| DATA_SET_DESCRIPTION |
Data Set Overview
=================
This data set contains reduced images of comet 9P/Tempel 1 acquired by
the Deep Impact Impactor Target Sensor Visible CCD (ITS) after the
impactor was released from the flyby spacecraft on July 3, 2005 UT.
The 9P/Tempel 1 data are described in 'Deep Impact: The Anticipated
Flight Data' by Klaasen, et al. (2005) [KLAASENTAL2005]. For more
details about the images taken around impact, refer to the ITS
encounter data summary document which provides a log of the exposures
beginning about 22 hours before impact.
A list of the comet observations in this data set is provided here:
Mid-Obs Exposure IDs
Date DOY Minimum Maximum Mission Activity
---------- --- ------- ------- ---------------------------------
2005-07-03 184 9000026 9000043 Release, Continuous Comet Imaging
2005-07-03 184 9000060 9000128 Continuous Comet Imaging
2005-07-04 185 9000130 9000710 Continuous Comet Imaging, Impact
Essential Reading
-----------------
The following documents, located on the Deep Impact Documentation
volume, DIDOC_0001, are essential for the understanding and
interpretation of this data set:
ANTICIPATED_FLIGHT_DATA.* : Anticipated flight data by Klaasen,
et al. (2005) [KLAASENTAL2005]
ITS_ENCOUNTER_DATA_SUMMARY.* : Image log beginning about 22 hours
before impact
INFLIGHT_CALIBRATION_SUMMARY.* : Summary of in-flight calibrations
CALIBRATION_DOC.* : Instrument calibration by Klaasen,
et al. (2006) [KLAASENETAL2006]
INSTRUMENTS_HAMPTON.* : Instrument paper by Hampton, et al.
(2005) [HAMPTONETAL2005]
MISSION_OVERVIEW_AHEARN.* : Mission overview by A'Hearn, et al.
(2005) [AHEARNETAL2005B]
SCLK_CORRELATION.* : Discussion of the discrepancy
between the spacecraft clocks and
UTC
AIDC_FLIGHT_HRIV_MRI_ITS.* : Description of the data set and
definitions of label keywords
Initial results from the encounter and impact were presented in
'Deep Impact: Excavating Comet Tempel 1' by by A'Hearn, et al.
(2005) [AHEARNETAL2005A].
Related Data Sets
-----------------
The following PDS data sets are related to this one:
DII-CAL-ITS-2-9P-CRUISE-V1.0 : Raw ITS cruise calibrations
DII-C-ITS-2-9P-ENCOUNTER-V1.0 : Raw ITS encounter data
DII-CAL-ITS-2-GROUND-TV3-V1.0 : ITS ground calibrations
DI-C-SPICE-6-V1.0 : SPICE kernels
Processing
==========
The reduced, two-dimensional FITS images in this data set were
generated by the Deep Impact calibration pipeline, maintained by the
project's Science Data Center (SDC) at Cornell University. The
pipeline performed the following reduction steps to produce the
images in this data set:
- Calibration of temperatures and voltages in the FITS headers
- Decompression of compressed images
- Correction for uneven bit weighting due to analog-to-digital
(ADC) conversion (a unit correction)
- Subtraction of a dark frame
- Removal of electronic cross-talk removal
- Application of a flat field
- Normalization of quadrant gains
- Removal of CCD transfer smear
- Conversion of data numbers to units of radiance for an absolute,
radiometric calibration
The resulting data were provided in physical units of radiance,
Watts/(meter**2 steradian micron). These data, designated by the
pneumonic ''RADREV'', were not cleaned and are considered reversible
because the calibration steps can be removed to get back to the
original, raw data numbers.
During the calibration process, the pipeline updated the
pixel-by-pixel image quality map, the first FITS extension, so the
following types of pixels could be identified:
- 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
- Pixels considered bad as indicated by bad pixel maps
The pipeline also created a FITS image extension for a signal-to-noise
ratio map. The calibration steps and files used to reduce each raw
image are listed in the PROCESSING_HISTORY_TEXT keyword in the PDS
data label for that image. For a detailed discussion of the
calibration pipeline and the resulting data, see the instrument
calibration document by Klaasen, et al. (2006) [KLAASENETAL2006].
Applied Coherent Technology Corporation in Herndon, VA, produced the
PDS data labels by extracting parameters from the FITS headers.
Data
====
CCD Images
----------
The two-dimensional, CCD images in this data set are in FITS format.
The primary data array contains the image, followed by two image
extensions that are pixel-by-pixel maps which provide additional
information about the primary 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 a signal-to-noise ratio for
each pixel in the primary image.
Each image FITS file is accompanied by a detached PDS label. For
more information about the FITS primary image and extensions, refer
to the instrument calibration document.
Compression
-----------
The calibration pipeline used one of four lossy lookup tables to
decompress raw images that were compressed onboard the spacecraft.
For information about data compression, see the Deep Impact
instruments document by Hampton, et al. (2005) [HAMPTONETAL2005] or
the instrument calibration paper by Klaasen, et al. (2006)
[KLAASENETAL2006].
True-Sky ``As Seen By Observer'' Display
----------------------------------------
A true-sky 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 increasing to the top. This convention is also
defined in the data labels:
SAMPLE_DISPLAY_DIRECTION = 'RIGHT'
LINE_DISPLAY_DIRECTION = 'UP'
The direction to Celestial North and Ecliptic North, measured
clockwise from the top of the displayed image, is provided in PDS
labels by CELESTIAL_NORTH_CLOCK_ANGLE and
SOLAR_NORTH_POLE_CLOCK_ANGLE, both of which assume the correct
display defined by SAMPLE_DISPLAY_DIRECTION and
LINE_DISPLAY_DIRECTION.
Using this convention for Tempel 1 approach images, ecliptic East is
toward the left, ecliptic North is up, and the Sun is to the right.
This was also the convention used then displaying ITS images of
Tempel 1 in published papers.
For a comparison of the orientation of ITS flight images with those
from ground-based calibrations as well as those from the High and
Medium Resolution Instrument CCDs (HRIV and MRI, respectively), see
the quadrant nomenclature section of the 'Deep Impact Instrument
Calibration' document.
File Naming Convention
----------------------
The naming convention for the image labels and FITS files is
IVcccccccccc_iiiiiii_nnn_RR.LBL or FIT where cccccccccc is the
spacecraft clock count at the mid-point of the observation, eeeeeee
is the exposure ID (OBSERVATION_ID in data labels). Up to 999
individual images could be commanded for one exposure ID.
Therefore, nnn in the file name provides the sequentially increasing
image number within an exposure ID and corresponds to IMAGE_NUMBER
in the data labels. For example, if two images were commanded for
exposure ID 9009001, the two FITS files names would be
IVcccccccccc_9009001_001_RR.FIT and IVcccccccccc_9009001_002_RR.FIT.
Finally, RR identifies the reduction level, RADREV: reversible data
in units of radiance (RADREV). This reduction level is described
in the instrument calibration document.
Parameters
==========
Data Units
----------
Reduced RADREV data are in units of radiance, W/(m**2 steradian
micron). The data are reversible and are not cleaned.
Imaging Modes
------------
For a description of the imaging modes, please see the Deep Impact
instrument paper by Hampton, et al. (2005) [HAMPTONETAL2005].
Most image modes had 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.
Time-Related Keywords
---------------------
All time-related keywords in the data labels, except
EARTH_RECEIVED_TIME, are based on the clock on board the flyby
spacecraft. EARTH_RECEIVED_TIME provides the UTC when an
Earth-based observer should have been able to see an event recorded
by the instrument.
The TIME_FROM_IMPACT_VALUE keyword in the data labels was based on
the estimated impact time at the flyby spacecraft of UTC
05:44:35.821 on 4 Jul 2005, as published by A'Hearn, et al. (2005)
[AHEARNETAL2005A].
Geometry-Related Keywords
-------------------------
The SDC attempted to calculate geometric parameters relating to the
target only when it was within several fields of view of the
instrument boresight. When these parameters could not be
calculated, the corresponding keywords in the PDS data labels were
set to a value of unknown (UNK).
Geometric parameters were computed using the best SPICE kernels
available at the time of calibration and the results were stored in
the FITS header. For impact images, geometric parameters such as
the target-to-spacecraft distance were computed with respect to the
impact site, not the center of the comet.
Geometric parameters provided in the data labels were computed at
the epoch specified by MID_IMAGE_TIME, except for the target-to-sun
and earth-observer-to-target parameters. Target-to-sun values were
calculated for the time when the light left the sun while
earth-observer-to-target were calculated for the time when the light
left the target.
Geometry-related parameters in the PDS data labels are uncertain at
a level of a few seconds because of a known 2-second discrepancy
between the clocks on board the flyby and impactor spacecraft and
between in-situ data and ground-based observations. The Deep Impact
project generated self-consistent SPICE kernels, outside of NAIF, to
help account for the discrepancy. The November 2005 version of
these kernels was used to calculate the image times and geometric
information in PDS data labels. After further analysis of the
timing problem in early 2006, improved self-consistent SPICE kernels
were generated by the Deep Impact project. The improved kernels
were included in the DI SPICE data set. For more information about
this discrepancy, please see the spacecraft clock correlation
document included on the Deep Impact document volume.
The SPICE kernels used to calculated the geometric parameters are
provided by the SPICE_FILE_NAME keyword in the PDS data labels. The
kernels are listed in the order they were loaded into memory for
processing.
Ancillary Data
==============
Geometric parameters included in the data labels were computed using
the November 2005 version of the SPICE data. These were the best
available data at the time of image creation. Since image creation,
some SPICE kernels were improved, and these were archived in the Deep
Impact SPICE data set.
Coordinate System
=================
Earth Mean Equator and Vernal Equinox of J2000 (EME J2000) was the
inertial reference system used to specify observational geometry
parameters in the data labels.
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