My fiancé has an 8inch dobsonian and I’d like to get him some additional stuff for it for Christmas, but I’m very unfamiliar with this hobby and the terminology even though I enjoy sitting with him and looking through the telescope.

I know he has a 25mm eyepiece, a 9mm celestron plossl and a 7mm celestron plossl.

We live in a bortle zone 7 but often travel to a 3-4 zone for better viewing conditions. I know he’d like to be able to view and find deep space objects better/more easily.

Any suggestions? If I can get super simple explanations of some of these things it would be really appreciated because again, I’m unfamiliar with the terminology.

Ideally looking to spend under $300-400 whether that be one thing or a combination of things.