As you mention you want to open an ASPX page in the tooltip, I mentioned you will need to use an IFRAME - please refer to the MSDN
for information on this HTML element.
As for the UpdatePanel control - for information, please click this link
However, you need not know much about the UpdatePanel - I only mentioned it as you will need to add the IFRAME to the UpdatePanel in the RadToolTip:
|protected void OnAjaxUpdate(object sender, ToolTipUpdateEventArgs args)
| HtmlGenericControl iframe = new HtmlGenericControl("IFRAME");
| iframe.Attributes["src"] = "Test.aspx";
In this code fragment, I use the AjaxUpdate handler of the RadToolTipManager to create an IFRAME, set its SRC attribute to the desired ASPX and add it to the RadToolTip that is about to show.
All the best,
the Telerik team