Focal point and relay device
When working on their teleportation technology, the Black Mesa science team discovered that the alien world Xen was somehow involved during the teleportation process. This unknown factor had previously prevented them from accurately predicting where any teleport event would lead to back on Earth.
To counter this, they discovered they could build a device which, when attached to the realm's strange crystalline structures, allowed them to create a focal point and relay that aided in the teleportation. A survey team was sent to the borderworld to set up the machine, but the researchers on Earth lost contact with them shortly after it was set up in place. The science team later discovered other methods of aiming the field and abandoned the device.
In appearance, the Focuser is a large mechanical apparatus with two rotating arms known as emitters attached to a large crystal. It is powered by a small portable generator. The arms are controlled from a nearby console which must be aligned to achieve a maximum signal strength to the teleportation equipment at Black Mesa. When the device is tuned, the affixed crystals begin to glow.
In Blue Shift, Barney Calhoun and his associates need to make use of the old teleporter equipment in the Section A-17 Prototype Labs in a plan to escape the facility, but the equipment does not have the ability to target an Earth destination in its current state. Following Dr. Rosenberg's instructions, Barney is sent to Xen to find and activate the Focuser as this older lab equipment requires it for accurate teleportation aiming on Earth. Once he finds the device, he has to power it on and manually align the emitters. When it’s fully operational, aliens alerted by the equipment's activation arrive at the scene and attempt to prevent Barney from returning to Earth.
Behind the scenes
In the Half-Life storyline, series writer Marc Laidlaw intended for the satellite featured in the original game to to be part of a larger array with three sections. He stated that one portion of the array was in the Lambda core, one was in Earth's orbit as a result of the delivery rocket launched by Gordon Freeman, and one was in Xen. The Focuser featured here in Blue Shift that was activated by Barney is said to be the Xen part of this satellite array.