Hello,
This sounds like you dont have enough back focus. Have a read of this
https://www.opticscentral.com.au/blo...s-with-a-dslr/
I too have a D600 and love it for astrophotography.
If your rings/connections are M42, I highly recommend to switch to DX (APS-C) mode. Dont use the 1.25 nose adapters.... they are pointless on these camera.
If your using 2"/M48 adapters you can use full frame.
Cheers
Wayne