function open_popup(url, winName, winWidth, winHeight){
		window.open(url,winName,'toolbar=0,location=0,directories=0,status=0,menubar=0,scrollbars=0,resizable=0,left='+centre(winWidth,screen.availWidth)+',top='+centre(winHeight,screen.availHeight)+',width='+winWidth+',height='+winHeight);
}
		
function centre(size,area){return (area/2)-(size/2);
}