Why does d750 have purple edges when shooting the starry sky?

The edge with strong contrast will appear purple edge and green edge because of dispersion, which is a physical phenomenon of the lens. Of course, expensive lenses will be improved, and dispersion will be corrected by technology, but the degree of improvement is not worth the price (except those who don't care about money).

Suggest post-processing, saving a sum of money.

PS opens the photo, the red mark is enlarged in ACR, and the other side has an inconspicuous green edge.

Select Lens Correction-Manual, adjust the parameters, and then compare whether the purple edge and the green edge are not obvious.