function popup(b, h, url) {

var breite = b;
var hoehe = h;

var links = (screen.width-breite)/2;
var oben = (screen.height-hoehe)/2;

Fenster = window.open(url ,"Fenstername", "width="+breite+",height="+hoehe+",left="+links+",top="+oben+",resizable=no");

Fenster.focus();

}