Active(Adaptive) Optics: Optical components or assemblies whose performance is monitored and controlled, and whose mirror shape (figure) is actively and continuously being deformed in order to compensate for disturbances (aberrations, static and dynamic perturbations), or adapt to changing conditions, system needs or mission requirements with a resulting improved optical system performance.

Ambient Light: Light present in the environment around a detecting instrument generated from sources outside the instrument (optical system) and whose presence is considered noise by the detector system.

Aperture:An opening or hole (having an aspect ratio of at least 5 to 1,usually 10 to 1 or greater),through which radiation passes.

Aspect Ratio: The ratio of width to height of an image or target.

Attenuation: The phenomenon of the loss of average light through an optical system (input optical power to output optical power).

Autocollimation: Technique of projecting a target (image) at infinity and receiving that target (image) after reflection from a flat mirror (or other flat reflecting surface)

Autoreflection: The use of a sighting instruments (viewing telescope) or a point source's own image, after reflection from a flat imrror (or other flat reflecting surface), to establish coarse preliminary alignment.

Auxiliary (Finder) Telescope: A small telescope mounted to the main optical structure that has lower magnificaition and a much larger field of view than the primary optical system and is used for acquisition.

Baffling: An opaque shielding item designed to reduce the effect of stray light (external or internal) from entering an optical system and being sensed by the detector system.

Baseline: Requirements agreed to with systems engineering or system test engineering.

Beam Expander: A telescope configuration designed to increase the diameter of a radiation beam.

Boresight: (a) To align the optomechanical axes of two instruments, (b) To align the optical axes of an instrument to the mechanical axes of that instrument, and (c) To align the optical axes, the mechanical axes, and the electro-optical (detector) or an instrument.

Cross-coupling: The motion in one axis as a result of an input to an other axis motion.

Crosstalk: The measurable leakage of optical energy (electrical signal detection) from one optical item to another.

Dark Noise (Current): The current or pulses (signal) produced even when a detector system is shielded for all known external optical radiation.

Distortion: The image is not a true-to-scale reproduction of the object.

Flatness: The measured deviation of a surface with respect to a reference plane.

Hysteresis: (a) The dependence of an item on its previous history, causing a permanent non-repeatability effec. (b) The error in a measurement between up-scale reading and down-scale reading or its ability to be repeatable between the up/down scale readings.

Interferometry: The study and utilization of the interference of light waves to measure optical surface quality and also to measure lenths or change in length (or thickness).

Jitter: The deviation from an angle, measured in a scanning direction and due to inherent random mechanical vibrations.

Line of sight (LOS): The straight line connecting an object and the objective lens of an optical instrument.

Linear Array Detector: A solid-state detector consisting of a single row of light-sensitive semiconductor devices.

Mirror (Mirrow Assembly): Intentionally left blank.

Modulator: An item that interupts a beam of light through use of acousto-optical, electro-optical, or electro-mechanical principles.

Optical Chopper: A mechanical or electrical-electromagnetic device for periodically interrupting a beam of light for a known brief interval time.

Optical Flat: A piece of material having one or both surfaces ground and polished plano,generally flat to less than a tenth of a wavelength of light.

Piezoelectric Effect: The interaction between electrical and mechanical stress-strain factors in a dielectric crystal material such that when it is compressed, a voltage is generated across it or when a voltage is applied across certain faces of the material, a mechanical distortion is produced.

Pig-tail: A short length of optical fiber, permanently fixed to a component (savior of the laser diode).

Pixel: The smallest resolvable area of an array type detector.

Quiescent (Period): The quietest testing period, usually at least two hours.

Range: The difference between maximum and minimum measurements of an instrument or system, while maintaining the required specifications.

Reflectivity: The ratio of the energy of the total radiation reflected from a surface to the total incident on that surface.

Repeatability: The measure of accuracy with which a predetermined or previous setting of an instrument can be discriminated and measured.

Resolution: The separation of closely related instrument movements, and indicaion of the degree to which they can be discriminated and measured.

Retroflection (Retroflector): Reflection in which radiation is returned in direction close to that from which it came, being performed over a wide field-of-regard, utilizing a corner of a cube with mirrored surfaces.

Scanning (Scanner): The successive measurement of the light level or other characteristics of the component of a picture area (scene view), following a given sequence.

Scattering: Change of the spatial distribution of a beam of radiation when it intercats with a surface, in which there is signal in an electro-optical system.

Shutter: A mechanical device that cuts off a beam of light by opening and closing at specified rate of speed.

Slit: An aperture, rectangular in shape, with a large length-to-width ratio, and fixed or adjustable in edge configuration.

Stability: The ability of an instrument to maintain positional accuracy with passage of time in a given ambient.

Stepper Motor: A positioning drive that rotates a fraction of a 360 degree turn when the motor coils are activated (rather than with a continuous rotation).

Straightness: The ability of a system to move an object along a perfectly straight path.

Strehl Ratio: The ratio of the illuminance at the peak of the diffraction pattern of an aberrated point image to that at the principle which greatly reduces normally experienced rotational "runout".

Striction: Friction that tends to prevent relative motion between two moveable parts at their null position.

Thermoelectric Cooling: A refrigeration method based on the Peltier effect, where an electric current is passed through a junction of two dissimilar metals joined in two places and heat is absorbed at the cold junction and dissipated at the hot junction.

Tracebility: The ability to identify and follow, step by step, the origin of technical information, usually at the "baseline" specifications or requirements.

Tracking Accuracy: Measurement of a translation stage's deviation from absolute straightness (that is, its angular motion in both the vertical and the horizontal planes.

Vignetting: The blockage of light in an optical system due to obstruction (mechanical) of a portion of a bundle of rays.

Wavefront: A wavefront, in the field of electromagnetic energy emanating from a source, is the surface connecting all field points that are equidistant from that source (A surface of constant phase).

Wind-up (torsional): An effect whereby a torque applied to the drive end of a shaft rotates it farther than the output end of the shaft.

Wobble: An irregular rocking or staggering type of motion that is caused by angular error between the datum axis and the actual rotaion axis.