ASP.Net Javascript "focus" Method Needs To Be Called Twice??

I am using the script below to open a new window and once opened,
redirect to that open window from the original window:

private void btnNewPDFWindow_Click(object sender, System.EventArgs e)

{

string NewPage = "NewPageZoom.aspx";

string ScriptBlockNewPage = "<script language='javascript'>var
NewPDFPage=window.open('" + NewPage + "','PDFPage');";

ScriptBlockNewPage = ScriptBlockNewPage +
"NewPDFPage.focus();</script>";

Response.Write(ScriptBlockNewPage);

}
For some reason, with the code above, I have to click the button twice
in order to get it to focus to the 2nd window again.
Any ideas?