function openwin_centered(aurl, awidth, aheight){
    var ascroll = 1;

    if (arguments.length > 3) {
      ascroll = arguments[3];
    }

    width=awidth;
    height=aheight;
 
    leftpos=(screen.width-width)/2;
    toppos=(screen.height-height)/2; 

    s='resizable=0,scrollbars='+ascroll+',status=0,titlebar=0,toolbar=0,directories=0,menubar=0,screenX='+leftpos+',screenY='+toppos+',top='+toppos+',left='+leftpos+',width='+width+',height='+height;
    win=window.open(aurl, '_blank', s);
  }