function center_popup(popup_url, popup_width, popup_height, popup_top_offset, popup_options) {

	// Position des Fensters berechnen
	var posx = (screen.availWidth - popup_width)/2;

	// offset wird z.B. wegen toolbar-Höhe abgezogen.
	var posy=(screen.availHeight - popup_height)/2-popup_top_offset;

	// Beistrich anfügen
	if (popup_options!='') popup_options = popup_options + ',';

	// Fenster öffnen und Fokus holen
	popup=window.open(popup_url,'',popup_options + 'width=' + popup_width + ',height=' + popup_height + ',screenX=' + posx+ ',screenY=' + posy + ',left=' + posx + ',top=' + posy + '');
	popup.focus();
	return(popup);
}