var done=false;
function launchViewer(filename,x,y)
 { 
 if (!done)
 {
  window.imagex=x;
  window.imagey=y;
  window.imagename=filename;
  viewer=window.open('http://www.amsterdamsebinnenstad.nl/viewer/z.html','VIEWER','scrollbars,status,resizable');
  viewer.opener=window;
  viewer.focus();
 }
 return done;
 }