PDS_VERSION_ID = PDS3 LABEL_REVISION_NOTE = " 2006-12-27 SOC:Carcich Initial version; 201609 RChen/EN work around catalog ingest. Delete this line next time." RECORD_TYPE = STREAM OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = "NH" INSTRUMENT_ID = "ALICE" OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "ALICE ULTRAVIOLET IMAGING SPECTROGRAPH" INSTRUMENT_TYPE = "SPECTROGRAPH" INSTRUMENT_DESC = " ######################################################################## ######################################################################## REQUIRED READING: - Stern et al. (2008) [STERNETAL2008] - SOC Instrument Interface Control Document (ICD) ######################################################################## ######################################################################## The ALICE description is adapted from Slater et al. (2005) [SLATERETAL2005] and Stern et al. (2005) [STERNETAL2005]. INSTRUMENT OVERVIEW ======== The New Horizons ALICE instrument is a lightweight (4.4 kg), low-power (4.4 Watt) imaging spectrograph aboard the New Horizons mission to Pluto/Charon and the Kuiper Belt. Its primary job is to determine the relative abundances of various species in Pluto's atmosphere. ALICE will also be used to search for an atmosphere around Pluto's moon, Charon, as well as the Kuiper Belt Objects (KBOs) that New Horizons hopes to fly by after Pluto-Charon, and it will make UV surface reflectivity measurements of all of these bodies as well. The instrument incorporates an off-axis telescope feeding a Rowland-circle spectrograph with a 520-1870A spectral passband, a spectral point spread function of 3-6A FWHM, and an instantaneous spatial field-of-view that is 6 degrees long. Different input apertures that feed the telescope allow for both airglow and solar occultation (SOCC) observations during the mission. The focal plane detector is an imaging microchannel plate (MCP) double delay-line detector with dual solar-blind opaque photocathodes (KBr and CsI) and a focal surface that matches the instrument's 15-cm diameter Rowland-circle. SPECIFICATIONS -------- NAME: ALICE DESCRIPTION: Ultraviolet Mapping Spectrograph PRINCIPAL INVESTIGATOR: Alan Stern, SwRI WAVELENGTH RANGE: 520 - 1870 Angstrom FIELD OF VIEW: 1.7 x 70 mRad (Note 1); 35 x 35 mRad (Note 2) ANGULAR RESOLUTION: 1.7 x 5.2 mRad WAVELENGTH OFFSET: 229.5+/-1.5 Angstrom at first pixel WAVELENGTH RESOLUTION: 1.815+/-0.004 Angstrom/pixel ACTIVE PIXELS: ~780 pixels (Note 3) Note 1: Slit Note 2: Solar occultation aperture/channel; SOCC; also sometimes SOC Note 3: Starting at an offset of 130 pixels from the first pixel DESCRIPTION -------- This ALICE is a lightweight (4.4 kg), low-power (4.4 W), ultraviolet spectrograph based on the ALICE instrument now in flight aboard the European Space Agency's Rosetta spacecraft. Its primary job will be to detect a variety of important atomic and molecular species in Pluto's atmosphere, and to determine their relative abundances so that a complete picture of Pluto's atmospheric composition can be determined for the first time. ALICE will also be used to search for an atmosphere around Pluto's moon, Charon, as well as the Kuiper Belt Objects (KBOs) New Horizons hopes to fly by after Pluto-Charon. Light can enter the telescope section through either a 40mm x 40mm entrance aperture (i.e. the airglow channel) or a stopped-down 1-mm diameter entrance aperture and flat relay mirror (i.e. the SOCC) and is collected and focused by an off-axis paraboloidal (OAP) primary mirror onto the spectrograph entrance slit. The OAP has a 120 mm focal length. Scientific Objectives ======== Upper atmospheric temperature and pressure profiles of Pluto Temperature and vertical temperature gradient should be measured to ~10% at a vertical resolution of ~100 km for atmospheric densities greater than ~10^9 cm-3. Search for atmospheric haze at a vertical resolution <5 km Mole fractions of N2, CO, CH4 and Ar in Pluto's upper atmosphere. Atmospheric escape rate from Pluto Minor atmospheric species at Pluto Search for an atmosphere of Charon Constrain escape rate from upper atmospheric structure Calibration ======== ALICE instrument calibration issues include the following: - Dark count rate - Spectral offsets i.e. which wavelength is sampled by each pixel - Spectral and spatial resolution and point-spread function - Scattered light characteristics, including H Lyman-alpha - Absolute effective area See Stern et al. (2008) for details. Operational Considerations ======== The New Horizons ALICE UV spectrometer was successfully launched on 19 January 2006 and is operating normally in space. All in flight performance tests to date have shown performance within specification; the pointing and airglow sensitivity tests completed in September 2006 were nominal. The main remaining tests to be performed were the testing of the solar occultation aperture after it was opened, and instrument mutual noise susceptibility testing: May 2007, Inter-instrument interference test: The ALICE contribution to the Interference test was completed successfully. No interference was found in any of the three acquisitions. The retrieved housekeeping (HK) data for temperatures, HVPS and countrate were all nominal and no outliers were observed. In particular, the background countrate had returned to normal after the elevated count rate at Jupiter. July 2007, SOCC door opening: Open status was confirmed by detection of the star Bellatrix close to the center of the box in the Y (along slit) direction. Events coincident with the door opening also were detected by SDC, i.e. the movement of the door caused impact-like events to be measured by the Student Dust Counter. All further tests during the ACOs (Annual CheckOuts, 2007-2014) were nominal. Detectors ======== The detector is a 2-D array of 1024 spectral pixels by 32 spatial pixels. Although there are 1024 spectral pixel columns across the detector array, only about 780 are active, starting at an offset of 130 columns from the first spectral column. The spectral passband quoted above only refers to the active pixels. See Stern et al. (2008) for details. Electronics ======== Major electronic subsystems are - Low-Voltage Power Supply Electronics. - Command-and-Data-Handling Electronics. - Decontamination Heaters. - High Voltage Power Supplies. See Stern et al. (2008) for details. Optics ======== Summary information is above; see Stern et al. (2008) for details. Operational Modes ======== Optics Aperture Modes --------------------- Alice has two separate entrance apertures that feed light to the telescope section of the instrument: an 'airglow' aperture, which allows measurement of emissions from atmospheric constituents, and an 'occultation' aperture, when either the Sun or a bright star is viewed through the atmosphere producing absorption by the atmospheric constituents. The Alice occultation mode will be used just after New Horizons passes behind Pluto and looks back at the Sun through Pluto's atmosphere. Data Acquisition Modes ---------------------- ALICE has two detector data collection modes: (i) Pixel List Mode; (ii) Histogram Imaging Mode. The ALICE flight software controls both modes. Data are collected in a dual-port acquisition memory that comprises two separate 32kx16-bit memory channels. In both modes, the instrument is turned on and photons hitting a pixel on the detector, that result in analog pulses above a set threshhold level, generate photon events that are then transferred to the flight software. Each photon event contains the pixel location of the event. The difference between the modes is in how the flight software stores the photon events in the memory channels. Pixel List Mode: continuous 1-D stream of photon events and time hacks ----------------------------------------------------------------------- In Pixel List Mode (PLM), the 2-D pixel array location of each single photon event from the detector is transferred from ALICE to the spacecraft memory as a stream of data; the data stream is also interspersed with timing information (time hacks) that can be used to constrain the time of each photon event. In PLM, each ALICE memory channel acts as a linear data stream buffer: while one memory channel is being written to with detector data, the other is written to spacecraft memory. Once the instrument fills the first memory channel, the roles switch and the detector data go to the second channel while the first channel is written to spacecraft memory. This double-buffering - called 'ping-pong' acquisition - allows continuous readout and storage of detector event data. In this mode, a single event is represented by one 16-bit word in instrument memory and can be either a detector photon event or a time hack event. Photon events occur stochastically in time and are generated by photons hitting the detector. Time hack events, referred to simply as 'time hacks,' occur at regular intervals in time and are generated by the ALICE flight software. The time hack interval is programmable and can be as short as 4ms. The PLM data stream comprises photon event locations interspersed with time hack values. Time hacks can be converted to timestamps in the series of events and may be used to provide temporal information about the PLM data stream and to constrain the time that any photon event occurs. Histogram Imaging Mode: summing of photon events into a 2-D spectrogram ------------------------------------------------------------------------ In Histogram Imaging Mode (HIM), the ALICE flight software sums detector photon event counts for each pixel over a specified period (exposure) and then writes the result out as a 2-D array i.e. a spectral-spatial image or spectrogram. In this mode, the 1-D 32k memory channel is treated as a 2-D 1024x32 array; each memory channel location thus accumulates the photon event count acquired at its corresponding pixel location in the detector array. While some have found it confusing to call this a histogram, it is only an extension of the normally 1-D arrangement of histogram bins into pixels representing bins in two dimensions. Note that the timing information present in the time hacks of the PLM data stream is neither generated nor saved in HIM. Therefore, while the PLM data stream can be analyzed to generate the equivalentof the HIM spectrogram, the HIM data cannot be used to generate the PLM data. Besides the collection and binning of detector events, HIM collects pulse-height distribution (PHD) data from the detector electronics. These PHD data are collected and binned into a 64-bin histogram that is stored within the first two rows of the detector histogram, in a location where no physical pixel within the detector active area exists (therefore, the PHD data does not interfere with the collected detector data). During ground processing, the Science Operations Center pipeline software reads these PHD data and then zeroes the relevant area of the input array before creating the data products. The same ALICE instrument software that controls the PLM ping-pong acquisitions also controls the HIM. See Stern et al. (2008) for further details. Measured Parameters =================== Pixel List Mode --------------- Each 16-bit word in the PLM data stream represents either the detector location of a photon event or the time of a time hack. One bit in each word identifies that word as a photon event or as a time hack. The meaning of the remaining fifteen bits depends on which type of event the word represents. Photon events use ten bits for the spectral detector position (0 to 1023) and five bits for the spatial detector (0 to 31). Time hack events use all fifteen bits to represent the number of 4ms time intervals since either the instrument was turned on or the most recent rollover of the time hack counter. Histogram Imaging Mode ---------------------- Accumulated count of photon events, which each generated an analog pulse above the set threshhold, at each pixel location for the duration of each exposure. MCP pulse-height distribution histogram. 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