The Instruments on UVIS, UVIS's Job, and the
Science of UV Light and Spectra
NOTE: Some of the more technical terms you'll
find below are defined in the UVIS Glossary. NASA also a great aerospace
A Little Background on Ultraviolet Light and Spectra/spectrum
UVIS's job is to measure Ultraviolet (UV) light, but first we need
to understand what UV is to appreciate UVIS's purpose.
UV light is part of the electromagnetic
spectrum. A spectrum 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 that ranges from the shortest
cosmic rays, through gamma rays, X-rays, ultraviolet light, visible
light, infrared radiation, microwave and other wavelengths of radio
energy. Spectra is the plural of spectrum. We see a natural spectrum
when we look at a rainbow.
Ultraviolet light (UV) is a portion of the electromagnetic spectrum
that is a shorter wavelength than visible light; roughly it lies within
a wavelength interval between 100 to 4000 angstroms. It is the same
light you see coming out of a "black light" that makes fluorescent
rocks glow bright colors. Most high-energy ultraviolet light is blocked
by the ozone layer of the Earth, but some gets through and causes skin
tanning. Extended exposure can lead to eye damage and skin cancer.
The UVIS Instrument's Job on Cassini
The Ultraviolet Imaging Spectrograph Subsystem (UVIS) is a set of
telescopes that measure ultraviolet (UV) light from Saturn, its rings
and its moons. It also does a few other neat things we'll talk about
UVIS is a remote sensing instrument (think sight--from very far away)
that captures UV light reflected off objects, like Saturn's rings or
clouds, and turns it into images that it sends back to scientists on
Earth. Studying these images helps us learn more about the structure
(shape) and composition (what it's made of) of these things as well
as the particles in the atmosphere and their temperatures too.
The UVIS is a spectrograph, which measures wavelengths of light (spectra)
and then displays the data as a graph. An imaging spectrograph,
like UVIS, can also turn the points on the graph into digital data
that can then be made into pictures. 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. This representation simulates what a being
with UV-sensitive eyes would see.
UVIS, which measures UV light between
the wavelengths of 55.8 to 190 nm (or 558-1900 Å, or angstroms) , uses two different types of
telescopes (EUV and FUV) to measure different levels of UV light.
As we said, the UVIS also does a few other things. There are a two
other components of the UVIS, the High Speed Photometer Channel (HSP),
and the Hydrogen-Dueterium Absorption Cell Channel (HDAC). These are
The UVIS model found on this site, consists of pieces
that represent each of the instruments on UVIS:
The Extreme Ultraviolet Channel (EUV) performs imaging spectroscopy
and spectroscopic measurements of the structure and composition of
the atmospheres of Titan and Saturn. It measures spectra between 55.8-118
nm (or 558-1180 Å) . The EUV consists of a telescope with a three-position
slit changer, a baffle system, and a spectrograph with a CODACON microchannel
plate detector and associated electronics.
The far ultraviolet channel (FUV) carries out
imaging spectroscopy and spectroscopic measurements of the structure
and composition of the atmospheres of Titan and Saturn and of the rings.
It measures spectra between 110-190 nm (or 1100-1900 Å ). The
FUV is similar to the EUV channel except for the grating ruling density,
optical coatings, and detector details. The FUV electronics are similar
to those for the EUV except for the addition of a high-voltage power
supply for the ion pump. The ion pump maintains a vacuum inside the
FUV channel. The EUV is open to space.
Both EUV and FUV ultraviolet channels are built into weight-relieved
aluminum cases, and each contains a reflecting telescope, a concave
grating spectrometer, and an imaging, pulse-counting detector.
The high-speed photometer channel (HSP) measures the amount of starlight
and sunlight that peek through Saturn's rings and around Titan and
Saturn, when a star or the Sun is behind them (called an occultation).
This helps us discover the vertical structure of clouds and gasses
in the atmosphere and active processes that move the particles in Saturn's
The HSP performs stellar occultation measurements of the structure
and density of material in the rings. The HSP measures light reflected
from its own parabolic mirror with a photomultiplier tube detector.
The hydrogen-deuterium absorption cell channel (HDAC) is used to measure
hydrogen and deuterium in the Saturn system using a hydrogen cell,
a deuterium cell, and a channel electron multiplier (CEM) detector
to record photons not absorbed in the cells. Both cells are made of
stainless steel coated with Teflon and are sealed at each end with
MgF2 windows. This helps us understand the origin and history of Titan's
Microprocessor Logic Assembly
All of these instruments are controlled by the Microprocessor Logic
Assembly, which is the brain of the instrument. This computer tells
the UVIS what and when to measure, and formats the data to be sent
back to Earth.
The UVIS microprocessor electronics and control subassembly consists
of input-output elements, power conditioning, science data and housekeeping
data collection electronics, and microprocessor control elements.
All of these instruments are mounted onto a baseplate, which attaches
the complete UVIS instrument to the Cassini spacecraft.