open popup to close, update div in parent window?

i have a form where a user can open a popup to add categories. when the window is closed, id like the dropdown list in the form to get refreshed via rjs.

however, i am having trouble updating the drop down since i cant seem to access the div id in the parent window from the popup. anyone know how?

THANKS FOR ANY HELP!

If you're planning on using RJS already, why not have the 'close' method send the categories to the server, and then the server can build up the <options> and send them down to the browser?