opening the aspx in new window while retaining PreviousPage.FindControl method

Hi
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 =
  &nb ...