Triggering PopupControlExtender from button click

I've got a simple scenario: I'm using a text box with a PopupControlExtender to display a calendar. Works great, but not the most intuitive way to bring up the calendar. I'd like to add an image button to the right of the text box that could ALSO trigger the opening of the calendar, in addition to the normal click on the text box.

I've tried using a call to this as the OnClientClick property, but nothing happens:

function OnButtonClick(){ var behavior = $find('pce'); behavior.showPopup();}

I'd appreciate any assistance on what seems like itshould beso obvious!

-- Jim --

what about using the AJAX toolkit's built-in calendar extender?