Here is a java script function that opens a new window (popup) and puts it on center of screen:

function PopupCenter(pageURL, title,w,h) {
var left = (screen.width/2)-(w/2);
var top = (screen.height/2)-(h/2);
var targetWin = window.open (pageURL, title, 'toolbar=no, location=no, directories=no, status=no, menubar=no, scrollbars=no, resizable=no, copyhistory=no, width='+w+', height='+h+', top='+top+', left='+left);
}


CLICK TO OPEN POPUP

function PopupCenter(pageURL, title,w,h) {
var left = (screen.width/2)-(w/2);
var top = (screen.height/2)-(h/2);
var targetWin = window.open (pageURL, title, ‘toolbar=no, location=no, directories=no, status=no, menubar=no, scrollbars=no, resizable=no, copyhistory=no, width=’+w+’, height=’+h+’, top=’+top+’, left=’+left);
}