in asp. page
<asp:Button ID="btnPhoto" runat="server" Text="Button" />
in c# Page_Load event
btnPhoto.Attributes.Add("onclick","window.open(' your page name comes here with
Extension ','cal','width=305, height=206, left=630, top=384,toolbar=no, menubar=no, scrollbars=no');");
OR
<asp:Button ID="btnPhoto" runat="server" Text="Button"
OnClientClick='window.open("your page name comes here with Extension","cal","width=900, height=300, toolbar=no, menubar=no, scrollbars=no")' />
