| DATA_SET_DESCRIPTION |
Data Set Overview
=================
This data set contains version 2.0 of calibrated spectral images of
comet 9P/Tempel 1 acquired by the Deep Impact High Resolution
Instrument Infrared Spectrometer (HRII) during the encounter phase of
the mission. Version 2.0 includes uncleaned and cleaned radiance data
with improved calibration and geometry. The data were collected from
20 June through 6 July 2005. Spectra taken before June 20, 2005, are
not included in this data set because the spectrometer was not
pointing at the target.
A summary of the comet observations in this data set 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 spectra were described in 'Deep Impact: The
Anticipated Flight Data' by Klaasen, et al. (2005) [KLAASENTAL2005].
For more details about the spectra taken around impact, refer 1) to
the HRII encounter data summary document which provides a log of the
exposures taken from 28 hours before impact through lookback and 2)
the HRII encounter pointing summary document which describes the
pointing and scan direction for exposures taken at encounter. These
documents are included on the Deep Impact Documentation volume.
Essential Reading
-----------------
The following documents, located on the Deep Impact Documentation
volume, DIDOC_1000, are essential for the understanding and
interpretation of this data set:
ANTICIPATED_FLIGHT_DATA.* : Anticipated flight data by Klaasen,
et al. (2005) [KLAASENTAL2005]
HRII_REDUCED_ENC_INDEX.* : Science-related index table for
this data set
HRII_ENCOUNTER_DATA_SUMMARY.* : Image log from 28 hours before
impact through lookback
HRII_ENCOUNTER_POINTING_SUMM.* : Pointing and scan directions for
encounter data from June 20 through
July 6
HRII_HRIV_CONTEXT_MAPS/* : HRII/HRIV context maps show where
the IR slit was located on the
closest HRIV CCD frame
HRII_CALIBRATION_LIMITATIONS.* : Discusses known deficiencies in
the IR calibration process that
reduced the spectral images
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
AICD_FLIGHT_HRII.* : 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:
DIF-CAL-HRII-2-9P-CRUISE-V1.0 : Raw HRII cruise calibrations
DIF-C-HRII-2-9P-ENCOUNTER-V1.0 : Raw HRII encounter data
DIF-C-HRII/HRIV/MRI-6-TEMPS-V1.0 : Instrument temperature data
DIF-CAL-HRII-2-GROUND-TV1-V1.0 : HRII pre-flight calib data
DIF-CAL-HRII/HRIV-2-GROUND-TV2-V1.0 : HRII pre-flight calib data
DIF-CAL-HRII/HRIV/MRI-2-GROUND-TV4-V1.0 : HRII pre-flight calib data
DI-C-SPICE-6-V1.0 : SPICE kernels
Processing
==========
The calibrated two-dimensional (wavelength and spatial) FITS image
spectra in this data set were generated by the Deep Impact calibration
pipeline, maintained by the project's Science Data Center (SDC) at
Cornell University. For these data (version 2.0), the method for dark
subtraction was revised which improved the calibration of the area
under the anti-saturation filter by about 10%. New, time-dependent
bad pixel maps were utilized, and the geometry was improved because
the final kernels from the Deep Impact SPICE archive were used.
The pipeline performed the following reduction and calibration steps
to produce the two types of spectral images (uncleaned and cleaned
radiance) in this data set:
- Calibration of temperatures and voltages in the FITS headers
- Decompression of compressed images
- Linearization of raw data numbers
- Subtraction of dark noise
- Removal of electronic cross-talk (a unit correction)
- Conversion of data numbers to units of radiance for an absolute,
radiometric calibration ('RADREV')
- Interpolation over bad and missing pixels for partially cleaned,
radiometric calibration ('RAD'); Cosmic rays were not removed
because the existing calibration routine was not robust
The flat-field correction was not reliable and was not applied during
the calibration process.
The uncleaned radiance data, designated by the mnemonic 'RADREV',
were provided in units of radiance as Watts/(meter**2 steradian
micron) and were considered reversible because the calibration steps
could be removed to get back to the original, raw data numbers.
The irreversibly cleaned radiance data, designated by the mnemonic
'RAD', were provided in units of radiance as Watts/(meter**2
steradian micron). Only the RADREV data were included in version
1.0 of this data set.
During the calibration process, the pipeline updated the
pixel-by-pixel image quality map, the first FITS extension,
to identify the following types of pixels:
- 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 (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. 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].
The reduced spectra were the best available data as of September
2006. There are known deficiencies for these spectra which are
discussed in the HRII calibration limitations document included
on the Deep Impact Documentation volume.
Applied Coherent Technology Corporation in Herndon, VA, produced the
PDS data labels by extracting parameters from the FITS headers.
Data
====
File Naming Convention
----------------------
The naming convention for the data labels and FITS files is
HIcccccccccc_eeeeeee_nnn_rr.LBL or FIT where cccccccccc is the
spacecraft clock count at the mid-point of the observation and
eeeeeee is the exposure ID (OBSERVATION_ID in data labels). Up to
999 individual images could 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 9009001, the first FITS
file name would be HIcccccccccc_9009001_001_RR.FIT and the last
would be HIcccccccccc_9009001_032_RR.FIT. Finally, rr identifies
the type of reduction:
RR for RADREV data (radiance units, reversible)
R for RAD data (radiance units, partially cleaned)
Spectral Images
---------------
The infrared spectral data were stored as FITS. The primary data
array contains the two-dimensional spectral image, with the fastest
varying axis corresponding to increasing wavelengths from 1.05 to
4.8 microns and the slowest varying axis corresponding to the
spatial dimension. Each FITS file includes four image extensions
that are two-dimensional pixel-by-pixel maps the provide additional
information about the spectral (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 labels define the purpose of each bit.
- The second extension provides the spectral registration or
wavelength for each pixel in the primary image. This
extension was required because the wavelength for each
pixel changed 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 was
required because the bandwidth for each pixel changed 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 label. For more
information about the FITS primary image and the extensions, refer
to the instrument calibration document included on the Deep Impact
Documentation volume.
Imaging Modes
-------------
A summary of the imaging modes is provided below. For a thorough
description of the modes, please see the Deep Impact instrument
paper by Hampton, et al. (2005) [HAMPTONETAL2005] included on the
Deep Impact Documentation volume.
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)
In this table, X-Size is the spectral dimension and Y-Size is the
spatial dimension.
Compression
-----------
All data files in this data set were uncompressed. If the
associated raw data file 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 lossy
lookup tables to decompress raw image. 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] included on the
Deep Impact Documentation volume.
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 PDS 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 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
around to lookback at the comet. For lookback images, ecliptic
North is toward the left, and the Sun is down.
Spectral Scans
--------------
Nearly all exposures of Tempel 1 during encounter were scans across
the nucleus or the coma, and these are identified in the HRII
encounter pointing summary document. To make scans, the spacecraft
was slewed while the IR detector recorded data. To work with
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.
Several exposures from encounter were stares, where the spectrometer
was pointed at a specific target location for the duration of the
exposure. Exposure IDs that were stares are also identified in the
HRII encounter pointing summary document included on the Deep Impact
Documentation volume.
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
relative boresight alignments section of the instrument calibration
document. 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 on the Deep Impact Documentation
volume.
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 was reset and read out on a
pixel-by-pixel basis, the read out order affects the time at which
each pixel was exposed, although each pixel had the same exposure
duration. For more information about the timing of the spectra, see
the IR focal plane and quadrant nomenclature sections of the
instrument calibration document.
Parameters
==========
Data Units
----------
Reduced RADREV and RAD data are in units of radiance,
W/(m**2 steradian micron).
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 be able to see an event recorded by
the instrument.
The TIME_FROM_IMPACT_VALUE keyword in the data labels was based
on the best estimate of the time of impact based on the clock
onboard the flyby spacecraft was UTC 05:44:34.265 on 4 Jul 2005.
The analysis that lead to this estimate is discussed in the
spacecraft clock correlation document included on the Deep Impact
Documentation volume.
Geometry-Related Keywords
-------------------------
The SOLAR_NORTH_POLE_CLOCK_ANGLE in the data labels specified the
the direction of ecliptic north as projected onto the image plane.
It is measured from the 'upward' direction, clockwise to the
direction toward ecliptic north when the image is displayed as
defined by the SAMPLE_DISPLAY_DIRECTION and LINE_DISPLAY_DIRECTION
keywords.
The SDC pipeline was not able to automatically determine the
proper geometric information for the target of choice in many
cases. When these parameters could not be computed, the
corresponding keywords in the PDS data labels were set to a
value of unknown (UNK). Geometry-related keywords for most
calibration targets were set to UNK.
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. After a
detailed analysis of the timing problem in early 2006, improved
self-consistent SPICE kernels were generated by the Deep Impact
project to correlate the spacecraft clocks; there is still a
1-2 second uncertainty between the in-situ data and the ground-
based observations and an uncertainty of about one half of a
second between the clocks on the flyby and impactor spacecraft.
These improved kernels were included in the DI SPICE data set
and were used to calculate the geometric parameters in the PDS
data labels. For more information about this discrepancy, please
see the spacecraft clock correlation report provided on the DI
documentation volume, DIDOC_0001.
The SPICE kernels used to calculated the geometric parameters are
provided by the SPICE_FILE_NAME keyword in the PDS data labels. The
kernels were listed in the order they were loaded into memory for
processing.
Ancillary Data
==============
Geometric parameters included in the data labels were computed using
the final version of the kernel files 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.
|
| CONFIDENCE_LEVEL_NOTE |
Confidence Level Overview
=========================
This data set, version 2.0, replaces version 1.0 that was delivered
to PDS in December 2005.
As noted above, the 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. For more information about this discrepancy, please
see the spacecraft clock correlation report included on the Deep
Impact Documentation volume.
The FITS files in this data set were reviewed internally by the Deep
Impact project and were used extensively by the science team.
Review
======
This data set was peer-reviewed in April 2007 and was accepted
for the PDS archive pending resolution of liens (completed in July
2007).
Data Coverage and Quality
=========================
There are no gaps in this data set. All raw spectral images of
Tempel 1 that were received on the ground were successfully reduced
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 the notes about image quality map in
the PDS data label or to the instrument calibration document.
Limitations
===========
Known Deficiencies for the IR Calibration
-----------------------------------------
There are known deficiencies with the version of the IR calibration
pipeline that reduced these data. The deficiencies are discussed in
the HRII calibration limitations document included on the Deep
Impact Documentation volume. Given these deficiencies, the most
reliably calibrated pixels are outside the anti-saturation filter
and between 2.0 and 4.6 microns.
HRI Telescope Focus
-------------------
Early images of stars using the HRI visible CCD indicated the HRI
telescope was out of focus. However, this focus problem did not
significantly affect the HRII instrument. For more details, please
see the instrument calibration paper by Klaasen, et al. (2006)
[KLAASENETAL2006] included on the Deep Impact Documentation volume.
Displaying Images
-----------------
Flight software overwrote the first 50 uncompressed (or 100
compressed) pixels of first quadrant read out from the instrument
with an image header. These header pixels were included in the
reduced FITS images. Since the values in these pixels vary
dramatically, it is recommended that the values of the MINIMUM and
MAXIMUM keywords in the PDS 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 reference
rows and columns located around the edge of the spectral image.
The location of the header pixels in a displayed FITS image
depends on the readout order of the instrument, as discussed in
the quadrant nomenclature section of the instrument calibration
paper included on the Deep Impact Documentation volume.
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