| DATA_SET_DESCRIPTION |
Data Set Overview
=================
This dataset contains raw, 1.05- to 4.8-micron spectral images
of comet C/Garradd (2009 P1) acquired by the High Resolution Infrared
Spectrometer on 26 March and 02-03 April 2012 during the Cruise 3 phase
of the EPOXI mission.
While DI Flyby spacecraft (DIF) was officially in hibernation after
the encounter with comet 103P/Hartley 2 in November 2010, it continued
to carry out observations of comets from a distance as the opportunity
arises. One such observing program was carried out in 2012 on comet
C/Garradd (2009 P1). The infrared spectra were obtained in two
separate observing sequences in the observing window (set by solar
elongation as seen from the spacecraft) on 26 March and 02-03 April
2012, when the comet was at approximately 2 AU from the sun outbound
and 1.4 AU from the spacecraft and at a phase angle of about 35
degrees. All observations consisted of spatial scans perpendicular to
the length of the slit (in order to ensure that the comet was imaged,
allowing for pointing uncertainties) consisting of 17 frames, obtained
at a scan rate of 2 slit-widths per frame with an integration time of
12 seconds per frame. Each frame is a single, long-slit spectrum
averaging over an effective area of 10x20 microradians. Each sequence
consisted of 64 scans, of 17 binned full frames (512x256-pixels) each.
The scans were taken at 15-minute intervals, beginning at approximately
07:34 UTC on 26 March 2012 and 08:19 UTC on 02 April 2012.
Visible-wavelength imaging was also obtained for context information
during these observing sessions, but those data are archived with all
the other Visible-wavelength imaging data. Initial results based on
these spectral data, including a detailed description of the scanning
technique, are presented in Feaga, et al. (2014) [FEAGAETAL2014].
Visible-wavelength imaging was also obtained for context information
during these observing sessions, but those data are archived with
all the other visible-wavelength observations. See the 'Related Data
Sets' section below.
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 processed
the raw 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.
HRII_2_EPOXI_GARRADD.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.
HRII_GARRADD_DATA_ARTIFACT.ASC
- This ASCII report briefly describes an artifact found in the
comet Garradd IR spectra on DOY 093, 2 April 2012, due to burst
noise on the sensor.
Related Data Sets
-----------------
The following PDS datasets are related to this one and may be useful
for research:
DIF-C-HRII-3/4-EPOXI-GARRADD-V1.0
- Calibrated HRII infrared spectral images of comet Garradd
DIF-C-HRIV-2-EPOXI-GARRADD-V1.0
DIF-C-HRIV-3/4-EPOXI-GARRADD-V1.0
- Raw and calibrated HRIV high-resolution CCD images comet Garradd
DIF-C-MRI-2-EPOXI-GARRADD-V1.0
DIF-C-MRI-3/4-EPOXI-GARRADD-V1.0
- Raw and calibrated MRI medium-resolution CCD images comet Garradd,
including context images for the IR scans
DIF-C/E/X-SPICE-6-V1.0
- EPOXI SPICE kernels
DIF-CAL-HRII/HRIV/MRI-6-EPOXI-TEMPS-V3.0
- 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.
N.B. The pipeline does not use these thermal data to calibrate
IR spectra of the target. Instead it uses instrument temperatures
recorded in the FITS headers.
DI-C-HRII/HRIV/MRI/ITS-6-DOC-SET-V4.0
- Deep Impact and EPOXI documentation set
Processing
==========
The raw two-dimensional (wavelength and spatial/along-slit) FITS
spectral images and PDS labels in this dataset 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
EPOXI SIS document. 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 consisting
of eight, single-bit flags to indicate the quality of each pixel in
the primary image. The data label provides a short description of
each single-bit flag. 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.
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 extension. 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 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), 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.
For a spectral scan, many frames are acquired 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 17 frames were
commanded for a scan with an exposure ID of 4000001, the first FITS
file name would be HI12032607_4000001_001.FIT and the last would be
HI12032607_4000001_017.FIT.
Image Compression
-----------------
All data products in this dataset are uncompressed. Specifically
all raw spectral images for this target were never compressed
on board the spacecraft.
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 HRII scan in this dataset 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 wavelength maps are provided as the second FITS
extension in the corresponding calibrated data products only.)
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].
In many cases, nearly simultaneous MRI images, located in the
dataset DIF-C-MRI-2-EPOXI-GARRADD-V1.0, were acquired with
each IR scan and should 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
----------
Raw spectral image data have units of raw data numbers.
Imaging Modes
-------------
One mode was used for all images in this dataset:
X-Size Y-Size Bin
Mode Name (pix) (pix) Type Comments
---- ------ ------ ----- ----- -------------------
1 BINFF 512 256 2x2 Binned full 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.
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.
Since the pole of comet Garradd is not well known, the pipeline
used the default SPICE kernel, GARRADD_0001.TPC, which specifies
a non-rotating body with the positive pole aligned with EMEJ2000.
Ancillary Data
==============
The timing and 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.
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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