How to open a page in new window using Response.Redirect

Use OnClientClick="form1.target='_blank';"

<form id="form1" runat="server">

<asp:ImageButton runat="server" ID="imgBtn" OnClick="imgBtn_Click"
OnClientClick="form1.target ='_blank';"/>
</form>

in the code behind:

protected void imgBtn_Click(object sender, ImageClickEventArgs e)
{

Response.Redirect("NewPage.aspx");

}