function openvenster(url,popup_width,popup_height,scrollbars) {
		var undefined;
		if (popup_width == undefined) popup_width=800;
		if (popup_height == undefined) popup_height=600;
		if (scrollbars == undefined) scrollbars='no';
		left_pos = (screen.availWidth/2) - (popup_width / 2);
		top_pos = (screen.availHeight/2) - (popup_height / 2);
		window.open(url,'popup','height=' + popup_height + ',left=' + left_pos + ',width=' + popup_width + ',scrollbars=' + scrollbars);
	}