Instrument Overview
===================
Juno-UVS, the ultraviolet instrument on the Juno mission to Jupiter, is
primarily based on the Alice instrument on the New Horizons (NH) mission to
the Pluto system and on the Lyman Alpha Mapping Project (LAMP) instrument
on the Lunar Reconnaissance Orbiter (LRO) mission currently in orbit around
the Moon. Juno-UVS is an imaging spectrograph with a spectral range in the
extreme-ultraviolet (EUV) and far-ultraviolet (FUV) of 68-210 nm. This
wavelength range was chosen to cover all important UV emissions from the H2
bands and the H Lyman series produced in Jupiter's auroras, while also
including longer wavelengths sensitive to the absorption signatures of
aurora-produced hydrocarbons. Juno- UVS will remotely sense Jupiter's
auroral morphology and brightness, providing context for in-situ
measurements, and will map the mean energy and flux of precipitating
auroral particles.
The Juno-UVS instrument was developed at SwRI and delivered to Lockheed
Martin for integration onto the Juno spacecraft before launch on August 5,
2011. The instrument consists of two main assemblies: (1) a shoebox sized
sensor, which includes a telescope section and a spectrograph & detector
section, and (2) an electronics box housed in the spacecraft vault. Besides
this changed configuration (LRO-LAMP and the Alices each consisted of a
single assembly), a number of changes have been incorporated to adapt the
instrument to the Juno mission. A main design driver for these differences
is Jupiter's harsh radiation environment.  Another major change is the
addition of a scan mirror, which allows the targeting of specific areas of
interest when the spinning spacecraft is close to Jupiter. In the following
sections we present the design and operation, the key changes from earlier
designs, calibration results, and initial in-flight results for Juno-UVS,
but we first begin with an overview of the science planned for Jupiter.
2 Scientific
  Much of the information in this instrument description is taken from the
UVS mission paper [GLADSTONEETAL2014].  See this paper for more details.
 
 
Scientific Objectives
=====================
  Juno-UVS's main objective is to provide context for the particles and
fields instruments (i.e., JADE, JEDI, Waves, and MAG) in the investigation
of Jupiter's polar magnetosphere (Fig. 1). During the typically 6-hour
near-perijove period of auroral observations, Juno-UVS will scan Jupiter's
auroral regions once per 30-s spacecraft spin to observe the morphology,
brightness, and spectral characteristics of Jupiter's far-ultraviolet (FUV)
auroral emissions, which are primarily comprised of the Lyman series of H
and the Lyman, Werner, and Rydberg band systems of H2. By obtaining
time-tagged pixel list data (where each photon event is assigned a unique
location, wavelength, and time), images and maps of the northern and
southern auroral regions will be reconstructed on the ground at a resolution
appropriate for the signal-to-noise ratio (SNR) of the spectral feature of
interest. Near the beginning and end of the near-perijove observation
period Juno-UVS will provide global snapshots of the northern and southern
auroral morphology from a range of several Jovian radii. Closer to Jupiter,
a scan mirror will be used to target the atmospheric region near the
expected location of the Juno magnetic field line footprint (based on
magnetic field models and the spacecraft orbit and spin axis). This will
allow a direct comparison of the precipitating particle fluxes measured by
JADE and JEDI with the FUV emissions they produce upon impacting Jupiter's
upper atmosphere, and how the particular region sampled by the spacecraft
relates to the rest of the magnetosphere. Other frequent targets will be the
magnetic field line footprints of the Galilean satellites (at a variety of
local times and from near nadir to positions near the limb), the polar
flares, and the main oval ansae (i.e., the locations where the main oval
emission cross the limb). During MWR orbits (when the Juno spin and orbit
planes coincide), the Juno-UVS scan mirror will be used much less, so that
simultaneous FUV data will be acquired from the same auroral regions
observed by the JIRAM near-IR instrument.
 
 
Calibration
===========
  After it was assembled and through with environmental testing, just prior
to delivery to Lockheed Martin for integration on the Juno spacecraft,
Juno-UVS underwent a series of tests to characterize its radiometric
performance. Specifically, the Juno-UVS flight instrument was tested in
SwRI's vacuum radiometric calibration chamber in order to determine the best
pre-launch values for: (1) dark count rate; (2) PSF and wavelength
calibration; (3) off-axis light scatter, and (4) effective area. Note that
some of these attributes (e.g., effective area) can be measured much more
accurately in flight (through stellar calibration), while other important
calibration data (e.g., flat field measurements) are obtained only after
launch. Table 3 shows a summary of the results for each of the ground
radiometric tests performed, along with the performance requirement. As
shown in this table, all the specified radiometric requirements measured
during the vacuum radiometric tests were met. Figure 7 shows Juno-UVS in
the test chamber just before starting radiometric calibration (during
2010 October 12-17).
 
 
Design Overview
==========================
  The scan mirror, OAP mirror, and diffraction grating are each constructed
from monolithic pieces of aluminum, coated with electroless nickel and
polished using low-scatter polishing techniques. The aluminum optics, in
conjunction with the aluminum housing, form an athermal optical design.
The scan mirror, OAP mirror, and diffraction grating are also each
overcoated with sputtered Al/MgF2 for optimum reflectivity within the
Juno-UVS spectral bandpass. Besides using low-scatter optics, additional
control of internal stray light is achieved using internal baffle vanes
within both the telescope and spectrograph sections of the housing, a
holographic diffraction grating with low scatter and near-zero line ghost
problems, and an internal housing with alodyned aluminum surfaces (Jelinsky
and Jelinsky 1987; Moldosanov et al. 1998). In addition, a zero order light
trap has a black anodized Al coating with very low surface reflectance at
EUV/FUV wavelengths. Figure 4 shows a labeled opto-mechanical schematic of
the interior of the Juno-UVS instrument, with light rays illustrating the
optical path.
 
 
Detector and Detector Electronics
=================================
  The Juno-UVS detector configuration includes an XDL microchannel plate
(MCP) detector scheme housed in a vacuum enclosure with a one-time opening
door containing a UV-grade fused-silica window (for limited UV throughput
during testing). The door was spring loaded for opening with a
wax-pellet-type push actuator. The vacuum enclosure has a vacuum pump port
and a small, highly polished region which functions as a zero-order
reflector (directing zero-order light from the instrument grating into the
zero-order trap on the side of the instrument housing). The vacuum
enclosure also utilizes four female connectors for the anode signals, and
two high-voltage (HV) connectors for the MCP and anode gap voltages.
  The detector's MCP configuration uses a Z-stack that is cylindrically
curved to match the 150-mm Rowland circle diameter to optimize spectral and
spatial focus across the Juno- UVS bandpass. The detector electronics
provide two stimulation pixels that can be turned on to check data
throughput and acquisition modes without the need to apply high voltage to
the MCP stack or to have light on the detector. The MCP pulse-height
information is output as 5 bits, which, together with the 11 bits of
spectral and 8 bits of spatial information, results in the 3-byte output
for every photon. The input surface of the Z-stack is coated with an opaque
photocathode of CsI (Siegmund 2000).
  A repeller grid above the curvedMCP Z-stack enhances the detective quantum
efficiency (DQE). Each of the three nested MCPs has a cylindrical 7.5-cm
radius of curvature matching the instrument's Rowland circle radius (i.e.,
15.0 cm diameter). The approximate resistance per MCP plate is ?130 M?.
The MCP format is 4.6 cm wide in the spectral axis by 3.0 cm height in the
spatial axis with 12-?m diameter pores and a length-to-diameter (L/D) ratio
of 80:1 per plate. The XDL anode is a rectangular format of 4.4 cm?3.0 cm.
The combination anode array and MCP sizes gives an active array format of
3.5 cm? 1.8 cm necessary to capture the entire 68?210 nm instrument
bandpass. The pixel readout format is 2048 pixels (spectral dimension)
?256 pixels (spatial dimension). The active area is 3.5 cm? 1.8 cm, with
?1500 spectral pixels and ?230 spatial pixels. The XDL anode uses two
orthogonal serpentine conductive strips for encoding an event's X-position
and Y-position. Each event (i.e., a cloud of electrons exiting the MCP) is
collected in equal amounts by the two strips, Charge is collected at each
end of each strip, and the difference in arrival time at each end of a given
strip is used to determine the event position (e.g., Siegmund et al. 1999).
  The detector electronics are composed of a separate electronics package
mounted directly behind the detector vacuum enclosure within the sensor
housing. Power to the detector electronics is supplied by the Juno-UVS low
voltage power supply (LVPS) and commandand- data handling electronics
(C&DH), both located in the electronics box (Ebox) in the spacecraft vault
(several meters of cable away from the Juno-UVS sensor housing). The
detector electronics are composed of five boards: (1) the amplifier board
with two fast amps for the X direction (spectral dimension) and two fast
amps for the Y direction (spatial dimension) and two charge amps for total
event charge; (2 & 3) a time amplitude converter (TAC) board for each axis,
X and Y , that encodes 2048 pixels in the X-axis and 256 pixels in the
Y-axis by event arrival time differences; (4) the digital board (DIG) that
provides the control signals and interface logic, and (5) a delay line
board to delay the End signals. The detector electronics also generate a
5-bit analog sum signal for each detected photon event that can be used for
generating a pulse-height distribution (PHD) via ground test or flight
analysis software. Pixel list data (i.e., a list of pixel x, y addresses)
is sent from the detector electronics to the C&DH electronics for further
processing. A commandable stimulation pulse generator is also included that
provides two stim pixels at two locations in the array; these are useful
in checking data throughput without HV and in correcting for temperature
effects on the wavelength scale.
  A UV photon impinging on the photocathode generates a charge that is
amplified by the microchannel plate Z-stack. The amplified charge cloud
leaves the back end of the microchannel plate and is accelerated across the
MCP-anode gap, impinging on the anode and generating pulses that propagate
in both the +X and -X directions and +Y and -Y directions along separate
integral delay lines to the detector electronics. The detector electronics
then output the X and Y pixel locations to the C&DH based on the time
delay between the two opposing pulses in each axis.
  The detector electronics require input DC voltages of ?7.3 V and +5.0 V.
The detector MCP high voltage is raised to a room temperature operational
voltage of about .4.2 kV.The gap between the MCP output and the anode array
requires a voltage drop of approximately .600 V. Both the MCP and the anode
gap voltages are supplied by the instrument's two redundant high voltage
power supplies (HVPS) located in the Ebox. The overall detector gain is
?2E7 (?25 %). At an expected average count rate of 2000 count/s, the amount
of charge pulled from the MCP as a function of time is ? 0.2 Coulomb/year.
 
 
Telecommanding
=================
  Juno-UVS operations are commanded using a set of 30 separate telecommands.
Telecommand processing handles the redundant telecommand channels and
includes error detection and recovery. Nominally, the spacecraft may send
up to two telecommand transactions to the instrument every 2 s cycle.
These are formatted as separate Internet Protocol/User Datagram Protocol
(IP/UDP) Packets, and include (among other items) time and nadir messages.
  The acceptance and completion status of the command execution is reported
in the housekeeping data. The instrument verifies incoming telecommands
before they can be executed; this basic verification includes a format and
checksum check of the telecommand. As mentioned, Juno-UVS has two redundant
telecommand interfaces, but after power-up, the active interface will be
determined and operations from that point on will only use that single
interface.
  In addition to the command verification mechanism, the instrument
implements two additional mechanisms to protect the instrument from
anomalous telecommands. Some commands are only allowed when the instrument
is in a specific state. In addition to this, a number of commands have been
declared 'critical'. For Juno-UVS, this means that within a nominal
30-second timeout period, a specific confirmation command has to be received
before the actual (critical) command execution starts. During most of the
in-flight operations, this timeout is short compared with the light travel
time to the spacecraft, meaning that the confirmation already has to be
issued before confirmation of the acceptance of the command has been
received on the ground. Still, this mechanism provides protection against
accidental execution of commands.
  The set of telecommands can be divided into three categories:
    General operations - These allow for the complete basic operational
commanding of the instrument. This includes setting and storing of
parameters and starting and stopping of the science acquisitions. This set
of seven commands allows for the full science operations of the instrument.
    Manual operations - Additional capabilities needed during commissioning
and instrument verification are provided by 15 additional telecommands that
allow for extended command options. Some of these commands may be used for
science operations depending on the situation.
    Memory functions - Software code management and maintenance and
additional debugging functions are provided by three general-purpose memory
functions that allow for verification, load, and dump of memory blocks.
  Whenever Juno-UVS detects errors while accepting or executing commands,
an error will be reported in the generated telemetry packet. This includes
an identifier for the telecommand (if available) and a general error code.
The error code continues to be reported in the telemetry data until another
error is detected or the instrument is reset. This simple form of error
reporting is limited to reporting a single error per HK cycle (i.e., at most
once per 2 s). An additional mechanism implementing a small error log is
available for more extensive problem investigation. The command code for any
successful command is also reported in the telemetry data, so the telemetry
registration can be used to reconstruct the received telecommands. Note
that the parameters of a telecommand are not included in this reporting.