Astrophotography Lense

I'm looking to upgrade to a good Astrophotography lens but I don't know anything about them. I've read a few things here and there about different ones. What ones would you guys recommend? I have a Nikon D3400.

Dark Horse

Here's my first go at using a star tracker to shoot the core of the MW. This was shot at 70mm, with an exposure time of 60". Didn't care for the color version, so I edited in black and white.