The Celestron RS-232 Cable is used to connect your PC or computer to NexStar computerized telescopes via a port in the hand control. The large adapter attaches to the serial port of your computer. If your computer does not have a serial port, you will need a Serial-to-USB Adapter (#18775) so that you can connect the RS-232 Cable via a USB port instead. The other end of the cable terminates in a phone-jack style plug, and simply snaps into the port designed for this purpose.
Once you have connected your computer and telescope, you can update your hand control firmware. The latest updates can be found in the Support section of this website.
Connecting a computer to your telescope will also allow you to navigate the telescope from your computer screen! There are many software programs available today that will not only display the night sky as it appears on any chosen observing night, but that will allow you to send your telescope to any object on the screen with the click of a button.