A photonics mast (or optronics mast^[1]) is a sensor on a submarine which functions similarly to a periscope without requiring a periscope tube, thus freeing design space during construction and limiting risks of water leakage in the event of damage. A photonics mast replaces the mechanical, line-of-sight viewing system with digital equipment, similar to a digital camera array, and it has fewer locational and dimensional constraints than a traditional periscope.

Unlike a periscope, it need not be located directly above its user, and it requires only a small pressure hull penetration for cabling. This allows the photonics mast to fit entirely within the sail of the submarine and means the control room need not be placed directly below the sail.

A photonics mast operates by rising above the water similarly to a telescoping car-antenna and provides information through an array of sensors, such as high-definition low-light and thermographic cameras. Images and information can appear on display panels for analysis. The photonics mast can also support the navigation, electronic warfare, and communications functions of a conventional optical-periscope mast.

The Royal Navy tested an optronic mast on the Trafalgar-class HMS Trenchant in 1998. Boats of the Astute class currently under construction as of 2012^[update] each have two optronic masts^[1] manufactured by Thales Optronics.

Use by United States Navy

In 2004, the United States Navy began fitting photonics masts to Virginia-class submarines.^[2]

According to the US Navy:^[3]

In Virginia-class boats, traditional periscopes have been supplanted by two Photonics Masts that house color, high-resolution black and white, and infrared digital cameras atop telescoping arms. With the removal of the barrel periscopes, the ships’ control room has been moved down one deck and away from the hull’s curvature, affording it more room and an improved layout that provides the commanding officer with enhanced situational awareness.

• US Navy Fact Sheet, Attack Submarines
• How Photonics Masts Will Work - from HowStuffWorks