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5 minutes is quite long even for a lens without autoguiding. So yes probably the fact of not autoguided.
You can do a Photoshop correction by lassoing that area, feathering it 20 pixels, duplicate layer, set it to darken mode then filters/other/offset and play with the vertical and horizontal numbers (they may need to be - numbers ie -1) until it looks better. You may need to boost slightly with curves as sometimes it dims the stars. Fine adjustment by opacity of the slider or edit/fade.
If you're not comfortable with the above I am happy to do it for you. It'd take me 2.5 minutes (I'll time myself!).
Greg.
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