Get the AutomationElement for a WPF Popup

I'm working on a project that uses a popup in an editor to provide objects in a ListBox for users to double click in order to add them to their document. I'm trying to create automated tests for the app using the Automation API provided by microsoft, but I can't figure out how to get the AutomationElement for a Popup control, since it's in a different tree than my editor and its controls. Does anybody know how to get the AutomationElement for a WPF Popup ?