function center_window(wurl, ww, wh, wtarget, wopt)
{
	var sw = screen.width / 2 - ( ww / 2);
	var sh = screen.height / 2 - ( wh / 2 );
	var winpara = 'width='+ww+',height='+wh+',left='+sw+',top='+sh;
	if (wopt) { winpara = winpara + ', ' + wopt; }
	return window.open(wurl, wtarget, winpara);
}