function popUp (pagina, width, height) {
	if(!width)
		width = 580; 
	if(!height)
		height = 550;
	
	var opciones="toolbar=no, location=no, directories=no, status=no, menubar=no, scrollbars=yes, resizable=yes, width="+width+", height="+height+", top="+(screen.height/2 - height/2)+", left="+(screen.width/2 - width/2);
	window.open(pagina,"",opciones);
}