DATA_SET_DESCRIPTION |
Data Set Overview : This dataset contains raw, 1.05- to 4.8-micron spectral images of comet 103/P Hartley 2 acquired by the High Resolution Infrared Spectrometer (HRII) from 01 October through 26 November 2010 during the Hartley 2 encounter phase of the EPOXI mission. Initial results based on these data are discussed by A'Hearn, et al. (2011) [AHEARNETAL2011]. The following list summarizes the comet observations in this dataset. Descriptive text for each activity is included below. Additionally, the HRII Hartley 2 Flyby (E-18 hours to E+2 days) Log in the DOCUMENT directory provides notes about each scan, such as frames containing the comet, that were recorded by the science team as the data arrived on the ground. -------------------------------------------------------------------------- Mid-Obs Exposure IDs Mission Activity Date/DOY Min Max (E : Encounter) -------------- ------- ------- ---------------------------------------- 2010-10-01/274 4000001 4000063 Approach imaging E-34 to E-8 days; to Coma scans every 30 minutes; 2010-10-27/300 Odd ExpIDs only, most repeated daily* 2010-10-28/301 4000001 4000063 Approach imaging E-8 days to E-18 hours; to Coma scans every hour; 2010-11-03/307 Odd ExpIDs only, most repeated daily** 2010-11-03/307 4000001 4000604 Flyby imaging E-18 to E-3 hours; to Scans every two hours; then reduced to 2010-11-04/308 one hour cadence ExpIDs are not repeated 2010-11-04/308 5000000 5008002 Flyby imaging E-2 to E+1.5 hours; Near-continuous scans of coma/nucleus; Nadir imaging at closest approach; ExpIDs are not repeated 2010-11-04/308 4000700 4010100 Flyby imaging E+2 hours to E+2 days; to Coma scans every 30 minutes; 2010-11-06/310 ExpIDs are not repeated 2010-11-06/310 4100001 4500023 Departure imaging E+2 to E+12 days; to Coma scans every ~15 minutes; 2010-11-16/320 Odd ExpIDs only; repeated daily 2010-11-16/320 4100001 4500011 Departure imaging E+12 to E+21 days; to Coma scans every 20 minutes; 2010-11-26/330 Odd ExpIDs only; repeated daily -------------------------------------------------------------------------- * Data acquired on 06 Oct were never downlinked due to a pointing problem with the Deep Space Network (DSN). ** First cycle of 28 Oct, only six hourly scans were taken before the sequence stopped and restarted at 'dosido' section for 7 hourly scans. Therefore the middle 11 scans were not acquired on purpose. Some frames in a scan may be missing because they were scheduled for transmission after the HGA was turned off, i.e. loss of signal. Any data the DSN received after this were considered extra credit. Hartley 2 Approach Imaging, E-34 to E-8 Days (Start HRII): From 01 to 28 October 2010, MRI and HRIV imaged 103P/Hartley 2 about every 5 minutes while the HRII spectrometer scanned for outbursts once every 30 minutes. The instruments observed the comet for 16 hours per day allowing for 8 hours of downlinking; the same sequence was repeated daily yielding one full cycle per day. Data from the 6th cycle on 06 October 2010 (DOY 279) were never downlinked because of a pointing problem with the Deep Space Network. Those data had to be erased on board the spacecraft to make room for the next daily cycle and could not be recovered. Hartley 2 Approach Imaging, E-8 Days to E-18 Hours: From 28 October to 03 November 2010, the MRI and HRIV imaged 103P/Hartley 2 continuously and HRII scanned the comet about every hour for 16 hours per day allowing for 8 hours of downlinking punctuated by hourly maneuvers, called dosido, to observe the comet. During this imaging phase there was only a single downlink of all images with zero margin; thus some images were occasionally lost as expected. The first cycle (DOY 300/301) was abbreviated such that the first comet-imaging session was only 6-hours long, followed by the standard 8-hour dosido. Hartley 2 Encounter Imaging, E-18 hours to E+2 Days: From 03 to 06 November 2010, the HRII, HRIV, and MRI performed high resolution encounter imaging of 103P/Hartley2. The HRIV and MRI instruments began sampling about once every two hours until one hour before encounter when the cadence changed to once every 15 minutes. At E-30 minutes the instruments began continuously imaging of the comet. At E+30 minutes simultaneous observing and data playback began with samples being taken every 30 minutes. During the encounter imaging period, HRII infrared scans occurred every two hours until four hours prior to encounter when the cadence increased to hourly then more frequently one hour before closest approach. About one hour after closest approach, regular infrared sampling at 30-minute intervals resumed. During this period, HRII obtained several full spectral maps of the coma with a scale < 250 m/pixel (the exposure IDs are provided): 5001000 (E-14 min, ALTFF/Mode5 scan at ~115 m/pixel) 5001002 (E-7 min, ALTFF/Mode5 scan at ~60 mmpixel 5006000 (E+7 min, ALTFF/Mode5 scan at ~50 m/pixel) 5007000 (E+14 min, ALTFF/Mode5 scan at ~115 m/pixel) Also HRII obtained a full spectral map of the nucleus with a scale < 100 m/pixel: 5005001 (E+3 min, BINSF2/Mode3 scan at ~30 m/pixel) Please note the comet is in fewer frames than expected at closest approach because there was an error in how the spacecraft was commanded to point during closest approach. However this unexpected offset enabled serendipitous imaging of cometary debris near the nucleus. Hartley 2 Departure Imaging, E+2 to E+12 Days: From 06 to 16 November 2010, the HRII spectrometer scanned 103P/Hartley 2 every ~15 minutes while the MRI CCD imaged the comet every 2 minutes and HRIV once every hour. Hartley 2 Departure Imaging, E+12 to E+21 Days: From 16 to 26 November 2010, the HRII spectrometer scanned 103P/Hartley 2 every 30 minutes, and HRIV performed rotation sampling at the same cadence. MRI performed rotation sampling every 30 minutes and imaging using gas filters every two to four hours. Data Acquisition Strategy ------------------------- The data acquisition strategy for the IR scans throughout the Hartley 2 encounter had to balance data volume limitations with desired sampling. The goal of the IR observations during approach and departure was to monitor the coma for any changes that occurred from one coma scan to another. Therefore, the field of view covered by a scan was selected such that for a reasonable outflow velocity, the scan would cover the distance traveled by any new material released from the nucleus since the previous observation. In order to meet the sampling and field of view criteria, the cadence of scans was selected and then scan rate of the spectrometer (that is, the slew rate of the spacecraft), perpendicular to the slit length, was set to be either one slit width per exposure frame or two slit widths per exposure frame: Mission Timeline Scan Rate ------------------------ ------------------------------------------ Approach Imaging E-34 to E-8 days 2 slit widths per exposure frame E-8 days to E-18 hours 1 slit width per exposure frame Flyby Imaging E-18 hrs to E+2 days 2 slit widths per exposure frame for these observations (exposure IDs): 4000200, 4000500, 5008002, and 4000700-4010100 excluding 4002700 and 4002800. All other observations are 1 slit width per exposure frame. Departure Imaging E+2 days to E+12 days 2 slit widths per exposure frame E+12 days to E+21 days 1 slit width per exposure frame Required Reading --------------- The documents listed below are essential for the understanding and interpretation of this dataset. Although a copy of each document is provided in the DOCUMENT directory of this dataset, the most recent version is archived in the Deep Impact and EPOXI documentation set, DI-C-HRII/HRIV/MRI/ITS-6-DOC-SET-V3.0, available online at http://pds.nasa.gov. EPOXI_SIS.PDF - The Archive Volume and Data Product Software Interface Specifications document (SIS) describes the EPOXI datasets, the science data products, and defines keywords in the PDS labels. HARTLEY2_CAL_PIPELINE_SUMM.PDF - The EPOXI Hartley 2 Calibration Pipeline Summary provides an overview the calibration pipeline as of June 2011 used for processing data acquired during the Hartley 2 Encounter. The document also discusses known limitations of the calibration pipeline with respect to the HRII, HRIV, and MRI instruments. For a thorough discussion of the pipeline refer to EPOXI Instrument Calibration by Klaasen, et. al. (2011, in preparation) [KLAASENETAL2011]. INSTRUMENTS_HAMPTON.PDF - The Deep Impact instruments paper by Hampton, et al. (2005) [HAMPTONETAL2005] provides very detailed descriptions of the instruments. HRII_HARTLEY2_FLYBY_LOG.PDF - This log provides notes recorded by the science team as each Flyby exposure (scan) acquired from E-18 to E+48 hours was received on the ground. Annotations include data quality and a list of frames within each scan that appeared to contain the comet. HRII_2_EPOXI_HARTLEY2.TAB - This ASCII table provides image parameters such as the mid-obs Julian date, exposure time, mission activity type, and description or purpose for each observation (i.e., data product) in this dataset. This file is very useful for determining which data files to work with. Related Data Sets ----------------- The following PDS datasets are related to this one and may be useful for calibration purposes: DIF-C-HRII-3/4-EPOXI-HARTLEY2-V1.0 - Calibrated HRII spectral images of comet Hartley 2 DIF-CAL-HRII-2-EPOXI-CALIBRATIONS-V2.0 - Raw HRII in-flight calibrations from 2007 to 2011 DIF-C-HRIV-2-EPOXI-HARTLEY2-V1.0 DIF-C-HRIV-3/4-EPOXI-HARTLEY2-V1.0 - Raw and calibrated HRIV images of comet Hartley 2 DIF-C-MRI-2-EPOXI-HARTLEY2-V1.0 DIF-C-MRI-3/4-EPOXI-HARTLEY2-V1.0 - Raw and calibrated MRI images of comet Hartley 2 DIF-C/E/X-SPICE-6-V1.0 - EPOXI SPICE kernels DIF-CAL-HRII/HRIV/MRI-6-EPOXI-TEMPS-V2.0 - HRII, HRIV, and MRI instrument thermal telemetry data for EPOXI which may be useful for determining how temperature fluctuations affect the science instruments, in particular the IR spectrometer DI-C-HRII/HRIV/MRI/ITS-6-DOC-SET-V3.0 - Deep Impact and EPOXI documentation set including a draft of the Deep Impact instrument calibration paper by Klaasen, et al. (2008) [KLAASENETAL2006] Processing : The raw two-dimensional (wavelength and spatial/along-slit) FITS spectral images and PDS labels in this dataset were generated by the Deep Impact/EPOXI data pipeline, maintained by the project's Science Data Center (SDC) at Cornell University. The FITS data were assembled from raw telemetry packets sent down by the flyby spacecraft. Information from the embedded spacecraft header (the first 100 bytes of quadrant A image data) was extracted and stored in the primary FITS header. Geometric parameters were computed using the best available SPICE kernels and the results were also stored in the FITS header. If telemetry packets were missing, the corresponding pixels were flagged as missing in the quality map included as a FITS image extension. The quadrant nomenclature and the image quality map are described in the EPOXI SIS document. The SDC did not apply any type of correction or decompression algorithm to the raw data. Data : FITS Images and PDS Labels -------------------------- Each raw spectral image is stored as FITS. The primary data unit contains the two-dimensional spectral image, with the fastest varying axis corresponding to increasing wavelengths from about 1.05 to 4.8 microns and the slowest varying axis corresponding to the spatial or along-slit dimension. The primary array is followed by one image extension that contains a two-dimensional pixel-by-pixel quality map. This extension uses one byte of eight bit flags to indicate the quality of each pixel in the primary image. The data label provides a short description of each bit. For more information about the FITS primary image and its extension or for examples of how to access and use the quality flags, refer to the EPOXI SIS document. Each FITS file is accompanied by a detached PDS data label. The EPOXI SIS document provides definitions for the keywords found in a PDS data label. Many values in a label were extracted from FITS image header keywords which are defined in the document EPOXI_FITS_KEYWORD_DESC.ASC found in the Deep Impact and EPOXI documentation dataset, DI-C-HRII/HRIV/MRI/ITS-6-DOC-SET-V3.0. File Naming Convention ---------------------- The naming convention for the raw data labels and FITS files is HIyymmddhh_eeeeeee_nnn.LBL or FIT where 'HI' identifies the HRII instrument, yymmddhh provides the UTC year, month, day, and hour at the mid-point of the observation, eeeeeee is the exposure ID (OBSERVATION_ID in data labels), and nnn provides the image number (IMAGE_NUMBER in the data labels) within the exposure ID. Up to 999 individual images can be commanded for one exposure ID. Spectral scans often had 8 or more frames for one specific exposure. Therefore, nnn in the file name provides the sequentially increasing frame number within an exposure ID and corresponds to IMAGE_NUMBER in the data labels. For example, if 30 frames were commanded for a scan with an exposure ID of 5000000, the first FITS file name would be HI10110412_5000000_001.FIT and the last would be HI10110412_5000000_030.FIT. Image Compression ----------------- Raw data numbers for HRII frames could be compressed on board the flyby spacecraft by use of a lookup table then downlinked, processed, and archived in the same format. However this dataset does not contain compressed products because all Hartley 2 spectral images were never compressed. Image Orientation ----------------- A true-sky 'as seen by the observer' view is achieved by displaying the image using the standard FITS convention: the fastest-varying axis (samples or wavelength) increasing to the right in the display window and the slowest-varying axis (lines or spatial/along-slit) increasing to the top. This convention is identified in the data labels: the SAMPLE_DISPLAY_DIRECTION keyword is set to RIGHT and LINE_DISPLAY_DIRECTION to UP. The direction to celestial north, ecliptic north, and the Sun is provided in data labels by CELESTIAL_NORTH_CLOCK_ANGLE, ECLIPTIC_NORTH_CLOCK_ANGLE, and SUN_DIRECTION_CLOCK_ANGLE keywords and are measured clockwise from the top of the image when is displayed in the correct orientation as defined by SAMPLE_DISPLAY_DIRECTION and LINE_DISPLAY_DIRECTION. Please note the aspect of the North celestial pole in an image can be computed by adding 90 degrees to the boresight declination given by DECLINATION in the data labels. For a comparison of the orientation FITS image data from the three science instruments, see the quadrant nomenclature section of the the EPOXI SIS document. Spectral Scans -------------- Each HRII scan of Hartley 2 consists of multiple frames within one exposure ID (OBSERVATION_ID in the data labels). To work with these spectral scans, it is recommended that all frames for one exposure ID be stacked into a three-dimensional cube. Then, a spatial-spatial map can be produced for a specific wavelength by selecting the appropriate spectral column from the image cube. Spectral wavelengths are provided by the second FITS extension, the spectral registration (wavelength) map. IR Slit Location ---------------- For a comparison of the relative locations of the IR slit with respect to the fields of view of the Medium Resolution Instrument Visible CCD (MRI) and the High Resolution Instrument Visible CCD (HRIV), see the instrument alignment section of the EPOXI SIS document or Klaasen, et al. (2011) [KLAASENETAL2011]. There are no visible CCD context images provided in this dataset to aid in orienting the IR slit location with the nucleus during a particular observation. In many cases, nearly simultaneous MRI frames, located in the dataset DIF-C-MRI-3/4-EPOXI-HARTLEY2-V1.0, were acquired during the IR scans and may provide field of view context for the slit location. Utilizing the infrared data scans themselves is currently the best way to determine the slit location on the nucleus. The user can create a three-dimensional cube as described above then use the ~2.4-micron spatial-spatial map to determine where the slit was pointed during that particular scan. Geometry values, such as right ascension and declination, given in the data labels are for the instrument boresight and do not easily give positional information along the slit for tying a pixel to a specific point on the nucleus. Timing for Spectra ------------------ It is important to note that the readout order of the IR detector affects the timing of the spectra. When a HRII spectral image is displayed using the true-sky convention, the wavelength increases horizontally to the right and the spatial or along-slit direction is vertical. In this orientation, the IR detector was read out from the left and right edges and toward the center and starting with the first row at the bottom and ending with the last row at the top of the display. Since the detector is reset and read out on a pixel-by-pixel basis, the read out order affects the time at which each pixel is exposed although each pixel has the same exposure duration -- except for the ALTFF mode that has different read and reset causing the effective exposure time to vary with line number, i.e., along the slit in the spatial direction. Additionally, the end of the spectrometer slit that always points roughly towards the sun is the first line to be readout and the last line to be read out is furthest from the sun, assuming the spacecraft is in its usual orientation with the solar panels pointing roughly toward the sun. For more information about the timing of the spectra, see the zero exposure background section of the EPOXI instrument calibration paper by Klaasen, et al. (2011) [KLAASENETAL2011]. A brief discussion about how the calibration pipeline handles the ALTFF mode is included in the EPOXI Hartley 2 Calibration Pipeline Summary document. Parameters : Data Units ---------- Raw spectral image data have units of raw data numbers. Imaging Modes ------------- A summary of the imaging modes is provided here. For more information see the EPOXI SIS and EPOXI Hartley 2 Calibration Pipeline Summary documents, Hampton, et al. (2005) [HAMPTONETAL2005] and Klaasen, et al. (2011) [KLAASENETAL2011]. In the table below, X-Size is the spectral dimension and Y-Size is the spatial dimension. X-Size Y-Size Bin Mode Name (pix) (pix) Type Comments ---- ------ ------ ----- ----- ------------------------------------ 1 BINFF 512 256 2x2 Binned full frame 2 BINSF1 512 126 2x2 Binned sub-frame 3 BINSF2 512 64 2x2 Binned sub-frame 4 UBFF 1024 512 1x1 Unbinned full frame 5 ALTFF 512 256 2x2 Alternate mode 1 (min. exposure time is 1/2 of mode 1) 6 DIAG 1024 512 1x1 Diagnostic, one reset frame followed by a separate read frame such that odd IMAGE_NUMBERs are reset frames and even IMAGE_NUMBERs are read frames 7 MEMCK 1024 512 1x1 Memory Check By utilizing the different imaging modes of the HRII instrument, the observational requirements for desired exposure times were met. Note, of the 7 modes, only modes 1-6 were used for the encounter with comet Hartley 2. Subframe modes are binned (2x2), reduce the spatial (LINE) extent of the image FOV, and have a shorter readout time which reduces the exposure time for bright objects and keeps the detector from saturating. Time- and Geometry-Related Keywords ----------------------------------- All time-related keywords in the data labels, except EARTH_OBSERVER_MID_TIME, are based on the clock on board the flyby spacecraft. EARTH_OBSERVER_MID_TIME provides the UTC when an Earth-based observer should have been able to see an event recorded by the instrument. The SDC pipeline was not able to automatically determine the proper geometric information for the target of choice in some cases. When these parameters could not be computed, the corresponding keywords in the data labels are set to a value of unknown, 'UNK'. Also if GEOMETRY_QUALITY_FLAG is set to 'BAD' or GEOMETRY_TYPE is set to 'PREDICTED' in the PDS labels, then this indicates the geometry values may not be accurate and should be used with caution. The value 'N/A' is used for some geometry-related keywords in the data labels because these parameters are not applicable. Observational geometry parameters provided in the data labels were computed at the epoch specified by the mid-obs UTC, IMAGE_MID_TIME, in the data labels. The exceptions are the target-to-sun values evaluated at the time light left the target that reached the spacecraft at mid-obs time, and the earth-observer-to-target values evaluated at the time the light that left the target, which reached the spacecraft at mid-obs time, reached Earth. Ancillary Data : The geometric parameters included in the data labels and FITS headers were computed using the best available SPICE kernels at the time the data products were generated. The kernels are archived in the EPOXI SPICE dataset, DIF-C/E/X-SPICE-6-V1.0. Coordinate System : Earth Mean Equator and Vernal Equinox of J2000 (EME J2000) is the inertial reference system used to specify observational geometry parameters in the data labels. Software : The observations in this dataset are in standard FITS format with PDS labels, and can be viewed by a number of PDS-provided and commercial programs. For this reason no special software is provided with this dataset.
|