| DATA_SET_DESCRIPTION |
Data Set Overview
=================
This dataset contains calibrated, 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].
Version 2 includes a new per-pixel linearity correction treatment and
its propagation through the calibration steps (i.e., bad-pixel maps,
flat-field file update, revised spectral calibration curve), new
mode-dependent master darks, an optimized scaling factor for the master
dark, and a refinement in the absolute spectral calibration curve. Also
the sub-frame master darks now include the effect of glow from
saturated border pixel and non-image (reference) pixels at the image
edges, which have no scientific value for calibrated products, are
now set to zero. These improvements are described in the EPOXI
Calibration Pipeline Summary document in this dataset and by Klaasen,
et al. (2013) [KLAASENETAL2011].
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 to hours 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 30 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 m/pixel
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-V4.0, 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.
EPOXI_CAL_PIPELINE_SUMM.PDF
- The EPOXI Calibration Pipeline Summary provides an overview
of the final version of the calibration pipeline that generated
the data products in this dataset. For a thorough discussion
of the pipeline, see 'EPOXI Instrument Calibration' by Klaasen,
et al. (2013) [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_3_4_EPOXI_HARTLEY2.TAB
- This ASCII table provides image parameters such as the mid-obs
Julian date, exposure time, image mode, 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 research:
DIF-C-HRII-2-EPOXI-HARTLEY2-V1.0
- Raw HRII comet Hartley 2 observations
DIF-CAL-HRII-2-EPOXI-CALIBRATIONS-V2.0
- Raw HRII in-flight calibrations from 2007 to 2011
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.
N.B. The pipeline does not use these thermal data to calibrate
IR spectra of Hartley 2. Instead it uses instrument temperatures
recoreded in the FITS headers.
DI-C-HRII/HRIV/MRI/ITS-6-DOC-SET-V4.0
- Deep Impact and EPOXI documentation set
Processing
==========
The calibrated two-dimensional (wavelength and spatial/along-slit) FITS
spectral images and PDS labels in this dataset were generated by the
Deep Impact/EPOXI calibration pipeline, maintained by the project's
Science Data Center (SDC) at Cornell University. The final version of
the pipeline for HRII processing, dated January 2013, was used. Known
limitations and deficiencies of the pipeline and the resulting data are
discussed in the EPOXI Calibration Pipeline Summary document in this
dataset and by Klaasen, et al. (2013) [KLAASENETAL2011].
For HRII spectra, the pipeline generates two types of calibrated
products:
- Uncleaned radiance data provided in units of
Watts/(meter**2 steradian micron) and identified by the mnemonic
'RADREV'. The RADREV data are considered to be reversible
because the calibration steps can be backed out to return to the
original, raw data numbers.
- Irreversibly cleaned radiance data provided in units of
Watts/(meter**2 steradian micron) and identified by the mnemonic
'RAD'. The RAD data are considered to be irreversible because
the calibration steps, such as smoothing over bad pixels, cannot
easily be backed out to return to the original, raw data
numbers.
The calibration pipeline performed the following processes, in the
order listed, on the raw HRII FITS data to produce the RADREV and
RAD products found in this data set (the process uses the image
mode to select the appropriate set of calibration files):
- Calibration of temperatures and voltages in the FITS header
- Per-pixel linearization of raw data numbers (version 1.0 used
per-quadrant linearization)
- Subtraction of dark noise, derived for per-quadrant linearization
(the prisms/spectral imaging module and IR focal plane array
temperatures, OPTBENT and IRFPAT in the FITS header, are used
for scaling if dark modeling is required)
- Division by a flat field, derived for per-quadrant linearization
- Determine spectral registration and bandwidth for each pixel
(using OPTBENT from FITS headers)
- Conversion of data numbers to units of radiance for an absolute,
radiometric calibration that is reversible (RADREV) and that was
derived for per-quadrant linearization
- Interpolation over bad and missing pixels identified in the
RADREV data to make a partially cleaned, irreversible, radiometric
calibration with units of radiance (RAD); Steps for despiking
(i.e., cosmic ray removal) and denoising the data which are part
of the RAD stream were not performed because the existing routines
are not robust.
- Set non-image pixels at the left, right, and bottom edges to zero in
the RADREV and RAD products. The 'real data' window of an image
is given by CALWINDW in the FITS header. If edge pixels need to be
analyzed, the original DN values can be found in the raw products
located in the PDS dataset, DIF-C-HRII-2-EPOXI-HARTLEY2-V1.0.
As part of the calibration process, the pipeline updated the per-pixel
image quality map, the first FITS extension, to identify:
- Pixels where the raw value was saturated,
- Pixels where the analog-to-digital converter was saturated,
- Pixels that were ultra-compressed and thus contain very little
information, and
- Pixels considered to be anomalous as indicated by bad pixel
maps derived for per-pixel linearity (missing pixels were
identified when the raw FITS files were created).
The pipeline also created FITS image extensions for a spectral
registration (wavelength) map, a spectral resolution (bandwidth) map,
and a signal-to-noise ratio map, which are briefly described in the
next section. The calibration steps and files applied to each raw
image are listed in the PROCESSING_HISTORY_TEXT keyword in the PDS
data label.
Data
====
FITS Images and PDS Labels
--------------------------
Each calibrated 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 image is
followed by four image extensions that are two-dimensional
pixel-by-pixel maps providing additional information about the
spectral image:
- The first extension uses one byte of eight, bit flags to
describe the quality of each pixel in the primary image.
The PDS data label defines the purpose of each bit.
- The second extension provides the spectral registration or
wavelength for each pixel in the primary image. This
extension is required because the wavelength for each
pixel changes as the temperature of the instrument
increased or decreased.
- The third extension provides the spectral bandwidth for
each pixel in the primary image. This extension is
required because the bandwidth for each pixel changes as
the temperature of the instrument increased or decreased.
- The fourth extension provides a signal-to-noise ratio for
each pixel in the primary image.
Each FITS file is accompanied by a detached PDS data label. The
EPOXI SIS document provides definitions for the keywords found in
a data label and provides more information about the FITS primary
image and the extensions. Many values in a data 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-V4.0.
File Naming Convention
----------------------
The naming convention for calibrated data labels and FITS files is
HIyymmddhh_eeeeeee_nnn_rr.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), nnn provides the image number
(IMAGE_NUMBER in the data labels) within the exposure ID, and
rr identifies the type of reduction:
RR for RADREV data (reversibly calibrated, radiance units)
R for RAD data (partially cleaned RADREV data, radiance units)
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 1000002, the first FITS file name would be
HI10110412_5000000_001_RR.FIT and the last would be
HI10110412_5000000_030_RR.FIT.
Image Compression
-----------------
All data products in this dataset are uncompressed. Specifically
all raw 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. (2013) [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. (2013) [KLAASENETAL2011]. A brief discussion
about how the calibration pipeline handles the ALTFF mode is included
in the EPOXI Calibration Pipeline Summary document.
Parameters
==========
Data Units
----------
The calibrated RADREV and RAD image data have units of radiance,
W/(m**2 steradian micron).
Imaging Modes
-------------
A summary of the imaging modes is provided here. For more
information see the EPOXI SIS and EPOXI Calibration Pipeline Summary
documents, Hampton, et al. (2005) [HAMPTONETAL2005], and Klaasen, et
al. (2013) [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.
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