Redirect to another page after form filling

Hi,
The code below submits an email and when it's sent, a message comes up thanking the person while the form is still visiable. What code do I  need to add so when the person submits and presses the button, they are redirected to another aspx page where a thank you note is.
Many thanks
Zuke
using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Web.Mail;
using System.Net.Mime;
using System.IO;
using System.Text;
public partial class Pages_contact : System.Web.UI.Page
{

StringBuilder str_mailer = new StringBuilder();
protected void Page_Load(object sender, EventArgs e)
{ ...