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About two years ago I was at a spot with a beautiful night sky with no moon so I tried to get some pictures of the sky.

I wasn't well equipped back then, I had a 35mm f1.8 lens and Nikon D50. I didn't shoot RAW back then. I had to manually try to find infinity focus as the camera could not do it.

These were shot at f2.5 and ISO 1600 and I cropped them using two stars as a guide.

You can see that the colors of the same stars vary a lot. On bottom left, you can see a star go from magenta to purple. I think this goes way beyond what can be explained by ISO noise.

What causes the randomness in colors in those pictures?

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Individual star close-ups. Remember that this is taken from a JPEG file:

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    Are you sure they are "fake" ? The farther the distance of a star from you, the more the light of the star shifts to the red side of the spectrum. The differences in colors may or may not be caused by one factor, Could be physics or could be camera, or both. OR? May not be a star at all. – Alaska Man Aug 16 at 17:07
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    @AlaskaMan The OP's point is that some stars have a very different color between the two shots. – xenoid Aug 16 at 17:17
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    @AlaskaMan, Sounds like you are talking about Hubble's Law, which says that the light from distant galaxies is red-shifted, on average, by an amount proportional to their distance from our own galaxy. It doesn't apply to stars within our own galaxy. (i.e., does not apply to any of the stars that you can see in the sky with your naked eye.) But if you stare at the night sky, you probably will notice that some stars actually are different colors from others. en.wikipedia.org/wiki/Stellar_classification – Solomon Slow Aug 16 at 22:25
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    Red-shift doesn't really come in to play with stars that are visible in the night sky. The shift occurs when an object is receding at a noticeable percentage of the speed of light. Spectral shift can be detecting using very sensitive instruments ... but not with a typical camera. Stars in our own galaxy aren't approaching or receding at high enough speeds to be noticed by a typical camera. – Tim Campbell Aug 17 at 2:11
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    @AlaskaMan: Green stars do not occur. – Joshua Aug 17 at 18:35
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If you are focusing well, stars are comparatively likely not to occupy significantly more than a single pixel. But pixels are covered with a regular grid of color filters, the Bayer filter typically using an RGGB arrangement for 2×2 cell grids. A so-called demosaicing algorithm making use of redundancy/correlation of luminosity information then tries to reconstruct RGB information. But if a star lights only a single pixel, there is no redundancy/correlation to work with for estimating the color distribution.

So if you want a good estimate of colors, you'd need to defocus a bit so that stars get a chance to touch more than a single pixel. Systematically, you can do that by using diffraction, namely very small apertures. Ironically, it may also help if your sensor resolution is better than what your camera optics may be able to deliver.

You may also try recording raw images and then playing with various demosaicing algorithms: some may work better with the inherently problematic situation (possibly by leaning to a stronger default behavior of preferring to guess "white" in the absence of better information).

One thing also worth noting is that color filters have wide selectivity, and demosaicing algorithms tend to assume correlations between colors due to the image elements mostly being reflective and sharing a common illuminant. That assumption does not work on a star picture because every star has its own independent color spectrum. So this can be a reason that more complex demosaicing algorithms usually considered to be superior can actually work worse in this situation, making differences to the exact alignment to the pixel grid produce worse color variation than a different algorithm would.

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    Stars always occupy more than a single pixel (you would have to have an exceptionally low resolution camera with massive pixels ... so that they look more like tiles). This is because while stars are so distant they technically qualify as a "point source" of light, that light travels as a wave and the wave nature of light causes the stars to focus as an Airy Disk (named for Sir George Airy - the astronomer who discovered the effect.). They always occupy several pixels on any modern camera with even modest sensor resolution. A Bayer Matrix will not cause a color shift in stars. – Tim Campbell Aug 17 at 2:18
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    @Tim Campbell: you are confusing the effects of light travel with the diffraction limit (the latter is what is creating the Airy Disk image on the sensor). At F2.5 (as the OP wrote), you are nowhere near the diffraction limit of your optical path so the Airy Disk is really small. The antialiasing filter will likely make more spread, but it's still such a low amount that the demosaicing does not get enough point spread to work well. And with regard to "modern camera with modest sensor resolution": the OP specifies a 6MP DX sensor camera. – user94588 Aug 17 at 6:30
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    @TimCampbell In a camera, the most relevant contributor to diffraction (which of course is due to the wave nature of the light) is the aperture. For a DX size sensor at aperture F2.5 on a 6MP sensor, diffraction makes for an Airy Disk with the main lobe in subpixel size. Since that is likely to lead to Moiré patterns, sensors with less than miniscule pixels are typically preceded by antialiasing filters causing a larger diffraction spread. But again, the main lobe tends to be somewhat less than pixel size, and this particular camera has tested as not suppressing Moiré patterns well. – user94588 Aug 17 at 15:42
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    Yep. This is why astronomers almost never use sensors with mosaic filters. If we want a color image, we use a monochrome sensor and combine exposures taken through multiple filters. Or we use multiple sensors. – John Doty Aug 17 at 18:55
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    @TimCampbell "Occupying a fair number of pixels" does not help a lot when the main weight is focused on very few. The "individual star closeups" don't appear to deal with color-changing stars but instead all show a white core. Without any further indication from where they have been taken (both picture and location in it), they don't show much more than a color gradient consistent with chromatic aberration that an old camera does not correct. "I cropped them using two stars as a guide" does not sound like identical framing, so interactions of CA with demosaicing would vary. – user94588 Aug 17 at 21:16
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I can tell 3 common reasons for weird/fake colors in astrophotography:

  • Chromatic aberration makes some starts appear white in the center, but their borders blue or red, depending what of those two are out of focus.
  • Demosaicing algorithms tends to fail for bright white objects against a dark background, and you see red or blue in one border of some stars. Noise makes it worse. See these examples.
  • Automatic white balance: If you are using no filter, just set WB in daylight.

Of course, stars may be red or blue, and most of the cases is OK to get those colors.

F/2.8 is a bit wide open and I'd expect some optical aberrations in the corner of the image. If posible, shoot with f/5.6 or f/8.0.

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ISO 1600 is the upper limit of D50's sensitivity, therefore the picture is likely to get a bit noisy. Noise is not guaranteed to be uniform across color channels, so it may manifest as color changes.

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On the second shot the stars seem to have all shifted towards blue, so I would assume a camera auto-white balance trying to make sense of a mostly black picture that makes it very sensitive to minor changes. Is the color temperature coded in the EXIF data?

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    It only says "White balance: sunny". White balance sounds like a possible explanation, but it does not seem like the change was uniform across all the stars. – Tomáš Zato - Reinstate Monica Aug 16 at 18:33

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