Close parent window and open same link in pop up window

Question

I want to close parent window and open same url in pop up window on user login. 

i have write following code for it:

Page.ClientScript.RegisterStartupScript(this.GetType(), "newWindow", String.Format("<script>window.open('{0}','mywindow','width=900,height=600,menubar=No, toolbar=no, scrollbars=yes');</script>", "Default.aspx"), false);

but it shows logout window when popup open.