| DATA_SET_DESCRIPTION |
Data Set Overview
=================
This data set contains version 1.0 of raw, 1.05- to 4.8-micron spectra
of Earth acquired by the High Resolution Infrared Spectrometer (HRII)
during the EPOCh phase of the EPOXI mission. Three sets of
observations were acquired on 18-19 March, 28-29 May, and 4-5 June
2008 to characterize Earth as an analog for extrasolar planets. Each
observing period lasted approximately 24 hours, and spectra were
acquired twice per hour. During the observing period in May, the Moon
transited across Earth as seen from the spacecraft.
At every half hour of each observing period, the spacecraft slewed
across the Earth while the IR spectrometer recorded data; these
scans were performed three times within several minutes, alternating
direction from south-to-north and north-to-south. Each scan
consisted of eight 128x256 binned subframes. Each half-hour set
alternated between slower scans with longer frame exposure times
and faster scans with shorter frame durations.
Additional Earth observations are being planned for the mission
because two observing periods that were scheduled from late March
through early May 2008 were canceled due to a telecommunications
anomaly on board the spacecraft. These data will be added to a
future version of this data set.
Required Reading
---------------
The following documents are essential for the understanding and
interpretation of this data set. Please note the most recent
version of these documents, including other formats such as ASCII
text, can be found in the Deep Impact and EPOXI documentation data set,
DI-C-HRII-HRIV-MRI-ITS-6-DOC-SET-V2.0.
EPOXI_SIS.PDF
- The Archive Volume and Data Product Software Interface
Specifications document (SIS) describes the the data set, the
science data products, and defines keywords in the PDS labels.
CALIBRATION_PAPER_DRAFT.PDF
- The Deep Impact instrument calibration paper by Klaasen, et al.
(2008) [KLAASENETAL2006] describes how the instruments were
calibrated for Deep Impact and similarly for EPOXI and explains
the calibration process used for both missions. The published
version should be available online in the Review of Scientific
Instruments by the American Institute of Physics. The EPOXI
archive provides only an incomplete draft.
INSTRUMENTS_HAMPTON.PDF
- The Deep Impact instruments paper by Hampton, et al. (2005)
[HAMPTONETAL2005] provides very detailed descriptions of the
instruments.
EPOCH_EARTH_OBS.PDF
- This document describes of the EPOCh Earth observations
although most of the information is captured in this data set
catalog file you are reading.
EPOCH_EARTH_SEQ_2008.PDF
- This document provides pointing and sequencing information
for the EPOCh Earth observations in 2008, including descriptions
of the HRII scans of Earth (scan direction, rate, etc.).
EPOCH_OVERVIEW.PDF
- This presentation provides an overview of the EPOCh phase of
the EPOXI mission.
HRII_2_EPOXI_EARTH.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 data set. This file is very useful for determining which
data files to work with.
Publications of the scientific results from the Earth observations
in this data set include Cowan, et al. (2009) [COWANETAL2009] and
Livengood, et al. (2009) [LIVENGOODETAL2009].
Related Data Sets
-----------------
The following PDS data sets are related to this one and may be useful
for research:
DIF-E-HRII-3/4-EPOXI-EARTH-V1.0
- Calibrated HRII Earth observations
DIF-CAL-HRII-2-EPOXI-CALIBRATIONS-V1.0
- Raw HRII dark frames that bracket each set of Earth observations
in this data set
DIF-E-HRIV-2-EPOXI-EARTH-V1.0
DIF-E-HRIV-3/4-EPOXI-EARTH-V1.0
- Raw and calibrated HRIV visible CCD Earth observations at
350, 450, 550, 650, 750, 850, and 950 nm, covering the same
three observing periods as this data set
DIF-E-MRI-2-EPOXI-EARTH-V1.0
DIF-E-MRI-3/4-EPOXI-EARTH-V1.0
- Raw and calibrated MRI visible CCD context images of Earth at
750 nm, covering only the March 2008 observing period.
DI-C-HRII-HRIV-MRI-ITS-6-DOC-SET-V2.0
- Deep Impact and EPOXI documentation set
DIF-C/E/X-SPICE-6-V1.0
- EPOXI SPICE kernels
DIF-CAL-HRII/HRIV/MRI-6-EPOXI-TEMPS-V1.0
- 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
Processing
==========
The raw two-dimensional (wavelength and spatial/along-slit) FITS
spectral images and PDS labels in this data set 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
Deep Impact instrument calibration document and the EPOXI SIS document
included in this data set. 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 or the Deep Impact
instrument calibration document.
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.
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 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 32 frames were commanded for a scan
with an exposure ID of 1000001, the first FITS file name would be
HI08060416_1000001_001.FIT and the last would be
HI08060416_1000001_032.FIT.
Image Compression
-----------------
Although 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, Earth spectra acquired
during the time period covered by this data set were never
compressed. Therefore the COMPRESSED_IMAGE_VALUE keyword in the
data labels is always set to 'UNCOMPRESSED'. For more information
about this topic, see the image compression section of the Deep
Impact instrument calibration documents.
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
Deep Impact instrument calibration document. Also the EPOXI SIS
has a brief discussion of this topic.
Spectral Scans
--------------
Each IR scan across the Earth 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 relative boresight alignments section of the
Deep Impact instrument calibration document.
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. 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
pointing roughly toward the sun. For more information about the
timing of the spectra, see the IR focal plane and quadrant
nomenclature sections of the Deep Impact instrument calibration
document.
Parameters
==========
Data Units
----------
Raw spectral image data have units of raw data numbers.
Imaging Modes
-------------
One HRII image mode was used for all Earth spectra:
X-Size Y-Size Bin
Mode Name (pix) (pix) Type Comments
---- ------ ------ ----- ----- ---------------------------------
2 BINSF1 512 126 2x2 Binned sub-frame
In the table above, X-Size is the spectral dimension and Y-Size is
the spatial dimension. For a thorough description of the imaging
modes, please see the Deep Impact instruments document or the Deep
Impact instrument calibration document. Also the EPOXI SIS has a
brief discussion of this topic.
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.
For Earth observations, sub-spacecraft and sub-solar longitude and
latitude coordinates (planetocentric, body-fixed rotating) are
provided, when available, in the data labels by
SUB_SPACECRAFT_LONGITUDE, SUB_SPACECRAFT_LATITUDE,
SUB_SOLAR_LONGITUDE, and SUB_SOLAR_LATITUDE.
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 for certain
calibration targets.
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
that were calculated for the time when the light arrived at the
target and the earth-observer-to-target values that were calculated
for the time when the light left the target.
The flyby spacecraft clock SPICE kernels (SCLK) used to convert to
UTC and to calculate geometry-related parameters for this data set
have a known accuracy of no better than 0.5 seconds. However as
this data set was being produced, the mission operations team
figured out how to correct raw clock correlation data for the
flyby spacecraft to allow timing fits that are accurate to at
least the sub-second level. The project plans to generate a
complete, corrected set of correlations since launch. This will
ultimately result in a future version of a SCLK kernel that will
retroactively change correlation for **all** Deep Impact and EPOXI
data. When this kernel is available, it will be added to the
SPICE data sets for the two missions and posted on the NAIF/SPICE
web site at http://naif.jpl.nasa.gov/naif/.
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. NAIF used these kernels to produce the
EPOXI SPICE data set, 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, unless specified otherwise (e.g,
SUB_SPACECRAFT_LONGITUDE).
Software
========
The observations in this data set 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
data set.
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