If you can't get rid of it, you can leave in the vignetting, or you can color correct the magenta fringing.
Try ISO 3200 or less if you are experiencing this.
The solution here is to use a low enough ISO to prevent or at least minimize the amplification noise. This happens because the amount of light hitting the edges, in particular the corners, of the frame is much less than the amount of light hitting the center of the frame due to the vignetting of the lens, and if the light level is dark enough then you're not overcoming the amplification/circuitry noise of the sensor and camera, so boosting the dark areas shows that noise. In some cameras, if you're star stacking with shutter speeds of around 10 seconds or less and your lens has strong vignetting at the aperture you're using, you could end up with very strong magenta color noise issues along the edges of the frame when you apply vignette correction in your raw editor, which brightens the dark edges, revealing the color noise. Blending the static foreground shot with the star tracker shot would require dealing with blending the blurred foreground of the star tracker shot with the sharp foreground of the static shot.
I do this anyways even without a tracker so that I can get detail and low noise in my foreground. This is fine if you’re just doing shots of the sky without a foreground, but if you’re capturing the foreground then it will blur in the star tracked exposures, so if you want a sharp foreground you’ll need a separate exposure (or more than one) of the foreground with the tracker turned off. You can certainly do this, but that requires lugging around the star tracker, and polar aligning the tracker every time you move your tripod. Before we get into star stacking, you might be wondering if you could just us a star tracker, a device that sits on top of your tripod and turns with the rotation of the earth so that your camera can follow the stars, capturing long exposures with no star movement.
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