function popup() {
   var w = screen.width; //fixed width
   var h = screen.height; //fixed height

   leftPosition = 0;		// centering horizontal position to middle of screen
   topPosition = 0;	// centering vertical position to middle of screen
   if ( h < 800) {
      var windowprops = 'width=' + w + ',height=' + h + ',top='+ topPosition +',left='+ leftPosition +',toolbar=no,location=no,directories=no,status=no,scrollbars=yes,menubar=no'; //set popup window properties
      var url = "main.php";
   }
   else {
      var windowprops = 'width=' + w + ',height=' + h + ',top='+ topPosition +',left='+ leftPosition +',toolbar=no,location=no,directories=no,status=no,scrollbars=no,menubar=no'; //set popup window properties
      var url = "main.php";
   }

   var popup = window.open(url,'history',windowprops); // open popup window with properties
//popup.moveTo(0,0)
//popup.resizeTo(w,h);
//popup.focus(); // focus on window
}