After a small success in shooting M31 with a 105mm lens I thought I would join the Big Boys (an Gals) and get a really outstanding shot with a true telephoto. Well....not so much. Such dreams can suffer an early death quickly. I spent four or five hours shooting M31 with a 600mm lens (at 450mm) and star tracker (first time). I shot about 120 lights and various darks, flats and bias shots. With palms sweaty from anticipation I got home at about 5:00 AM and ran them through Sequator and opened the image for further processing. There was an obvious problem, it was a blurry mess!
On site there was nothing but the stars to focus on so I use live view to focus as best I could manage. Before I spend another four to five hours reshooting I thought I would get the wisdom of some of you more experienced folks.
Attached is a jpeg file from the session. The edges of the stars are sharp but obviously they are not what is needed for a good results. The star tracker seemed to do a credible job. Am I right that this is a focus issue? If so, I have since confirmed that the center of the infinity mark on the lens barrel is indeed accurate. That said, I understand that if one uses the hyperfocal distance (plus a little further out) everything beyond the near Dof will be sharp. Is this a fair statement for use in astrophotos also? Any help will be much appreciated!

60sec, 480mm, f6.3, Nikon D750 all on manual control, VR off.