Rugged design implies a severe service environment. Typical severe service environments include mechanical shock, high inertial loading, mechanical vibration, temperature extremes, nuclear radiation, environmental contamination, and user abuse. Effective design requires precise knowledge of both the actual service environment and the effects of the service environment on the system.
Lightweight is a controversial word, but is usually taken to mean systems that are lower in mass than conventional systems. For example, a lightweight telescope primary mirror is defined as being lighter than a solid mirror of the same diameter and stiffness. A key parameter in lightweight systems is the ratio of stiffness to weight. This parameter affects response to both inertial and dynamic loading.
New materials include both newly developed materials, such as the metal matrix composites, and non-traditional metallic materials, such as titanium. Structural optimization is applied to improve stiffness to weight. Telescope primary mirrors are optimized by contouring the back of the mirror; support structures are optimized so that parts of the structure serve more than one purpose. Insensitive design includes traditional structural practice and clever optical design. An Airy point supported beam is highly resistant to dynamic and inertial loading. Tube bending is compensated by passive optical design in relay systems. Isolation from environmental effects includes the use of enclosures and vibration isolation systems. Finally, active control is used to reduce weight and stiffness of systems, while still maintaining optical alignment through the use of position sensors and actuators.
Design examples of rugged and lightweight systems include cryogenic space optics, ultra-lightweight telescopes, and large astronomical astrographic lenses. A 0.5 m, 15 kg, fused silica mirror for the Space Infrared Astronomical Facility operated successfully at 9 K, demonstrating the use of materials selection, structural optimization, and isolation from temperature effects. A series of ultra-lightweight telescopes, up to 0.4 m aperture, employs new metal matrix composite materials, and structural optimization. Passive optical compensation for temperature effects, non-traditional metallic materials, and isolation are effectively employed in a 2 m focal length, f/10 astrometric objective.