Web Forms :: Opening The Aspx In New Window While Retaining PreviousPage.FindControl Method?

I want to open a aspx page in new window.

I am openning a web page using JavaScript open .

StringBuilder cstext3 = new StringBuilder();
cstext3.Append("<script type=text/javascript> function OpenPreviewPage() {");
cstext3.Append("open('Preview.aspx');");
cstext3.Append("} </script>");

And executing following at button click on .cs page

protected void btnPreview_Click(object sender, ImageClickEventArgs e)
{
/*This will call the OpenPreviewPage method of java script from .cs file */
string javaScript =
"<script language=JavaScript>
" +
"OpenPreviewPage();
" +
"</script>";
RegisterStartupScript("ShowPreviewPage", javaScript);
}

The problem is the PreviousPage tag in preview page is null.

repPFareDetail = PreviousPage.FindControl("repFareDetail") as Repeater;

Is there any other way to open the aspx in new window by retaining the PreviousPage.FindControl method.