Pop up windows

hello,
i have just done the tutorial on this site to create a pop up window, with said properties.

function Launch(page) {
OpenWin = this.open(page, “CtrlWindow”, “toolbar=no,menubar=no,location=no,scrollbars=no,resizable=no,width=550,height=250”);
}

i was just wondering if and how can you set the positioning of the window, on the screen. ideally i would like it to be popping up in the centre.

i am pretty new to this so any help would be appreciated.

thanks.

ed__would