Instrument Overview
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Juno-UVS, the ultraviolet instrument on the Juno mission to Jupiter, isprimarily based on the Alice instrument on the New Horizons (NH) mission tothe Pluto system and on the Lyman Alpha Mapping Project (LAMP) instrumenton the Lunar Reconnaissance Orbiter (LRO) mission currently in orbit aroundthe Moon. Juno-UVS is an imaging spectrograph with a spectral range in theextreme-ultraviolet (EUV) and far-ultraviolet (FUV) of 68-210 nm. Thiswavelength range was chosen to cover all important UV emissions from the H2bands and the H Lyman series produced in Jupiter's auroras, while alsoincluding longer wavelengths sensitive to the absorption signatures ofaurora-produced hydrocarbons. Juno- UVS will remotely sense Jupiter'sauroral morphology and brightness, providing context for in-situmeasurements, and will map the mean energy and flux of precipitatingauroral particles.
The Juno-UVS instrument was developed at SwRI and delivered to LockheedMartin for integration onto the Juno spacecraft before launch on August 5,2011. The instrument consists of two main assemblies: (1) a shoebox sizedsensor, which includes a telescope section and a spectrograph & detectorsection, and (2) an electronics box housed in the spacecraft vault. Besidesthis changed configuration (LRO-LAMP and the Alices each consisted of asingle assembly), a number of changes have been incorporated to adapt theinstrument to the Juno mission. A main design driver for these differencesis Jupiter's harsh radiation environment. Another major change is theaddition of a scan mirror, which allows the targeting of specific areas ofinterest when the spinning spacecraft is close to Jupiter. In the followingsections we present the design and operation, the key changes from earlierdesigns, 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 theUVS mission paper [GLADSTONEETAL2014]. See this paper for more details.


Scientific Objectives
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Juno-UVS's main objective is to provide context for the particles andfields instruments (i.e., JADE, JEDI, Waves, and MAG) in the investigationof Jupiter's polar magnetosphere (Fig. 1). During the typically 6-hournear-perijove period of auroral observations, Juno-UVS will scan Jupiter'sauroral 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 Hand the Lyman, Werner, and Rydberg band systems of H2. By obtainingtime-tagged pixel list data (where each photon event is assigned a uniquelocation, wavelength, and time), images and maps of the northern andsouthern auroral regions will be reconstructed on the ground at a resolutionappropriate for the signal-to-noise ratio (SNR) of the spectral feature ofinterest. Near the beginning and end of the near-perijove observationperiod Juno-UVS will provide global snapshots of the northern and southernauroral morphology from a range of several Jovian radii. Closer to Jupiter,a scan mirror will be used to target the atmospheric region near theexpected location of the Juno magnetic field line footprint (based onmagnetic field models and the spacecraft orbit and spin axis). This willallow a direct comparison of the precipitating particle fluxes measured byJADE and JEDI with the FUV emissions they produce upon impacting Jupiter'supper atmosphere, and how the particular region sampled by the spacecraftrelates to the rest of the magnetosphere. Other frequent targets will be themagnetic field line footprints of the Galilean satellites (at a variety oflocal times and from near nadir to positions near the limb), the polarflares, and the main oval ansae (i.e., the locations where the main ovalemission cross the limb). During MWR orbits (when the Juno spin and orbitplanes coincide), the Juno-UVS scan mirror will be used much less, so thatsimultaneous FUV data will be acquired from the same auroral regionsobserved by the JIRAM near-IR instrument.


Calibration
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After it was assembled and through with environmental testing, just priorto delivery to Lockheed Martin for integration on the Juno spacecraft,Juno-UVS underwent a series of tests to characterize its radiometricperformance. Specifically, the Juno-UVS flight instrument was tested inSwRI's vacuum radiometric calibration chamber in order to determine the bestpre-launch values for: (1) dark count rate; (2) PSF and wavelengthcalibration; (3) off-axis light scatter, and (4) effective area. Note thatsome of these attributes (e.g., effective area) can be measured much moreaccurately in flight (through stellar calibration), while other importantcalibration data (e.g., flat field measurements) are obtained only afterlaunch. Table 3 shows a summary of the results for each of the groundradiometric tests performed, along with the performance requirement. Asshown in this table, all the specified radiometric requirements measuredduring the vacuum radiometric tests were met. Figure 7 shows Juno-UVS inthe test chamber just before starting radiometric calibration (during2010 October 12-17).


Design Overview
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The scan mirror, OAP mirror, and diffraction grating are each constructedfrom monolithic pieces of aluminum, coated with electroless nickel andpolished using low-scatter polishing techniques. The aluminum optics, inconjunction with the aluminum housing, form an athermal optical design.
The scan mirror, OAP mirror, and diffraction grating are also eachovercoated with sputtered Al/MgF2 for optimum reflectivity within theJuno-UVS spectral bandpass. Besides using low-scatter optics, additionalcontrol of internal stray light is achieved using internal baffle vaneswithin both the telescope and spectrograph sections of the housing, aholographic diffraction grating with low scatter and near-zero line ghostproblems, and an internal housing with alodyned aluminum surfaces (Jelinskyand Jelinsky 1987; Moldosanov et al. 1998). In addition, a zero order lighttrap has a black anodized Al coating with very low surface reflectance atEUV/FUV wavelengths. Figure 4 shows a labeled opto-mechanical schematic ofthe interior of the Juno-UVS instrument, with light rays illustrating theoptical path.


Detector and Detector Electronics
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The Juno-UVS detector configuration includes an XDL microchannel plate(MCP) detector scheme housed in a vacuum enclosure with a one-time openingdoor containing a UV-grade fused-silica window (for limited UV throughputduring testing). The door was spring loaded for opening with awax-pellet-type push actuator. The vacuum enclosure has a vacuum pump portand a small, highly polished region which functions as a zero-orderreflector (directing zero-order light from the instrument grating into thezero-order trap on the side of the instrument housing). The vacuumenclosure also utilizes four female connectors for the anode signals, andtwo high-voltage (HV) connectors for the MCP and anode gap voltages.
The detector's MCP configuration uses a Z-stack that is cylindricallycurved to match the 150-mm Rowland circle diameter to optimize spectral andspatial focus across the Juno- UVS bandpass. The detector electronicsprovide two stimulation pixels that can be turned on to check datathroughput and acquisition modes without the need to apply high voltage tothe MCP stack or to have light on the detector. The MCP pulse-heightinformation is output as 5 bits, which, together with the 11 bits ofspectral and 8 bits of spatial information, results in the 3-byte outputfor every photon. The input surface of the Z-stack is coated with an opaquephotocathode of CsI (Siegmund 2000).
A repeller grid above the curvedMCP Z-stack enhances the detective quantumefficiency (DQE). Each of the three nested MCPs has a cylindrical 7.5-cmradius 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 thespatial axis with 12-?m diameter pores and a length-to-diameter (L/D) ratioof 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 of3.5 cm? 1.8 cm necessary to capture the entire 68?210 nm instrumentbandpass. 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 twoorthogonal serpentine conductive strips for encoding an event's X-positionand Y-position. Each event (i.e., a cloud of electrons exiting the MCP) iscollected in equal amounts by the two strips, Charge is collected at eachend of each strip, and the difference in arrival time at each end of a givenstrip is used to determine the event position (e.g., Siegmund et al. 1999).
The detector electronics are composed of a separate electronics packagemounted directly behind the detector vacuum enclosure within the sensorhousing. Power to the detector electronics is supplied by the Juno-UVS lowvoltage 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). Thedetector electronics are composed of five boards: (1) the amplifier boardwith two fast amps for the X direction (spectral dimension) and two fastamps for the Y direction (spatial dimension) and two charge amps for totalevent 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 theY-axis by event arrival time differences; (4) the digital board (DIG) thatprovides the control signals and interface logic, and (5) a delay lineboard to delay the End signals. The detector electronics also generate a5-bit analog sum signal for each detected photon event that can be used forgenerating a pulse-height distribution (PHD) via ground test or flightanalysis 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 furtherprocessing. A commandable stimulation pulse generator is also included thatprovides two stim pixels at two locations in the array; these are usefulin checking data throughput without HV and in correcting for temperatureeffects on the wavelength scale.
A UV photon impinging on the photocathode generates a charge that isamplified by the microchannel plate Z-stack. The amplified charge cloudleaves the back end of the microchannel plate and is accelerated across theMCP-anode gap, impinging on the anode and generating pulses that propagatein both the +X and -X directions and +Y and -Y directions along separateintegral delay lines to the detector electronics. The detector electronicsthen output the X and Y pixel locations to the C&DH based on the timedelay 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 operationalvoltage of about .4.2 kV.The gap between the MCP output and the anode arrayrequires a voltage drop of approximately .600 V. Both the MCP and the anodegap voltages are supplied by the instrument's two redundant high voltagepower supplies (HVPS) located in the Ebox. The overall detector gain is?2E7 (?25 %). At an expected average count rate of 2000 count/s, the amountof charge pulled from the MCP as a function of time is ? 0.2 Coulomb/year.


Telecommanding
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Juno-UVS operations are commanded using a set of 30 separate telecommands.
Telecommand processing handles the redundant telecommand channels andincludes error detection and recovery. Nominally, the spacecraft may sendup 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 reportedin the housekeeping data. The instrument verifies incoming telecommandsbefore they can be executed; this basic verification includes a format andchecksum check of the telecommand. As mentioned, Juno-UVS has two redundanttelecommand interfaces, but after power-up, the active interface will bedetermined and operations from that point on will only use that singleinterface.
In addition to the command verification mechanism, the instrumentimplements two additional mechanisms to protect the instrument fromanomalous telecommands. Some commands are only allowed when the instrumentis in a specific state. In addition to this, a number of commands have beendeclared 'critical'. For Juno-UVS, this means that within a nominal30-second timeout period, a specific confirmation command has to be receivedbefore the actual (critical) command execution starts. During most of thein-flight operations, this timeout is short compared with the light traveltime to the spacecraft, meaning that the confirmation already has to beissued before confirmation of the acceptance of the command has beenreceived on the ground. Still, this mechanism provides protection againstaccidental execution of commands.
The set of telecommands can be divided into three categories: General operations - These allow for the complete basic operationalcommanding of the instrument. This includes setting and storing ofparameters and starting and stopping of the science acquisitions. This setof seven commands allows for the full science operations of the instrument.
Manual operations - Additional capabilities needed during commissioningand instrument verification are provided by 15 additional telecommands thatallow for extended command options. Some of these commands may be used forscience operations depending on the situation.
Memory functions - Software code management and maintenance andadditional debugging functions are provided by three general-purpose memoryfunctions 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 includesan identifier for the telecommand (if available) and a general error code.
The error code continues to be reported in the telemetry data until anothererror is detected or the instrument is reset. This simple form of errorreporting is limited to reporting a single error per HK cycle (i.e., at mostonce per 2 s). An additional mechanism implementing a small error log isavailable for more extensive problem investigation. The command code for anysuccessful command is also reported in the telemetry data, so the telemetryregistration can be used to reconstruct the received telecommands. Notethat the parameters of a telecommand are not included in this reporting.