function popup (w, h, file) {
	x = screen.availWidth / 2 - w / 2;
	y = screen.availHeight / 2 - h / 2;
	popupWindow = window.open(file, 'popup', 'width = '+ w + ', height = ' + h + ', left = ' + x + ', top = ' + y + ', screenX = ' + x + ', screenY = ' + y);
	popupWindow.focus();
}