Are you using a stable tripod? Like really a stable one not a $10 walmart one? Was it windy? Do you use a delay or remote shutter so that the button press doesn't create blur? All of these definitely matter when shooting an exposure for that long. Most likely there's this slightly little camera shake that is throwing you off that you're not realizing. Are you also shooting raw? Do you have sharp glass? 600m @ F6.3 would require some real expensive glass to get it to look sharp. Full sensor camera body? also a factor when doing astrophotography.

If all this fails and you're sure you're doing everything right in those aspects (not doubting you! just make sure!) you could try a technique called image stacking in which you take multiple photos over a range of F-stops and focuses in which you stack together using a photo editor that creates a single, fully composed image. Good luck!