function popup() {
window.open('/kaarttrekken.php', 'popup', 'width=750, height=450, toolbar=0, scrollbars=0, location=0, status=0, menubar=0, resizable=0, screenX=' + ((screen.width - 750) / 2) + ', screenY=' + ((screen.height - 450) / 2) + ', top=' + ((screen.height - 450) / 2) + ', left=' + ((screen.width - 750) / 2) + '')
}