function openwindow(url)
{
var new_window = window.open(url,'popup','width=400,height=514,scrollbars=no,menubar=no,status=no,toolbar=no,location=no,resize=no');
breedte = screen.availWidth
hoogte = screen.availHeight
toppos = (hoogte - 514) / 2
leftpos = (breedte - 400) / 2
new_window.moveTo(leftpos,toppos) 
new_window.focus();
}