| DATA_SET_DESCRIPTION |
Data Set Overview
=================
This data set set contains version 1.0 of raw narrow band filter
images (350-950 nm) of Earth acquired by the Deep Impact High
Resolution Visible CCD 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. HRIV images were acquired once per hour with the filters
centered on 350, 750 and 950 nm, whereas the 450-, 550-, 650-, and
850-nm data were taken every 15 minutes. During the observing period
in May, the Moon transited across Earth as seen from the spacecraft.
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.
HRIV_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-HRIV-3/4-EPOXI-EARTH-V1.0
- Calibrated HRIV Earth observations
DIF-CAL-HRIV-2-EPOXI-CALIBRATIONS-V1.0
- Raw HRIV dark frames taken at the end of each set of Earth
observations in this data set
DIF-E-HRII-2-EPOXI-EARTH-V1.0
DIF-E-HRII-3/4-EPOXI-EARTH-V1.0
- Raw and calibrated 1.05- to 4.8-micron HRI IR spectra of Earth,
covering the same observing period 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 FITS CCD 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 HRIV image is stored as FITS. The primary data unit contains
the two-dimensional CCD image. It 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
HVyymmddhh_eeeeeee_nnn.LBL or FIT where 'HV' identifies the HRIV
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 or frames can be commanded for one
exposure ID. 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 HV08060416_1000001_001.FIT and the
last would be HV08060416_1000001_032.FIT.
Image Compression
-----------------
Although raw data numbers for HRIV 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 images 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) increasing to the right in the display
window and the slowest-varying axis (lines)
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.
Instrument Alignment
--------------------
For a comparison of the field of view and the relative boresight
alignment of HRIV to the Medium Resolution Instrument Visible CCD
(MRI) and the slit of the High Resolution IR Imaging Spectrometer
(HRII), see the relative boresight alignments section of the Deep
Impact instrument calibration document.
Parameters
==========
Data Units
----------
Raw image data have units of raw data numbers.
Imaging Modes
-------------
One HRIV image mode was used for all Earth observations:
X-Size Y-Size
Mode Name (pix) (pix) Comments
---- ------ ------ ------ ---------------------------------------
2 SF1 512 512 Sub-frame, shuttered
All modes are unbinned. 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.
Most image modes have a set of bias overclock rows and columns,
located around the edges of the image array. All overclock pixels
were excluded from the calculation of the values for MINIMUM,
MAXIMUM, MEDIAN, and STANDARD_DEVIATION in the data labels. These
overclock areas described in the Deep Impact instruments document
and the Deep Impact instrument calibration document.
Filters
-------
A list of the characteristics of the HRIV filters used for the Earth
observations is provided below. For more information about the
filters, see the Deep Impact instruments document or the Deep Impact
instrument calibration document. Also the EPOXI SIS has a brief
discussion of this topic.
Filter Center Width
# Name (nm) (nm) Comments
- ---------- ----- ----- -------------------------------
2 BLUE 450 100
3 GREEN 550 100
4 VIOLET 350 100 Shortpass coating
5 IR 950 100 Longpass
7 RED 750 100
8 NIR 850 100
9 ORANGE 650 100
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.
|
| CONFIDENCE_LEVEL_NOTE |
Confidence Level Overview
=========================
The data files in this data set were reviewed internally by the EPOXI
project.
Review
======
This data set is archived at the PDS Small Bodies Node (SBN) and the
Multi-Mission Archive at STScI (MAST). It passed a peer review held
by SBN on 23 July 2009; MAST personnel participated.
Data Coverage and Quality
=========================
There are no unexpected gaps in this data set. All Earth observations
received on the ground were processed and included in this data set.
Horizontal striping through some images indicates missing data. The
image quality map extension identifies where pixels are missing. If
the second most-significant bit of a pixel in the image quality map is
turned on, then data for the corresponding image pixel is missing. For
more information, refer to EPOXI SIS document.
Limitations
===========
Predicted observational geometry
--------------------------------
Some data products for the Earth observation on 28-29 May
(2008/149-150) have geometry values based on predicted (estimated)
pointing C-kernels found in the EPOXI SPICE archive. Memory
limitations on board the spacecraft caused some attitude information
to be overwritten, and thus reconstructed (final and accurate)
pointing data for part of this observing period were not generated
nor available to the data pipeline . However the EPOCh team
provided a file of geometry from JPL Horizons that they used for
analysis of the May Earth data. See EPOCH_EARTH_GEOM_2008MAY.ASC
located in the DOCUMENT/ directory of this data set.
HRI Telescope Focus
-------------------
Images of stars acquired early during the Deep Impact mission in
2005 indicated the HRI telescope was out of focus. In-flight
bakeouts during late February and early March 2005 reduced the
defocus from about 1.0 cm to about 0.6 cm, resulting in a decrease
in the width of stars from about 12 pixels to 9 pixels. For more
details, please see the Deep Impact instrument calibration paper by
Klaasen, et al. (2006) [KLAASENETAL2006] and the Deep Impact image
restoration paper by Lindler, et al. (2007) [LINDLERETAL2007].
CCD Horizontal Gap
------------------
Calibration analysis combining Deep Impact and early EPOXI data
determined the two halves of the HRIV CCD - the boundary being the
two horizontal central lines 511 and 512 (zero based) - while
physically consistent across the boundary, are biased during
integration so that the centers of the two halves are apparently
1/6 pixel closer to the center, and the two boundary rows show a
decrease in sensitivity of 1/6. Reconstructed image files space
all lines evenly, so the true image is erroneously vertically
pushed apart by 1/3 pixel at its center in these reconstructions.
When making science measurements from HRIV images, one must
therefore be very careful to properly account for the two flaws
introduced by the apparently narrow central lines on the CCD - a
geometric error that separates the image by an extra 1/3 pixel at
the horizontal quadrant boundary, and 2) insertion of extra total
radiance into calibrated images due to the flat-field correction,
which corrects for an apparent radiance deficit in the two central
rows because of the smaller number of photons actually incident on
those rows.
Displaying Images
-----------------
Flight software writes an image header over the first 100 bytes of
quadrant A. These image header pixels were included in the raw
FITS images. Since the values in these pixels vary dramatically,
it is recommended that the values of the MINIMUM and MAXIMUM
keywords in the data label (or the MINPVAL and MAXPVAL in the FITS
header) be used to scale an image for display because these values
exclude the header bytes as well as the overclock rows and columns
located around the edge of the CCD image. For more information,
see the quadrant nomenclature section of the Deep Impact instrument
calibration document or the EPOXI SIS document.
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