at 13:44
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    @vsis Quote: you'll see a wide spectrum of colors in the stars. - But you won't see any green ones, unless your white balance is off. – SurpriseDog Jul 23 at 20:04
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    Neowise may be greenish, but it's hardly as GREEN as shown in the example. – Michael C Jul 23 at 22:21
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    It looked like a whitish blur when I looked at it through my 10x50 binoculars ¯_(ツ)_/¯ – SurpriseDog Jul 24 at 13:36
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When doing astrophotography, only use RAW -- avoid JPEG. Astrophotography images require a lot of post-processing and JPEG images don't have the adjustment latitude (JPEG is mostly for "final output" -- when no further adjustments are necessary).

In RAW, there is no white balance (the white balance is simply recorded as a preference in meta-data but is not actually processed in the image. Post processing software usually ignores it ... but some post processing applications will attempt to use it.)

The comet appears green because it is green.

As the comet (or any comet) "melts" as it nears the sun, it gives off a lot of dust, but much of the ices are molecules that have very low melting points (e.g. CO2 ice melts at roughly -57°C (rounded value)).

These gasses absorb ane re-emit electromagnetic radiation and this causes them to "glow". The colors you see are based on the elements involved.

Here's a shot (mostly unprocessed with the exception of slightly adjusting the background) that I captured Friday, July 24. But even though binoculars, the "hair" (coma) around the head of the comet is distinctly "green". (I did not want to post a processed shot because that can be misleading).

Comet C/2020 F3 (NEOWISE)

Image captured using Canon 60Da using 200mm at f/2.8 and ISO 800. The camera was on a polar-aligned tracking mount (Losmandy StarLapse)

Note the very strongly 'green' coma around the head of the comet. This is not an artifact of the camera ... it appears this when observed through binoculars.

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  • syfy.com/syfywire/… I found this article that adds further information to the color of C/2020 F3. Also, this is a great photo! – Jonas Jul 25 at 14:17
  • If your shot is "unprocessed", how is it in color? Something had to set the color channel multipliers and, thus, the white balance. – Michael C Aug 7 at 23:26
  • Hi Michael, the image was shot RAW (5 min exposure at the settings above). Of course it had to be converted to JPEG for upload. That means I opened it on the computer (which de-mosaics the data) and export as JPEG. I did reduce the black level so the background would be less gray ... but that was the only adjustment performed by me. – Tim Campbell Aug 8 at 15:35
  • In other words you allowed either your camera or your app to do "Auto WB" for you. – Michael C Aug 9 at 9:51
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Because when set to "Auto" White Balance, your camera does not have enough information in an almost totally dark frame to guess very well.

For astro work, I usually set the WB to 5200K with no correction in either direction on the Green ←→ Magenta axis. You're always free to change it in development if you're saving raw files. You should always save raw files when doing astrophotography.

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  • Thanks, I'll give it a try. I do have the raw files so maybe I can still do some post-processing. – Glorfindel Jul 23 at 10:58
  • @Glorfindel Keep in mind that the actual raw information is a set of monochromatic luminance values. There is no color until that information has been interpreted by several processing steps. What you see on your screen when you open a raw file is not "THE raw file", it is one of a near infinite number of possible interpretations of the raw data. You are free to reinterpret the raw data using any set of processing instructions you desire. – Michael C Jul 23 at 22:26
  • At night you hardly see color, especially on objects as faint as the net is meanwhile. Yet a white balance issue seems most likely. Without calibrations or reference white level shift the colors till they fit your aesthetics – planetmaker Jul 24 at 3:20
  • White balance is a bit different in AP. I often do WP based on selection a region of the background -- which should be neutral -- vs. any "foreground" subject in the image. Also, I usually use PixInsight (PI) to process my astrophotography (although I did not use PI on the image I posted above). But PI allows selection of the debayering algorithm from a RAW image. They strongly suggest using a debayering algorithm called VNG because this algorithm is well-suited for under-exposed data (which is most AP images). – Tim Campbell Aug 8 at 20:29
  • I was going to add... if there is a G2 star in the image (but that's hard to know) then you can pick that star as a WB source (assuming it wasn't over-exposed.) – Tim Campbell Aug 8 at 20:50

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