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I am a beginner at photography, I tried to click photographs of the night sky. The image has a weird reddish cloud at the center, I first thought it might be because of light pollution but then I realized if it were due to ambient then the sides of the image would have the light but here its from the center. I think it might because of some interstellar cloud but my friend says there's no interstellar cloud in that direction of the sky. Can anyone please explain what happened?enter image description here

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Light pollution reflecting off very thin clouds or other types of haze in an otherwise crystal clear sky can cause such effects.

enter image description here
The brightest light on the horizon is not a sunset, it is light pollution from a city about 30 miles away. Another, smaller city at about the same distance is seen closer to the center of the frame.

enter image description here
A few minutes later with the camera pointed almost straight up at the zenith. Notice the light pollution reflecting of the wispy cloud at the bottom of the frame.

Both images were edited to be viewed in a dark room with a dark background (i.e. no white border). Opening them at native resolution in a separate frame with no light colored border will allow you to see them better. Scaling errors can make many of the stars dimmer/not visible when viewed at non-native resolution.

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I think it’s a combination of several effects: Fixed pattern noise, the CMOS chip contains millions of photo sites and each as a built-in amplifier. Low light lever shots call for a higher level of amplification. Since all work at slightly different levels, this is the pattern you get. Thy making an exposure or two with the lens cap on. See if these exposures duplicate this pattern. Also, I think this is ambient light pollution. Street lights and the like illumining dust and water vapor. You can get optical filters from astronomy sites that mitigate.

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First, what red cloud? I took your picture, extended the lightest and darkest parts to the full 0 to 1 range, then non-linearly brightened the dark areas while keeping the ends of the range fixed. Now we can at least guess what you are talking about:

There are probably multiple contributing causes for this:

  1. Light thru a lens, especially at wide apertures, is not even across the picture. The center will be brighter. Usually the effect is slight enough that you don't notice. Keep in mind I had to do some extreme manipulation of the image to make it obvious. If the same had been applied to the picture of a ordinary scene, it would look horrible.

  2. The sky is not uniform. Even what looks like "clear" sky can still have haze. Just because there aren't obvious clouds doesn't mean that the sky is completely transparent. There is nothing unusual about a little more haze in one area than another.

  3. Light pollution isn't uniform.

  4. The universe isn't evenly lit. It seems like there were more or brighter stars in the middle of your picture. This general increased brightness would also cause more scattered light from high altitude haze in that direction.

All in all, there is nothing unusual or even unexpected here, especially considering how slight the overall effect is.

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