https://www.cloudynights.com/topic/476781-m45-halo-trouble/
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Removing Purple Star Halos
https://www.skyandtelescope.com/astronomy-blogs/astrophotography-jerry-lodriguss/purple-star-halos-astrophotography/Purple halos around stars can be a problem when shooting with inexpensive camera lenses and telescopes. They come about when refractive optics — optics that contain lenses — don’t focus blue and red wavelengths of light at the same place as the rest of the visual spectrum.
Because these wavelengths focus at different distances based on the refractive index of the glass, star images at red and blue wavelengths will be out of focus and appear larger than the in-focus star images in green light. These out-of-focus red and blue images combine to form a purple color that appears as a halo around bright stars.
Refractive optics in achromatic telescopes focus the wavelengths to which the eye is most sensitive, in the green portion of the spectrum. Because the eye is not as sensitive to the long red and short blue wavelengths, their lack of color correction is not as noticeable visually. But digital cameras will record these wavelengths, particularly in the long exposures common to deep-sky astrophotography.
觀星論壇 
