Close javascript window in code

Hello,
I have written a script in my application that opens a popup calendar as follows:

  Dim myScript As String = "<script language='JavaScript'>window.open('Calendar.aspx?ReturnTo=" & txtStartDate.ID & "', 'cal','width=180,height=160,left=270,top=180');</script>"

  RegisterStartupScript("popupCal", myScript)

This part works fine, and the window closes when the user clicks a date. The problem I am having is that I want to close the window when the user exits the page. I have tried using a function linked to onBodyUnload and a direct script (below) but neither work - failing as a Javascript error stating that the object 'cal' is undefined.

  Dim myScript As String = "<script language='JavaScript'>cal.close();</script>"

  RegisterStartupScript("closeCal", myScript)

Any help with my existing solution or an alternative would be greatly appreciated.
Cheers,
mcg