Always ask for authentication when using popup winow on IE6

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C#

ASP.NET

Javascript

Java

I am facing a problem with popup window on IE6. It always redirect to login page.
- In Web.config, I configured all files are required to login:

<authentication mode="Forms">
    <forms name="frmLogin" loginUrl="/Default.aspx" defaultUrl="/Home"></forms>
</authentication>
<authorization>
    <deny users="?" />
</authorization>

- When user input correct username and password, I use the following to provide authentication:

FormsAuthentication.RedirectFromLoginPage(username, false);

By using the above code, user could view all pages.
But in IE6 & 7:
- It's ok if user view page with the same window
- But if i open it pop up window using javascript as below, it asks for authentication again:

window.open(url, null, "width=1050, height=600, scrollbars=yes");

Please help me to fix it.
Thanks