| DATA_SET_DESCRIPTION |
Data Set Overview
=================
This dataset contains calibrated, 1.05- to 4.8-micron spectral images
of comet 9P/Tempel 1 the acquired by the High Resolution Infrared
Spectrometer (HRII) from 20 June through 06 July 2005 during the
encounter phase of the Deep Impact mission.
Version 3.0 was calibrated by the EPOXI mission pipeline and includes
the correct interpretation of microsecond counter of the spacecraft
clocks that previously introduced maximum errors of about 40
milliseconds in the earlier observation times, a corrected IR absolute
spectral calibration step that previously inflated all spectra by a
factor of 2, and an upgrade to the ALTFF line-dependent integration
time. Version 3.0 also includes a new flat-field file derived from
EPOXI lunar calibrations, improved quadrant-averaged linearity
coefficients, and a refinement in the absolute spectral calibration
curve. For Version 3.0, the EPOXI convention for constructing
PRODUCT_IDs and data file names was used. For more information see
the EPOXI Calibration Pipeline Summary document in this dataset and
Klaasen, et al. (2013) [KLAASENETAL2011].
A summary of the comet observations in this dataset is provided here:
Mid-Obs Exposure IDs
Date DOY Minimum Maximum Mission Activity
---------- --- ------- ------- --------------------------
2005-06-20 171 6002005 6002005 Daily comet imaging
2005-06-21 172 6002100 6002105 Daily comet imaging
2005-06-22 173 6002200 6002205 Daily comet imaging
2005-06-23 174 6002300 6002305 Daily comet imaging
2005-06-24 175 6002400 6002405 Daily comet imaging
2005-06-25 176 6002500 6002504 Daily comet imaging
2005-06-26 177 6002600 6002603 Daily comet imaging
2005-06-27 178 8000000 8000004 Continuous comet imaging
2005-06-28 179 8000005 8100004 Continuous comet imaging
2005-06-29 180 8100005 8300000 Continuous comet imaging
2005-06-30 181 8400000 8400005 Continuous comet imaging
2005-07-01 182 8400006 8500009 Continuous comet imaging
2005-07-02 183 8500009 8800003 Continuous comet imaging
2005-07-03 184 9000000 9000021 Continuous comet imaging
2005-07-04 185 9000022 9000029 Continuous comet imaging
9000030 9000039 Pre-impact scans
9000040 9000068 Impact imaging
9010000 9070002 Lookback imaging
2005-07-05 186 9080000 9110002 Lookback imaging
2005-07-06 187 9120000 9150002 Lookback imaging
The 9P/Tempel 1 data are described in 'Deep Impact: The Anticipated
Flight Data' by Klaasen, et al. (2005) [KLAASENETAL2005]. Initial
results from the encounter and impact were presented in 'Deep Impact:
Excavating Comet Tempel 1' by by A'Hearn, et al. (2005)
[AHEARNETAL2005A].
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
including this Deep Impact dataset recalibrated by the EPOXI
pipeline, 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].
CALIBRATION_PAPER_DRAFT.PDF
- This incomplete draft of 'Deep Impact Instrument Calibration'
by Klaasen, et al. (2008) [KLAASENETAL2006] explains how the
instruments were calibrated for the Deep Impact mission. It
also describes the Deep Impact calibration pipeline, which was
the basis for the EPOXI calibration pipeline.
INSTRUMENTS_HAMPTON.PDF
- The Deep Impact instruments paper by Hampton, et al. (2005)
[HAMPTONETAL2005] provides very detailed descriptions of the
instruments.
HRII_ENCOUNTER_DATA_SUMMARY.PDF
- This log provides notes and data quality recorded by the science
team for each HRII exposure, beginning 28 hours before impact and
continuing through the lookback period.
HRII_ENCOUNTER_POINTING.PDF
- This log provides general pointing and scanning information for
each set of exposure IDs commanded for the HRII spectrometer from
20 June through 07 July 2005.
HRII_3_4_DI_TEMPEL1.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-CAL-HRII-2-GROUND-TV1-V1.0
DIF-CAL-HRII/HRIV-2-GROUND-TV2-V1.0
DIF-CAL-HRII/HRIV/MRI-2-GROUND-TV4-V1.0
- Raw HRII pre-flight calibration spectra from the first and second
thermal vacuum tests in 2002 and the fourth one in 2003
DIF-CAL-HRII-2-9P-CRUISE-V1.0
- Raw HRII cruise calibration spectra
DIF-C-HRII-2-9P-ENCOUNTER-V1.0
- Raw HRII spectra of comet Tempel 1 and calibration sources
DIF-C-HRII/HRIV/MRI-6-TEMPS-V1.0
- HRII, HRIV, and MRI instrument thermal telemetry data from the
Deep Impact mission which may be useful for determining how
temperature fluctuations affect the science instruments, in
particular the HRII spectrometer
DI-C-SPICE-6-V1.0
- Deep Impact SPICE kernels
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 in late
2013 by the EPOXI data pipeline, maintained by the project's Science
Data Center (SDC) at Cornell University. 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] and in 'Deep Impact Instrument
Calibration' by Klaasen, et al. (2008) [KLAASENETAL2006].
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
- Quadrant-averaged linearization of raw data numbers
- Subtraction of dark noise, derived using quadrant-averaged
linearization of either in scene dark frames, specific
exposure-ID darks, or optimized mode- dependent master dark
frames (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 from quadrant-averaged
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 from quadrant-based 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-2-9P-ENCOUNTER-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 consisting of eight,
single-bit flags to describe the quality of each pixel
in the primary image. The PDS data label defines the
purpose of each single-bit flag.
- 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 the 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 or frames can be commanded for one
exposure ID. Spectral scans often had 32 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 50
frames were commanded for a scan with an exposure ID of 9000028, the
first FITS file name would be HI05070404_9000028_001_RR.FIT and the
last would be HI05070404_9000028_050_RR.FIT.
This convention is the one used by the EPOXI pipeline to construct
data product file names and PRODUCT_IDs. To translate between the
EPOXI and Deep Impact conventions, refer to the
HRII_TRANSLATE_PRODUCT_ID.LBL and HRII_TRANSLATE_PRODUCT_ID.TAB
files located in the DOCUMENT directory of this dataset.
Image Compression
-----------------
All calibrated data products are uncompressed. If an associated
raw data product was compressed on board the flyby spacecraft (and
thus received on the ground and archived as compressed) then the
calibration pipeline used one of four 8-bit lookup tables to
decompress the raw image. For more information, see the EPOXI
Calibration Pipeline Summary document as well as Hampton, et al.
(2005) [HAMPTONETAL2005], Klaasen, et al. (2008) [KLAASENETAL2006]
and Klaasen, et al. (2013) [KLAASENETAL2011].
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 it 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.
Using this convention to display an approach image of Tempel 1,
ecliptic North is toward the right and the Sun is down. After
impact, the Flyby spacecraft came out of shield mode and turned
back to observe at the comet. For lookback images, ecliptic
North is toward the left and the Sun is down.
For a comparison of the orientation of HRII flight images with those
from ground-based calibrations as well as those from the Medium
Resolution Instrument CCD (MRI) and the High Resolution Instrument
CCD (HRIV) CCD (ITS), see the quadrant nomenclature section in
Klaasen, et al. (2008) [KLAASENETAL2006] and Klaasen, et al. (2013)
[KLAASENETAL2011].
Spectral Scans
--------------
Each HRII scan of Tempel 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
CCD (MRI) and the High Resolution Instrument CCD (HRI), see the
instrument alignment section of Klaasen, et al. (2008)
[KLAASENETAL2006]. To visually inspect where the IR slit was
estimated to be on the nucleus of Tempel 1 during impact and
lookback, see the HRII/HRIV context maps included Deep Impact and
EPOXI documentation dataset, DI-C-HRII/HRIV/MRI/ITS-6-DOC-SET-V4.0.
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 Hampton, et al. (2005) [HAMPTONETAL2005], Klaasen, et al. (2008)
[KLAASENETAL2006] and Klaasen, et al. (2013) [KLAASENETAL2011]. In
the table below, X-Size is the spectral dimension and Y-Size is the
spatial dimension along the slit.
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 timing and geometric parameters included in the data labels and
FITS headers were computed using the final version of the kernel
files archived in the Deep Impact SPICE dataset DI-C-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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