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Cassini-UVIS Header

Glossary

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Aberration-corrected toroidal grating -A kind of diffraction grating (see Diffraction Grating)

Absorption cell - A cell that is filled with a particular gas that absorbs light of particular wavelengths.

Angstrom - A unit of measurement, equaling 10 -10 meter or 10 -8 centimeters, usually used to express short wavelengths.

Aperture -An adjustable opening in an instrument (like a camera) that controls the amount of light that can enter.

ASI   - The Italian Space Agency

Attitude control - The act of controlling the position of a spacecraft relative to the direction in which it's moving.

Baffle - A device that regulates the flow of light so that it all comes from a particular direction.

Channel - A specific frequency band for transmitting or receiving signals, (i.e. different signal frequencies give you different television channels). For UVIS, a part of the instrument sensitive to a particular frequency band.

Channel electron multiplier (CEM) - A light-sensitive device that amplifies dim signals.

CODACON (Coded Anode Converter) - A devise, at the output of the detector, that converts the data from the spectrometer into an image format.

Command and Data Subsystem (CDS) - Cassini's main computer that commands and collects data from the various instruments and packages it to be sent back to NASA?

Concave grating spectrometer - A spectrometer whose diffraction grating has a concave shape.

Detector - A light-sensitive device that produces electrical signals when illuminated.

Diffraction Grating - A grooved optical element that has been deformed to reflect light of many colors. It acts like a prism to produce a spectrum.

Electromagnetic Spectrum - See "Spectra". Also see graphic of spectrum.

ESA - European Space Agency

Filament current controllers - Controls the amount of current sent to the HDAC filaments, and thus, their temperature.

Focal plane - The location where the rays from a distant object merge:   This is where we place the detector.

FPSCAN - A Cassini observation where the spacecraft turns slowly to fields and particles all around it.

Grating ruling density - The number of grooves per mm on the diffraction grating.

GSE (UVIS Ground Support Equipment) - Equipment used for testing and operating UVIS before launch.

Imaging microchannel plate detector - A light detector formed from glass full of many small channels that creates an image of the light shining on it.

Imaging, pulse-counting detector -   Each bit of light that falls on the detector creates a pulse of electricity. See "imaging microchannel plate detector".

Imaging spectroscopy -   Imaging spectroscopy is what UVIS does--it takes spectrographic measurements and turns them into a graph, but it can also turn the points on the graph into digital data that can then be made into pictures (images). Because humans can't see UV wavelengths of light, we assign different "false colors" to the different wavelengths that allows us to produce a picture out of the data that our eyes can appreciate.

Ion pump - A small vacuum pump that works by ionizing air molecules.

JPL - NASA's Jet Propulsion Laboratory in Pasadena.

LASP - Laboratory for Atmospheric Research in Boulder, Colorado - A research facility of the University of Colorado and the builders of the UVIS instrument.

Logic - Computer circuitry

Lyman-alpha emission - Light emitted by hydrogen atoms at a wavelength of 1216Å.

Magnetosphere - A magnetic environment surrounding an object, like a planet, in a fashion similar to an atmosphere.

MDT - Mountain Daylight Time -- The time in Boulder, Colorado, where LASP is. (Except in winter when it's MST -- Mountain Standard Time).

MgF 2 windows - Windows made from crystals of magnesium fluoride.

Microchannel plate detector - (see above under "Imaging...")

Nanometer - A unit of measurement that is one billionth (10 -9 ) of a meter.

NASA - National Aeronautics and Space Administration, headquartered in Washington, DC with field centers across the country.

Neutrals - Particles, objects or systems that have no electrical charge.

Objective lens - A lens that focuses light on the detector.

Optical coatings - Coatings specifically made for the lenses of light-sensitive devices.

Orbiter - A spacecraft whose purpose is to orbit around an object like a planet or a satellite (moon).

Payload - The instruments, crew or equipment carried by a spacecraft.

Photochemistry - The chemistry of the effects of light on chemical systems.

Photometer - An instrument for measuring the intensity of light.

Photomultiplier tube detector - A vacuum tube that amplifies and detects weak light signals.

Photons - Packets of radiant energy:   quanta of light.

Pulse- amplifier-discriminator - The part of UVIS that counts responses (pulses) to light falling on the detector. It discriminates between light and background noise.

Reflecting telescope - A telescope where mirrors are the optical elements, not lenses.

Remote sensing - Gathering information from far away; like the UVIS instrument will gather data on ultraviolet light in the saturnian system and send it back to Earth.

Remote Sensing Palette (on Orbiter) - The area on Cassini to which experiments (like the UVIS instrument) are secured into place.

Resonance absorption cells - Cells filled with gas that absorbs at its atoms' resonant frequencies (same as "absorption cell")

Satellite - A body that revolves around another body like a Saturn's moon Titan revolving around Saturn; or a man-made object revolving around a body, like Explorer 1 orbiting Earth. We call the moons of planets satellites instead of moons so as not to confuse them with the Earth's Moon.

Signal-to-noise-ratio (SNR) - A ratio to measures the clarity of a data source or transmission, usually a ratio of the signal amplitude and the noise amplitude. The higher the SNR, the less the interference with reception.

Slew - To turn something on an axis or to turn sharply or veer. Cassini slews to point its cameras at targets of interest.

Slit changer (entrance slit) - There are three slits each on UVIS EUV and FUV telescopes and that allow light into the instrument to be measured. The slit changer switches between them to measure with different spectral resolutions.

Solar blind CsI photocathode - The active element that is insensitive to reflected sunlight only seeing light with shorter wavelength.

Solar flux - The amount of light from the Sun.

Solar or stellar occultation - Watching the Sun (or a star) pass behind an object of interest, like a planet or Saturn's rings. Observing solar occultations help us discover things like the vertical structure of clouds, gases in atmospheres and active processes that move the particles in Saturn's rings.

SOPC (UVIS Science Operations and Planning Computer) - The computer we use to plan observations and send them to JPL.

Spectra, Spectrum - Spectra is the plural of spectrum, which is a series of energies (like light) arranged according to wavelength, or frequency. The electromagnetic spectrum is an array of radiation that is divided into a number of sub-portions, where the boundaries are only vaguely defined. They extend from the shortest cosmic rays, through gamma rays, X-rays, ultraviolet light, visible light, infrared radiation, microwave and all other wavelengths of radio energy.

Spectral resolution - The ability to distinguish two spectral features close to one another; the smallest difference in wavelength between two such distinguished features.

Spectrograph - A spectroscope that measures wavelengths of light (spectra) and then displays the data as a graph. UVIS is an imaging spectrograph, which means it can also display the points of the graph as a picture (see Imaging Sepctrosopy).

Spectroscope - A machine (instrument) for producing and observing spectra.

Spectroscopic measurements - The measurements taken by a spectrograph.

Spectrometer - A spectroscope equipped with the ability to measure wavelengths.

Telemetry Data Server (TDS) - Telemetry data are the spacecraft observations. They are stored temporarily on the TDS.

Ultraviolet (UV)- (Extreme Ultraviolet and Far Ultraviolet)

Ultraviolet light (UV) is a portion of the complete electromagnetic spectrum Ultraviolet is a portion of the spectrum that is a shorter wavelength than visible light; roughly, with a wavelength interval from 100 to 4000 angstroms. Ultraviolet radiation from the Sun is responsible for many complex photochemical reactions like the formation of the ozone layer. Extreme and far ultraviolet wavelengths are different portions of the ultraviolet portion of the spectrum, with extreme being between 55.8-118 nanometers and far being between 110-190 nanometers.

Undispersed (zero-order) light - A specular reflection off the diffraction grating which acts like a mirror.

Uplink Product Generation System (UPGS) - A process consisting of manpower, software and hardware that generates the sequencing of the UVIS instrument according to the plan of the science team.

UTC - Coordinated Universal Time, also known as Greenwich Mean Time (GMT), Greenwich Mean Time is based upon the time at the zero degree meridian that crossed through Greenwich, England and is used as a world time and date standard.

Weight-relieved (aluminum cases) - UVIS is built out of aluminum that has been carved out to the dimensions necessary in order to be as light as possible.

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