| DATA_SET_DESCRIPTION |
Data Set Overview
=================
This dataset contains calibrated, 1.05- to 4.8-micron spectral images
of comet C/ISON (2012 S1) acquired by the High Resolution Infrared
Spectrometer on 16-17 February 2013 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
arose. One such observing program was carried out in 2013 on comet
C/ISON (2012 S1) to monitor for outbursts. The infrared spectra were
obtained in a single observing sequence in the observing window (set
by solar elongation as seen from the spacecraft) on 16-17 February
2103, when the comet was approximately 4.75 AU from the Sun inbound,
4.43 AU from the spacecraft, and at a phase angle of about 12.6
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
11 seconds per frame. Each frame is a single, long-slit spectrum
averaging over an effective area of 10x20 microradians. (The same
scanning technique was used to observe comet C/Garradd (2009 P1) and
is described in detail by Feaga, et al. (2014) [FEAGAETAL2014]).
The sequence, which began at approximately 08:31 UTC on 16 February
and continued for about 37.5 hours, consisted of 150 scans of 17
binned full frames (512x256-pixels). Visible-wavelength imaging was
also obtained for context information during this observing session,
but those data are archived separately. 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_3_4_EPOXI_ISON.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.
Related Data Sets
-----------------
The following PDS datasets are related to this one and may be useful
for research:
DIF-C-HRII-2-EPOXI-ISON-V1.0
- Raw HRII infrared spectral images of comet ISON
DIF-C-MRI-2-EPOXI-ISON-V1.0
DIF-C-MRI-3/4-EPOXI-ISON-V1.0
- Raw and calibrated MRI medium-resolution CCD images comet ISON,
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 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. The final version of
the pipeline for HRII processing, dated January 2013, was used. Known
limitations and deficiencies of the pipeline and the resulting data are
discussed in the EPOXI Calibration Pipeline Summary document in this
dataset and by Klaasen, et al. (2013) [KLAASENETAL2011].
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, in the
order listed, on the raw HRII FITS data to produce the RADREV and
RAD products found in this dataset (the pipeline uses the image
mode to select the appropriate set of calibration files):
- Calibration of temperatures and voltages in the FITS header
- Per-pixel linearization of raw data numbers
- Subtraction of dark noise, derived for per-pixel linearization
(the prisms/spectral imaging module and IR focal plane array
temperatures, OPTBENT and IRFPAT in the FITS header, are used
for scaling if dark modeling is required)
- Division by a flat field, derived for per-pixel linearization
- Determine 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-C-HRII-2-EPOXI-ISON-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 calibrated 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.
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 HI13021608_4000001_001_RR.FIT and the last would
be HI13021608_4000001_017_RR.FIT.
Image Compression
-----------------
All data products in this dataset are uncompressed. Specifically
all raw spectral images, from which these data products are derived,
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 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].
In many cases, nearly simultaneous MRI images, located in the
dataset DIF-C-MRI-2-EPOXI-ISON-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
----------
The calibrated RADREV and RAD spectral image data 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
---- ------ ------ ----- ----- -------------------
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 ISON is not well known, the pipeline
used the default SPICE kernel, ISON_0000.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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