I tried to establish a VNC connection to 'lectura' from home, but it couldn't make the connection. I didn't get a useful error message, but I did note that VNC told me to connect to "lectura.cs.arizona.edu:73" instead of the usual single-digit number.

VNC connections to 'lectura' from outside the CS department are limited to certain ports for security reasons. If you run 'vncserver' and the display number assigned is > 64, you won't be able to connect.

People sometimes forget to shutdown their VNC sessions (with vncserver -kill :nn) so the range of allowed display numbers gets used up.  To see if you already have a VNC session running: run 'ps -u' and see if 'Xvnc' is in the list. If so, reconnect to the existing session.

You may be able to use other machines besides 'lectura'.  Check with your instructor to see if this is possible for your class.  The alternate machines are fd01, fd02, fd03, fd04, fd05, fd06, fd07, and fd08.  Although not as powerful as 'lectura', they normally have less load.

An alternate (and more secure) way to connect is by tunneling through SSH. See "Tunneling VNC with SSH" at http://www.cs.arizona.edu/computing/access/x11vnc.html .