AJAX :: Hiding TargetControlID After Postback Processes?

I'm firing the my ModalPopupExtender from a server side button click (bthAuth) and then need to hide that button after my processing is complete. When btnAuth.Visible = False is called it displays the ModalPopupExtender. If I remove this piece of code the window goes away like it should.

<cc1:ModalPopupExtender ID="ModalPopupExtender1" runat="server" TargetControlID="btnAuth" BackgroundCssClass="modalBackground" PopupControlID="ModalPanel"> </cc1:ModalPopupExtender>

btnAuth_Click():

bunch of stuff here()

btnAuth.Visible = False -- This is what is causing the ModalPopupExtender to stay displayed.

On Page Load:

btnAuth.OnClientClick = String.Format("fnClickUpdate('{0}','{1}')", btnAuth.UniqueID, "")

Function:

<script type="text/javascript">
function fnClickUpdate(sender, e)
{
__doPostBack(sender,e);
}
</script>