The celestron nexstar rs 232 cable connects to your computerised telescope to a laptop or pc for control via various planetarium software programs the large adapter attaches to the serial port of your computer and the other end plugs into the telescopes hand control via a phone jack plug 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 once you have connected your computer and telescope