Voyager UVS VG1INST.CAT
Return to UVS data set page.
PDS_VERSION_ID = PDS3
RECORD_TYPE = STREAM
LABEL_REVISION_NOTE = "M.R. SHOWALTER, 2002-12-27"
OBJECT = INSTRUMENT
INSTRUMENT_HOST_ID = VG1
INSTRUMENT_ID = UVS
OBJECT = INSTRUMENT_INFORMATION
INSTRUMENT_NAME = "ULTRAVIOLET SPECTROMETER"
INSTRUMENT_TYPE = "ULTRAVIOLET SPECTROMETER"
INSTRUMENT_DESC = "
INSTRUMENT: ULTRAVIOLET SPECTROMETER
SPACECRAFT: VOYAGER 1 & 2
Instrument Information
======================
Instrument Id : UVS
Instrument Host Id : { VG1, VG2 }
Pi PDS User Id : ALBROADFOOT
Instrument Name : ULTRAVIOLET SPECTROMETER
Instrument Type : ULTRAVIOLET SPECTROMETER
Build Date : N/A
Instrument Mass : 4.52
Instrument Length : 43.18
Instrument Width : 14.78
Instrument Height : 17.15
Instrument Serial Number : 3
Instrument Manufacturer Name : N/A
Instrument Description
======================
The Voyager 1 and 2 Ultraviolet Spectrometers are nearly
identical instruments. This discussion applies to both, except
in a few instances in which important differences between the
two are noted explicitly. The Voyager 1 Ultraviolet
Spectrometer (UVS) is a compact Wadsworth mounted objective
grating spectrometer that covers the wavelength range of 0.0535
to 0.1702 micron (0.0513 to 0.1680 micron for the Voyager 2
UVS). It records the entire spectrum within this range in a
single exposure. It has no moving parts. A mechanical
'collimator' consisting of a series of 13 aperture plates
defines the main 'airglow' field of view (FOV) of 0.10 degrees
degrees full-width-half-maximum (FWHM) x 0.87 degrees in length.
Light passing through the collimator strikes the concave
diffraction grating at near normal incidence. The grating
disperses and focuses light onto a 1-d array detector that
records individual photoevents. An auxiliary field of view for
solar occultation experiments is offset 20 degrees from the
airglow field by a small mirror near the front of the
collimator. Using this 'occultation port', the UVS can view the
Sun without pointing the main field, and those of other
coaligned instruments, directly at the Sun. The occultation
field is 0.25 FWHM x 0.87 degrees. A sunshade prevents
illumination of the main entrance aperture by the sun during
occultation observations.
The UVS has two spectral resolutions depending on the nature of
the source. An extended monochromatic source that fills the
FOV ideally produces a triangular intensity distribution of 0.1
degree FWHM. (The actual response function is slightly rounded
at the top and base, but a triangle is a satisfactory
approximation for most applications.) The 0.1 degree
corresponds to width of 3.5 anodes, or 0.0033 microns. This
inherent spectral resolution may often be improved by spectral
analysis. A monochromatic point source is imaged onto a width
of about 1 anode for a practical resolution of about 0.0015
microns. Precise measurements of the relative response as a
function of position within the FOV have been made by rastering
the FOV across a star. At wavelengths longward of 0.1350
microns there is a slight (~10%) asymmetry in the response on
either side of the center of the FOV.
The effective sizes of the entrance apertures are (airglow
port) 21.2 and (occultation port) 0.75 cm**2.
The anode array is scanned at a rate of 3125 scans per second
and the results are added into an internal memory. The UVS
transmits the contents of this memory to the flight data system
(FDS) on command of the FDS. The FDS retrieves values for a
pair of channels each 5 msec, and so reads a complete spectrum
from the UVS in 0.32 sec. For the fastest transmitted data
rate (OC-1, see below) used for occultation observations, this
readout proceeds continuously, producing a series of spectra
separated by 0.32 sec. For lower data rates, the memory is
read in bursts of 0.32 sec separated by the appropriate
intervals. During these intervals, the UVS integrates the
spectrum in its internal memory. As the data is transferred to
the FDS it is logarithmically compressed from 16 to 10 bits.
The FDS determines the rate at which spectra are read from the
UVS after being integrated internally in the instrument memory.
Most planetary observations are made at one of two data rates,
OC-1 (0.32 sec spectra, for occultation measurements) and GS-3
(3.84 sec spectra, for emission spectroscopy). Slower rates
are used from time to time. Rates and their designations are:
Name Mode # Integration time (sec)
OC-1 1 0.32
GS-3 2 3.84
CR-1 3 12
CR-2 4 48
CR-4 6 192
CR-6 8 720
CR-5T 9 240
UV-5A 10 3.84
A description of the UVS investigation is given by
[BROADFOOTETAL1977]. Performance and analysis techniques are
described by [BROADFOOTETAL1981].
Scientific Objectives
=====================
The primary goal of the UVS is to study the composition and
structure of the atmospheres of the outer planets and their
satellites. Secondary goals include the study of
magnetospheric particle populations, magnetosphere-atmosphere
interactions, the composition and distribution of the
interplanetary wind, determinations of the solar flux, and
stellar astronomy.
Operational Considerations
==========================
The Voyager UVS instruments have operated nearly continuously
since launch in 1977. With the singular exception of a
decrease in the Voyager 1 microchannel plates (MCP) gain, due
to a high radiation-induced count rate during passage through
the inner Jovian magnetosphere, both instruments have remained
photometrically stable at a better than 3% level since 1977.
In-flight performance of the UVS from launch through the 1979
Jupiter encounters is reviewed in [BROADFOOTETAL1981] and an up
to date description of astronomical observations is contained
in [HOLBERG1990] and [LINICK&HOLBERG1991].
Calibration Description
=======================
Laboratory calibration of the UVS included measurements of:
1) sensitivity at a number of wavelengths throughout the
spectral range,
2) response to scattered light,
3) off-axis response, including collimator transmission
function, and
4) intrinsic dark count rate.
In-flight calibration has included assessments of absolute
sensitivity by comparisons with stars, and measurements of the
FOV response profile using stars.
Before the absolute calibration can be applied to a measured
spectrum, three or four spectral analysis steps are required.
These are flat field correction, dark count subtraction, and
descattering, and (sometimes) sky background removal.
Channel-to-channel variations in sensitivity result from
variations in effective count threshold among channels.
Applying a 'fixed pattern noise' (FPN) correction adjusts the
signal levels to their equivalents for a common threshold in
all channels. The FPN correction involves a channel-by-channel
multiplication by a correction spectrum. The correction
spectra are different for Voyager 1 and Voyager 2. In fact,
two spectra are in use for Voyager 1. The first is used for
data acquired before Jupiter encounter and the second for data
after encounter. The two differ to account for changes in the
response of the Voyager 1 UVS induced by its operation in the
intense Jovian radiation environment.
Channels 3 and 4 have large FPN corrections, i.e. they are
less sensitive than the others. Therefore the statistical
accuracy of the signal in these channels is lower than in the
other channels.
Dark Counts
-----------
In interplanetary space, detector dark counts arise mainly
from effects of gamma radiation from the radioisotope
thermoelectric generators that power the spacecraft. The
count rate is about 0.02 counts per channel per second. The
shape is approximately flat in wavelength, with a step near
the edge of the filter in the detector. The shape is
accurately known from long observations of the calibration
plate mounted on the spacecraft. Almost no photon signal
(except for a weak reflection of sky-background H Lyman
alpha) is recorded during these observations. The absolute
level varies slightly with scan platform position, because
rotating the scan platform changes the shielding mass between
the UVS detector and the generators. For data acquired
outside a planetary magnetosphere, subtracting a scaled dark
spectrum from a calibration plate observation is usually a
satisfactory correction for dark counting.
Within a planetary magnetosphere, the dark count rate can
include contributions from high-energy particles. For lower
levels, scaling a calibration plate spectrum is again
satisfactory, but for higher levels the shape of the dark
spectrum changes and another method must be used to
estimate dark levels. The best alternative is to use a
spectrum taken at nearly the same time with no significant
source in the FOV. Satellite observations often fill this
need.
Descattering
------------
Light scattered within the instrument illuminates channels
outside the ideal transmission function of the collimator.
The effects of scattering are removed by a process called
descattering. Descattering is accomplished through the use
of a matrix operator, a 126x126 element matrix which
describes the response of detector channel 'j' to the
measured signal at channel 'i'. This scattering matrix is
completely empirical, having been determined from laboratory
measurements of 50 individual emission lines covering the
entire passband. Descattering is a linear operation. Dark
counts must be subtracted prior to descattering as the
descattering algorithm is based on the assumption that only
photon events are present. Descattering will also correct
for second order response. Therefore, if the spectrum to be
descattered contains artifacts, such as anomalously high or
low counts in channels 3 or 4, a corresponding error will be
introduced in the vicinity of both the first and second order
positions.
Sky Background Subtraction
--------------------------
When the UVS slit is not completely filled by the disk of a
planet, the portion off the planet sees the sky background.
Fortunately, in the far UV the sky is generally quite dark
and diffuse starlight is seldom significant. However, bright
emissions at H Lyman alpha, Lyman beta, and He 0.0584 microns
from the interplanetary medium (IPM) often must be taken into
account. These lines arise from strong solar chromospheric
emission lines that are scattered from neutral H and He of
the local interstellar medium. The physics of this
'interstellar wind' is complex and leads to emission which is
inhomogeneous in space and variable in time. The IPM
responds to active regions of the solar chromosphere as the
sun rotates. This means that the sky brightness as seen by
the UVS can change noticeably on time scale of days. As with
instrumental dark counts, there are two standard means of
removing sky background: direct subtraction of an adjacent
sky background suitably scaled, if available, and
construction of a synthetic sky background spectrum.
Calibration
-----------
Calibrating the spectra converts them from count units to
absolute brightness units. This step has not been included in
the data processing because the correct procedure depends on
the type of source viewed. The data spectra represent count
rates after correction for fixed pattern noise, a background
subtraction, and descattering. Multiplying these spectra by
one of the calibration spectra converts it to brightness
units. There is a calibration spectrum corresponding to each
of the two source types, namely point sources and extended
sources.
Point Source: Multiplying a data spectrum by the calibration
spectrum VxPTCAL.TAB (x=1 for Voyager 1 and x=2 for Voyager 2)
converts the spectrum from counts/(channel) to
photons/(cm**2-Angstrom-time), where time is the integration
time of the spectrum.
Extended Source, continuum emission: Multiplying a data
spectrum that has been normalized to an integration time of 1
second by the calibration spectrum VxFLCAL.TAB (x=1 for
Voyager 1 and 2 for Voyager 2) converts the spectrum from
counts/(channel-second) to spectral brightness in units of
Rayleighs/Angstrom for a source that fills the field of view.
Extended Source, monochromatic emission: The finite spectral
resolution (about 35 A) of the spectrograph must be considered
in this case. For isolated lines (those that are not strongly
blended with emissions at nearby wavelengths) it is sufficient
to sum the channels that include light from the emission of
interest (about 7 channels) and multiply by the appropriate
calibration factor. This factor is the product of the
dispersion (9.26 Angstroms/channel) and the value in
VxFLCAL.TAB corresponding to the center channel of the
wavelength of interest. The resulting quantity is the
brightness of a monochromatic emission that fills the field of
view. For more complex spectra that include blended emissions,
the most accurate approach is spectral analysis by generating
synthetic spectra. This technique uses an iterative approach
to adjust an estimated brightness spectrum until the model
spectrum computed from it matches the observed spectrum. The
model can be fairly simple, but must include the triangular
transmission profile of the collimator and the instrument
sensitivity (calibration). The calibration factor described
earlier in this paragraph is the correct one to use for this
kind of synthesis.
Calibration and spectral analysis issues are discussed by
[HOLBERGETAL1982] and [HOLBERG1986].
Platform Mounting Descriptions
==============================
The UVS is mounted on the scan platform. The instrument is
approximately bore-sighted with the wide and narrow angle
television cameras, and with the PPS and IRIS instruments. The
alignment of the fields is not perfect; the following table
gives offsets of the centers of the UVS fields relative to the
centers of the ISS Narrow Angle Camera fields of view.
Elevation is positive to the right within the imaging field of
view, and cross-elevation is positive downward. The narrow axis
of the UVS slit is aligned with the elevation direction.
Instrument Elevation Cross-Elevation
--------------------------------------------------
Voyager 1 +0.010 deg -0.030 deg
+18.9 pixels -56.6 pixels
Voyager 2 0.0 deg +0.08 deg
0.0 pixels +150.9 pixels
Cone Offset Angle : UNK
Cross Cone Offset Angle : UNK
Twist Offset Angle : UNK
Principal Investigator
======================
The Principal Investigator for the ultraviolet spectrometer
instrument is A. L. Broadfoot.
Section 'UVS'
=============
Total Fovs : 2
Data Rate : VARIABLE
Sample Bits : 16
'UVS' Detectors
---------------
SPECTROMETER
'UVS' Electronics
-----------------
UVS
'UVS' Section Optic IDs
-----------------------
UVS
In modes
--------
PULSE COUNTING
PULSE INTEGRATION
HIGH VOLTAGE 0
HIGH VOLTAGE 1
HIGH VOLTAGE 2
HIGH VOLTAGE 3
HIGH VOLTAGE 4
HIGH VOLTAGE 5
HIGH VOLTAGE 6
HIGH VOLTAGE 7
'UVS' Section FOV Shape 'RECTANGULAR'
-------------------------------------
The occultation field is offset from the airglow field by a
small mirror. The offset is toward lower elevation. The
elevation offsets are:
Voyager 1 -19.53 deg
Voyager 2 -19.296 deg
Section Id : UVS
Fovs : 1
Horizontal Pixel Fov : N/A
Vertical Pixel Fov : N/A
Horizontal Fov : 0.86
Vertical Fov : 0.25
'UVS' Section Parameter 'SURFACE BRIGHTNESS'
--------------------------------------------
Sampling Parameter Name : TIME
Section Id : UVS
Instrument Parameter Unit : RAYLEIGHS
Minimum Instrument Parameter : 0.000000
Maximum Instrument Parameter : 0.000000
Minimum Sampling Parameter : 0.32
Maximum Sampling Parameter : 720
Sampling Parameter Unit : SECOND
'UVS' Section Parameter 'FLUX'
------------------------------
Sampling Parameter Name : TIME
Section Id : UVS
Instrument Parameter Unit : PHOTONS CM**-2 SEC**-1
Minimum Instrument Parameter : N/A
Maximum Instrument Parameter : N/A
Minimum Sampling Parameter : 0.32
Maximum Sampling Parameter : 720
Sampling Parameter Unit : SECOND
Instrument Detector 'SPECTROMETER'
==================================
The windowless, photoevent-counting detector consists of an
electron multiplier, a pair of microchannel plates (MCP) in
series, and a 128-element linear self-scanned readout array.
Photoelectrons ejected at the front of the MCP stack are
amplified by a factor of about 1E6, and the resulting charge
pulse is collected by the anode array. The 128 narrow aluminum
anodes, each 3 mm long, are deposited on 0.1-mm centers for a
collecting length (in the dispersion direction) of 13 mm.
The anodes are accessed sequentially by a shift register and
FET switches contained on the single integrated circuit. The
scanning circuitry discharges each anode into a charge
sensitive preamplifier. The charge pulse is digitized and the
information added into a shift register memory consisting of
128 16-bit words. The 128-anode array consists of two separate
interdigitated 64-anode arrays scanned by two shift registers.
The shift registers and memory are driven by a 200 kHz clock,
so that an individual anode is accessed every 320 microsecond.
The detector scan rate is therefore about 3125 Hz.
Wavelengths shorter than about 0.1250 micron strike the MCP
directly. Longer wavelengths first pass through a MgF2 filter
with a semi-transparent photocathode of CuI. This serves to
boost the quantum efficiency at long wavelengths and to reduce
the response to second-order light.
The detector is heavily shielded to reduce its response to
trapped particle radiation. A description of the detector may
be found in [BROADFOOT&SANDEL1977].
Detector Type : MICROCHANNEL PLATES WITH ANODE
ARRAY
Detector Aspect Ratio : N/A
Minimum Wavelength : 0.0535
Maximum Wavelength : 0.1702
Nominal Operating Temperature : 250
Instrument Electronics 'UVS'
============================
The UVS electronics is housed in an enclosure integral with the
optical section of instrument. Most of the electronics is in
the base of the instrument, but clock drive generators for the
anode array and the first stage of charge sensitive
preamplification of the analog signal processing electronics
are mounted in the detector housing so that they are near the
anode array. Elements of the electronics complement include:
(1) Low voltage power supply
(2) High voltage power supply
(3) Clock drive generator for the anode array
(4) Analog signal processing electronics including A/D
conversion
(5) 128x16 bit accumulation memory for spectrum
(6) FDS interface
The FDS interface sends data to the FDS on demand and accepts
mode commands from the FDS. The mode commands set the level of
the high voltage applied to the MCPs of the detector and set
the mode of analog signal processing (pulse counting or
integration).
Radiation-hard electronics components were used where possible,
and spot radiation shielding was used to reduce the fluence at
certain critical elements.
Instrument Optics 'UVS'
=======================
The optical system consists mainly of the mechanical collimator
and concave diffraction grating. The 13 aperture plates of the
collimator establish a field of 0.1x0.87 degree for the airglow
field and 0.25x0.87 degree for the occultation field. The 0.1
and 0.25 degree dimensions are in the dispersion direction, and
the 0.87 degree dimension is in the cross-dispersion direction.
The collimator provides separate light paths for the airglow
and occultation ports, and a small mirror diverts the
occultation field by 20 degrees from the airglow field.
The concave diffraction grating is a platinum coated replica,
ruled at 540 lines/mm, blazed at 0.0800 microns and having a
spherical radius of curvature of 400.1 mm. Dispersion in the
image plane is 0.00926 microns/mm, or 0.000926 microns/channel.
The grating substrate is a 4x6-cm rectangle, and the useful
ruled area is 21 square cm.
Telescope Diameter : 0.06
Telescope F Number : 4
Telescope Focal Length : 0.20
Telescope Resolution : UNK
Telescope Serial Number : UNK
Telescope T Number : UNK
Telescope T Number Error : UNK
Telescope Transmittance : UNK
Instrument Mode 'PULSE COUNTING'
================================
Two modes of electrical operation allow the detector to operate
in a photon-counting mode for low source intensities, or in an
integration mode for high source intensities. In the pulse
counting mode, the number in the corresponding memory location
is incremented by one if a charge above a fixed threshold is
detected on an anode. The access time of 320 microsec implies
that single random photoevents can be recorded on any one of
the anodes at a rate of about 300 Hz with a coincidence of 10%.
The pulse-counting mode is used for all measurements except
solar occultations.
Data Path Type : N/A
Gain Mode Id : N/A
Instrument Power Consumption : 3.2
In sections
-----------
UVS
Instrument Mode 'PULSE INTEGRATION'
===================================
In the pulse integration mode a 3-bit A-to-D converter is
introduced ahead of the adder. In this case the charge on each
anode is coarsely digitized and added to the previously
accumulated signal in memory. The statistics of sampling these
events is complicated by the logarithmic pulse height
distribution of the events. There is also a logarithmic
current limit function of the MCPs at the high event rates that
normally obtain when this mode is used. Because both these
characteristics lead to non-linear response, modeling of the
detector response is needed to restore linearity. The
integration mode is used for observing solar occultations.
Data Path Type : N/A
Gain Mode Id : N/A
Instrument Power Consumption : 3.2
In sections
-----------
UVS
Instrument Mode 'HIGH VOLTAGE 0'
================================
The gain of the electron multiplier can be adjusted by setting
the potential drop across the microchannel plates. The high
voltage level is commanded by the FDS. Level 0 corresponds to
high voltage off.
Data Path Type : N/A
Gain Mode Id : N/A
Instrument Power Consumption : 2.8
In sections
-----------
UVS
Instrument Mode 'HIGH VOLTAGE 1'
================================
The gain of the electron multiplier can be adjusted by setting
the potential drop across the microchannel plates. The high
voltage level is commanded by the FDS. Level 1 is used for
occultation and solar observations.
Data Path Type : N/A
Gain Mode Id : N/A
Instrument Power Consumption : 3.2
In sections
-----------
UVS
Instrument Mode 'HIGH VOLTAGE 2'
================================
The gain of the electron multiplier can be adjusted by setting
the potential drop across the microchannel plates. The high
voltage level is commanded by the FDS. Level 2 is used for
occultation and solar observations.
Data Path Type : N/A
Gain Mode Id : N/A
Instrument Power Consumption : 3.2
In sections
-----------
UVS
Instrument Mode 'HIGH VOLTAGE 3'
================================
The gain of the electron multiplier can be adjusted by setting
the potential drop across the microchannel plates. The high
voltage level is commanded by the FDS. Level 3 is used for
airglow observations and some occultation observations.
Data Path Type : N/A
Gain Mode Id : N/A
Instrument Power Consumption : 3.2
In sections
-----------
UVS
Instrument Mode 'HIGH VOLTAGE 4'
================================
The gain of the electron multiplier can be adjusted by setting
the potential drop across the microchannel plates. The high
voltage level is commanded by the FDS. Level 4 is intended to
recoup losses in supply output that could have occurred due to
radiation damage. It is not normally used for observations.
Data Path Type : N/A
Gain Mode Id : N/A
Instrument Power Consumption : 3.2
In sections
-----------
UVS
Instrument Mode 'HIGH VOLTAGE 5'
================================
The gain of the electron multiplier can be adjusted by setting
the potential drop across the microchannel plates. The high
voltage level is commanded by the FDS. Level 5 is intended to
recoup losses in supply output that could have occurred due to
radiation damage. It is not normally used for observations.
Data Path Type : N/A
Gain Mode Id : N/A
Instrument Power Consumption : 3.2
In sections
-----------
UVS
Instrument Mode 'HIGH VOLTAGE 6'
================================
The gain of the electron multiplier can be adjusted by setting
the potential drop across the microchannel plates. The high
voltage level is commanded by the FDS. Level 6 is intended to
recoup losses in supply output that could have occurred due to
radiation damage. It is not normally used for observations.
Data Path Type : N/A
Gain Mode Id : N/A
Instrument Power Consumption : 3.2
In sections
-----------
UVS
Instrument Mode 'HIGH VOLTAGE 7'
================================
The gain of the electron multiplier can be adjusted by setting
the potential drop across the microchannel plates. The high
voltage level is commanded by the FDS. Level 7 is intended to
recoup losses in supply output that could have occurred due to
radiation damage. It is not normally used for observations.
Data Path Type : N/A
Gain Mode Id : N/A
Instrument Power Consumption : 3.2
In sections
-----------
UVS "
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END
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