| DATA_SET_DESCRIPTION |
Data Set Overview
=================
This dataset contains calibrated, 1.05- to 4.8-micron spectra of Earth
acquired by the High Resolution Infrared Spectrometer (HRII) during
the EPOCh and Cruise 2 phases of the EPOXI mission. Five sets of
observations were acquired on 18-19 March, 28-29 May and 04-05 June
2008 and on 27-28 March and 04-05 October 2009 to characterize Earth
as an analog for extrasolar planets. Each observing period lasted
approximately 24 hours; every two hours spectra were acquired twice
within a twenty-minute interval. During the observing period in May
2008, the Moon transited across Earth as seen from the spacecraft.
HRII spectra were not acquired during the first attempt of an Earth
south polar observation on 27-28 September 2009 because fault
protection turned that instrument off; the full sequence was
successfully rerun on 04-05 October 2009. Version 2 corrects an
error in the IR absolute calibration that previously inflated all
spectra by a factor of 2. It also includes the application of an
improved flat field and a corrected spacecraft clock algorithm to
remove a known systematic error at the subsecond level in the
conversion of the spacecraft times to UTCs.
Every two hours the spacecraft was slewed across the disk of Earth
six times while the IR spectrometer recorded data. The scans were
performed in sets of three as the scan direction was alternated from
south-to-north, north-to-south, then south-to-north. Each scan
(exposure ID) consisted of eight 512x128 binned subframes. Each set of
three scans alternated between slower scans with longer frame exposure
times and faster scans with shorter frame durations. Each set of three
scans took about five minutes to acquire. The position of the IR slit
with respect to the disk of Earth at the start of each scan and the
scan rate and direction are provided in EPOCH_EARTH_SEQ_2008.PDF and,
EPOCH_EARTH_SEQ_2009.PDF located in the document directory.
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.
EPOCH_EARTH_OBS.PDF
- This document describes of the EPOCh Earth observations
although most of the information is captured in this dataset
catalog file you are reading.
EPOCH_EARTH_SEQ_2008.PDF
EPOCH_EARTH_SEQ_2009.PDF
- These documents provide pointing and sequencing information
for the EPOCh Earth observations in 2008 and 2009, 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_3_4_EPOXI_EARTH.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.
Publications of the scientific results from the Earth observations
in this dataset include Cowan, et al. (2009) [COWANETAL2009] and
Livengood, et al. (2009) [LIVENGOODETAL2009].
Related Data Sets
-----------------
The following PDS datasets are related to this one and may be useful
for research:
DIF-E-HRII-2-EPOXI-EARTH-V1.0
- Raw HRII Earth observations
DIF-CAL-HRII-2-EPOXI-CALIBRATIONS-V1.0
- Raw HRII dark frames that bracket each set of Earth observations
in this dataset
DIF-E-HRIV-2-EPOXI-EARTH-V1.0
DIF-E-HRIV-3/4-EPOXI-EARTH-V2.0
- Raw and calibrated HRIV visible CCD Earth observations at
350, 450, 550, 650, 750, 850, and 950 nm, covering the same
observing periods as this dataset
DIF-E-MRI-2-EPOXI-EARTH-V1.0
DIF-E-MRI-3/4-EPOXI-EARTH-V2.0
- Raw and calibrated MRI visible CCD context images of Earth at
750 nm, serving as context for the IR spectra and covering
the Mar 2008, Mar 2009, Sep 2009, and Oct 2009 observing
periods
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
DI-C-HRII/HRIV/MRI/ITS-6-DOC-SET-V4.0
- Deep Impact and EPOXI documentation set including a draft of the
Deep Impact instrument calibration paper by Klaasen, et al. (2008)
[KLAASENETAL2006]
Processing
==========
The 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. 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 on the raw
HRII FITS data to produce the RADREV and RAD products found in this
dataset:
- Calibration of temperatures and voltages in the FITS header
- Decompression of compressed images (Earth spectra were not
compressed)
- Per-quadrant linearization of raw data numbers
- Subtraction of dark noise
- Removal of electronic cross-talk (a unit correction)
- Division by a flat field, derived for per-quadrant linearization
- Assignment of 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-E-HRII-2-EPOXI-EARTH-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 raw 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 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_RR.FIT and the last would be
HI08060416_1000001_032_RR.FIT.
Image Compression
-----------------
All data products in this dataset are uncompressed. Specifically
all raw Earth 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 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.
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 IR scan across Earth consists of eight frames for 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-E-MRI-3/4-EPOXI-EARTH-V2.0,
were acquired during the IR scans and may provide field of view
context for the slit location.
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].
Parameters
==========
Data Units
----------
Calibrated RADREV and RAD spectral images have units of radiance,
W/(m**2 steradian micron).
Imaging Modes
-------------
One mode was used for all images in this dataset:
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 along the slit. For more information see
Hampton, et al. (2005) [HAMPTONETAL2005], Klaasen, et al. (2008)
[KLAASENETAL2006] and Klaasen, et al. (2013) [KLAASENETAL2011].
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.
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. Most kernels are available in the EPOXI
SPICE dataset, DIF-C/E/X-SPICE-6-V1.0; others that had not yet been
archived in the PDS when this dataset was produced are available online
at the Operational Flight Project Kernels website maintained by the
NASA Navigation and Ancillary Information Facility (NAIF),
http://naif.jpl.nasa.gov/naif/data_operational.html.
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 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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