The Phottix Aster Wireless Flash Trigger Set is used to remotely fire camera flashes. The Aster transmitter unti emits a wireless radio signal to the Aster receiver unit when the camera"s shutter is triggered. The signal fires the flash attached to the receiver, depending on the radio channel (4 separate channels) selected. The receiver can also be optically triggered with a camera"s built in flash or other flsh units. This allows you to take off camera flash photographs with radio triggering. The Aster receiver unit features two operating modes: In the W (wireless) mode the receiver unit is triggered by the wireless transmitter unit. In the L (light) mode the receiver unit is triggered optically by another flash. An optical sensor is on the back of the receiver unit. Four radio channels (for W wireless mode) are available to avoid interference. The transmitter and receiver units have an on/off switch and a LED to indicate operation and triggering. The receiver unit"s hotshoe mount will accept speedlight-type flahs units. The metal tripod thread underneath the receiver can be used for mounting the unit on a tripod and or light stand. The operating distance in the "wireless" mode is 100ft and 30ft in open areas in the L (light) mode. Flash supported modes are manual and non TTL auto.