function popup ( url, win_breite, win_hoehe, win_parameters, win_name ) {
	pos1 = ( screen.availWidth-win_breite) / 2;
	pos2 = ( screen.availHeight-win_hoehe ) / 2;
	parameters = "left=" + pos1 + ",top=" + pos2 + ",width=" + win_breite + ",height=" + win_hoehe + "," + win_parameters;
	win = window.open ( url, win_name, parameters );
	win.focus();
}