The UVIS Spectrum Dataset
==========================
    The UVIS instrument is part of the remote sensing payload of
 the Cassini orbiter spacecraft.  UVIS has two  spectrographic
 channels that provide images and spectra covering the ranges
 from 56 to 118 nm and 110 to 190 nm.  A third optical path
 with a solar  blind CsI photocathode is used for a high
 signal-to-noise ratio stellar  occultation by rings and
 atmospheres.  A separate hydrogen deuterium  absorption cell
 measures the relative abundance of deuterium and hydrogen
 from their lyman-alpha emission.  These channels are referred
 to as EUV,  FUV, HSP, and HDAC in this document.  The UVIS
 science objectives include  investigation of the chemistry,
 aerosols, clouds, and energy balance of the Titan and Saturn
 atmospheres; neutrals in the Saturn magnetosphere; the
 deuterium-to-hydrogen ratio for Titan and Saturn; icy
 satellite surface  properties; and the structure and
 evolution of Saturns rings.   basic instrument design adapts
 proven design concepts using a  grating spectrometer followed
 by a multi-element detector.  We chose to  use imaging, pulse-
 counting microchannel plate detectors because of more  than a
 decade of experience using this kind of detector equipped
 with a  CODACON readout anode.  The CODACON (Coded Anode
 Array Converter) acts as a photon locator.  The photon counts
 are accumulated in an external memory to build a picture that
 is periodically read out for transfer to the spacecraft memory
 and eventually, transmission to the ground.   The two dimensional
 format for the CODACON detectors allows simultaneous  spectral
 and one-dimensional spatial coverage.  The detector format is
 1024 x 64 (spectral by spatial).
    The Cassini HDAC consists of a channel electron Multiplier
 photodetector  equipped with 3 absorption cell filters:  a
 hydrogen cell, a deuterium cell  and an oxygen cell.  The
 oxygen cell is not utilized in flight.  The  hydrogen and
 deuterium cells function as adjustable absorption filters.
 In  each cell a hot tungsten filament disassociates the
 hydrogen and deuterium  molecules into atoms, producing an
 atomic density determined by each of 16  different filament
 temperatures.  These atoms resonantly absorb the  hydrogen
 and deuterium Lyman-alpha lines passing through the cells.
 Cycling the filaments on and off and comparing the
 differences in signal  gives a direct measurement of the
 relative hydrogen and deuterium signals.   Each cell has two
 filaments controlled by separate filament current
 regulators.  Only one filament at a time per cell is used
 during flight.   A Pulse Amplifier Discriminator detects
 photoelectrons from the CEM and  sends pulses to the UVIS
 instrument logic.   S contains a high speed photometer with
 an integration time as short  as 2.0 ms to observe stars
 occulted by the rings of Saturn.  The photon  counts
 collected from the photocathode are passed as a time ordered
 sequence to the instrument, then to spacecraft memory for
 transfer to the  ground.    The data in a UVIS observation
 are a copy of what was in the UVIS memory  buffer. That is,
 the observation consists of unprocessed experiment data
 stored in binary format. An observation belongs to one of
 four different  types of data product: a spectrum, a time
 series of spatial-spectral  images, a time series of detector
 counts, or an image at one wavelength.   Each observation has
 a unique identifier that associates it with a time  range and
 with the configuration of the instrument during that time.
 Each data product will contain one observation and will be
 completely  defined by a PDS label.  The objects will be
 correct in the sense that  they will conform to PDS formatting
 requirements and will be consistent  with data archived by the
 UVIS team.  They will be complete in that they  will represent
 all data taken by the UVIS instrument.  In addition, CODMAC
 level 3 data products will be derivable from the archived data
 and an  associated set of calibration data.
     The EUV and FUV channels can be read out to produce
 spectra.  Each  spectrum is generated by accumulating
 detector counts over a fixed time  interval.  The time
 interval is defined in the instrument configuration
 associated with the observation.  A spectrum consists of a
 sequence of  counts, each count being associated with a
 detector column (or columns). In the simplest case, a
 spectrum is a sequence of 1024 integers where  each integer
 is the number of counts taken during a fixed time interval
 by the detector at a particular wavelength and summed over
 the spatial  dimension of the detector.  In more complex
 cases, each integer in the  spectrum corresponds to a set of
 columns and is derived by summing over  both the spatial and
 spectral dimensions.  For example, if the binning defined on
 the spectral dimension is two, a spectrum will consist of
 512 integers, where each integer is derived by first summing
 1024  columns in the spatial dimension, then summing
 contiguous pairs of  detector lines in the spectral
 dimension.  A still more complex case  involves spectra
 derived from a set of sub regions of the detector.    In this
 case, the detector is divided into a set of active rectangular
 sub regions (windows).  Each window can be binned in the manner
 described above.  A data product (also referred to as an
 observation) is a sequence of spectra taken from a window
 during the same instrument  configuration.
    All instrument configuration information including
 window, bin, and integration time specifications will be
 contained in the PDS object label.
    For a more extensive description of the UVIS instrument
 see the file ROOT/DOCUMENT/UVIS.TXT on this archive
 volume.

Confidence Level Overview
=========================
The UVIS data objects are organized into separate observations.
Each observation contains data taken from one configuration of
the instrument.  There may be more than one observation generated
from one instrument configuration.  This may occur because science
data generated by the UVIS instrument is dropped when corrupted
data is detected or transmission failures occur.  The UVIS ground
system detects this and divides the data into two observations, one
terminated prior to the data drop the next beginning immediately
afterword.  The start time and duration of an observation correspond
to the actual times of the first and last records in the observation.
Incomplete observations are filled with zeroes.  Over 95% of data
taken by the instrument is contained in the UVIS archive.

Only one hardware feature affects UVIS data.  A light leak in the
instrument casing causes an increase in the background counts in
the lower half of the EUV channel wavelength range.  This effect is
detectable by visual inspection of a graph of the data.  No tools
for detection or correction of these background counts exists.

One anomaly in the flight software caused data errors to appear
when multiple windows which do not cover the entire EUV or FUV
detector are defined.  In an observation, the data errors appear
as randomly located spikes in detector count values, and one
completely incorrect spatial line located at a random spatial
index.  These errors are detectable by visual inspection of a
plot of the data.  The anomaly was fixed by a revison to the
software and effects data between launch and 2001-071 09:00:00 UTC

Review
======
This volume has completed a peer review by the PDS. The peer review panel
consisted of Lyle Huber, Mitch Gordon, Steven Adams, Ron Joyner and Mark
Vincent representing PDS, David Judd and Wayne Pryor from the UVIS team,
Diane Connor from the Cassini Project and Kurt Retherford and John Clarke
as outside users.