New mirror technologies to support Raytheon’s advanced electro-optical and infrared systems
Raytheon has enterprisewide technology networks established to communicate and coordinate technology needs and developments across the company. These networks help ensure discriminating technologies are available to our system solutions that, in turn, provide our customers with the highest performance capability at the lowest possible cost.
One particular technology network is the Multifunction Electro-Optical Systems Technology Network (MEOSTN) whose charter is to foster the advancement of electro-optical and infrared (EO/IR) technologies and promote enterprisewide communication, synergistic product development and technical reuse within the field. Mirrors play an important role in many EO/IR systems, both in the precise redirection of light and in component size and weight reduction. This article discusses two new Raytheon technologies in this area currently entering initial deployment with clear paths to expanding future roles across the company.
WIDE-FIELD STEERING MIRRORS
EO/IR turrets have become a standard feature on aircraft, providing magnified video images of the surface below for Intelligence, Surveillance and Reconnaissance (ISR) missions. They frequently require the ability to sweep large areas; for example, during rescue missions at sea when searching for survivors in life jackets on the ocean. Until recently, this required that the entire sensor turret slew to scan areas repeatedly, as well as have the ability to stabilize the optical images under conditions of shaking and vibration from the host aircraft. In fact, earlier generations of fast steering mirrors appeared in systems solely for image stabilization over small angles.
Raytheon has recently evolved the Fast Steering Mirror (FSM) beyond a single-task component into a multi-function scene scanning and image stabilization device (see Figure 1) able to sweep wide areas quickly, in much the same way as the human eye scans the page of a book without movement of the head.
The new breed of Raytheon FSMs are not only fast, but their integral position sensors make them precise, with low pointing noise and precise line of sight accuracy. As a result, EO/IR systems with this technology can produce sharp high resolution images of distant scenes free from jitter, while also having the ability to sweep large areas far more quickly than a practical imaging turret.
Raytheon’s current generation of EO/IR detectors are as wide as several inches across. The size of FSMs has also increased, taking advantage of the capabilities of these larger optics to produce high resolution images at extreme range. To account for the associated increase in mass of the larger mirrors, the new actuator mechanisms are reaction compensated to eliminate forces they would otherwise impart on the optical turret, causing shaking in the other instruments.
Use in airborne or space environments requires that Raytheon’s large diameter, wide field of motion mirrors be both rugged and reliable to support long duration missions under harsh conditions. These components are built to survive high accelerations and integrate with other systems having high operational reliability.
The practicality and effectiveness of Raytheon’s current generation wide field of motion FSMs have made them a capability multiplier in many programs. Today, larger mirrors with wider range of movement devices are under development, offering expanded capabilities and a clear path for future growth of the technology.
MERLOT® ULTRALIGHT METALLIC MIRRORS
Multispectral Reflective Lightweight Optics Technology, known as MERLOT, is a new material and design methodology that leverages free-form diamond point-turning capabilities to create high-performance, low-cost and lightweight reflective metallic mirrors and optical assemblies. The core of this technology is a new mirror blank material and manufacturing method to replace conventional aluminum 6061-based optical systems.
Raytheon originally developed MERLOT to meet weight and performance requirements for handheld targeting systems. In these types of systems, weight, toughness, material stability, optical quality and cost are all highly constrained. MERLOT opened new design space, leaving conventional aluminum 6061 for a 35 percent lighter weight material better adapted to low cost production in large numbers and an improved mirror process patented by Raytheon. The resulting mirrors provide diffraction-limited performance for improved daytime imaging, laser ranging and night vision target acquisition.
The blanks for MERLOT mirrors are produced by a type of low temperature, low thermal stress injection molding called thixomolding. With thixomolding, the mold for the mirror can have extensive light-weighting features such as carefully designed webs and machined mounting transitions (Figure 2), yielding blanks with a very efficient geometry absent of machining stresses imparted to the material. Starting with thin walled, weight-efficient shapes allows MERLOT mirrors to be stiffer than CNC Machined mirrors. A thixomolded alloy, due to its extremely fine grain and lack of harder elemental inclusions, can be diamond- point turned into high quality reflective mirrors (Figure 3).
MERLOT is tailored for standard diamond-point turning processes and holds a distinct advantage over aluminum 6061 in terms of diamond tool wear. With aluminum 6061, diamond tool wear is significant, forcing constant tool adjustments that increase cost and create undesirable and often unpredictable process variability. With MERLOT, diamond tool wear is virtually non-existent, providing tremendous opportunity for cost savings, uniform parts and reliable volume production. Additionally, the alloy is proven to be optically stable over time. Given the benefits it brings to metallic mirrors, MERLOT is an effective alternative to aluminum in reflective optics or optical assemblies seeking weight and cost reductions.
EO/IR devices and their associated mirrors, whether used as hand held instruments, seekers for missiles, imaging platforms on aircraft or sensors on spacecraft, are key to customer s’ mission success. The research, design and development, as well as manufacturing, deployment and support, of these assets continues at Raytheon as we develop practical solutions and decisive improvements in system capability and availability for our customers.
— Richard J. Wright, Andrew Bullard, John Anagnost, John P. Schaefer, Dan Vukobratovich, Scott Balaban