J,
It is a pvc Tee fitting. It looks like is is slip 2 side and threaded on the metal pipe side. I can't see the whole assembly but it will have to be sawn out and unscrewed off the metal pipe. The then a new T fitting screw back on the pipe with teflon tape or plumber's dope for sealing the threads. Once that is done, the assembly can be rebuilt onto the tee. Best to do everything threaded instead of pvc glued when reassembling. JMHO Aloha

Thanks Lehua, I guess I ought to just disassemble it first and then see how parts go together. I assume I get all the parts at a big box store.

Ja,

Yup. The Tee will have to be unscrewed off the copper pipe and a new one screwed back on with white Teflon tape on the threads. Then work out what you need from there. Ask if you need more help. Aloha

So I got the tee back on and I can attach the anti-siphon valve but I can see I won't have enough flexibility left to be able to reconnect the PVC pipe coming out of the valve to the pipe going into the ground. Since I always turn off the shutoff valve in the basement to my irrigation pipes between uses, I'm considering leaving out the valve which should allow me to make the connection. What do you think of this idea or is there some way to join short lengths of PVC pipe when you don't have a lot of wiggle room to get the couplings on?