| DATA_SET_DESCRIPTION |
Data Set Overview
=================
This dataset contains raw clear-filter, CN, OH and dust
continuum images of comet C/ISON (2012 S1) acquired by the Medium
Resolution Visible CCD (MRI) from 17 January through 06 March 2013
during the Cruise 3 phase of the EPOXI mission.
While the 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 and rotational
variability. Observations were obtained with the MRI CCD during a
series of windows from 17 January through 06 March as the comet
advanced from 5.09 AU to 4.55 AU from the Sun and at a distance of
5.28 AU to 3.96 AU from the spacecraft.
Hourly sequences were obtained between 17 January and 06 March with a
few gaps for spacecraft maneuvers and data downloads. During these
windows, the hourly MRI sequences consisted of 4 sets of broad- and
narrowband images: 2 sets each of 3 Clear, 1 CN, and 1 Green Continuum
followed by 2 sets each of 3 Clear and 1 OH (total of 18 images per
hourly sequence). Images were unbinned 256x256-pixel subframe
sections with the comet near the center of the frame.
A modified MRI imaging sequence was interspersed with the 1.05- to
4.8-micron infrared spectral imaging scans (archived separately; see
the 'Related Data Sets' section below) obtained 16-17 February.
During this window, the MRI sequence consisted of 1 Clear image
sampled every 15 minutes.
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 generated
the 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.
MRI_2_EPOXI_ISON.TAB
- This ASCII table provides image parameters such as the mid-obs
Julian date, exposure time, image mode, filter, 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-MRI-3/4-EPOXI-ISON-V1.0
- Calibrated MRI medium-resolution CCD images comet ISON,
including context images for the IR scans
DIF-C-HRII-2-EPOXI-ISON-V1.0
DIF-C-HRII-3/4-EPOXI-ISON-V1.0
- Raw and calibrated HRII infrared spectral images of comet ISON
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 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 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 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 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 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 the raw data labels and FITS files is
MVyymmddhh_eeeeeee_nnn.LBL or FIT where 'MV' identifies the MRI
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 3
frames were commanded for a scan with an exposure ID of 4010060, the
first FITS file name would be MV13011702_4010060_001.FIT and the
last would be MV13011702_4010060_003.FIT.
Image Compression
-----------------
All data products in this dataset are uncompressed. Specifically
all raw CCD images 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.
Instrument Alignment
--------------------
For a comparison of the field of view and the relative boresight
alignment of MRI to the High Resolution Instrument Visible CCD
(HRIV) and the slit of the High Resolution IR Imaging Spectrometer
(HRII), see the instrument alignment section of the EPOXI SIS
document or Klaasen, et al. (2011) [KLAASENETAL2011].
Parameters
==========
Data Units
----------
Raw image data are in units of raw data numbers.
Imaging Modes
-------------
One mode was used for all images in this dataset:
X-Size Y-Size
Mode Name (pix) (pix) Comments
---- ------ ------ ------ ----------------------
3 SF2S 256 256 Sub-frame, shuttered
For more information see Hampton, et al. (2005) [HAMPTONETAL2005],
Klaasen, et al. (2008) [KLAASENETAL2006] and Klaasen, et al. (2013)
[KLAASENETAL2011]. All modes are unbinned.
Filters
-------
Several MRI filters were used for the images in this dataset:
Filter Center Width
# Name (nm) (nm) Comments
- ---------- ----- ----- -------------------------------
1 CLEAR1 650 >700 For context; not band limited
3 GREEN_CONT 526 5.6 For dust in coma
6 CLEAR6 650 >700 For context; not band limited
7 CN 387 6.2 For CN in coma
9 OH 309 6.2 For OH in coma
For more information about the filters, see Hampton, et al. (2005)
[HAMPTONETAL2005], Klaasen, et al. (2008) [KLAASENETAL2006] and
Klaasen, et al. (2013) [KLAASENETAL2011]. For the effective center
wavelengths and the corresponding full-width-half-max values see
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.
|
| CONFIDENCE_LEVEL_NOTE |
Confidence Level Overview
=========================
The data files in this dataset were reviewed internally by the EPOXI
project.
Review
======
This dataset was peer reviewed and certified for scientific use on
21 March 2014.
Data Coverage and Quality
=========================
There are no unexpected gaps in this dataset. All observations
received on the ground were processed and included in this dataset.
Any 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 EPOXI SIS document.
Limitations
===========
Timing
------
The EPOXI project plans to generate a complete and highly accurate
set of UTC 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 datasets for
the two missions and posted on the NAIF/SPICE web site at
http://naif.jpl.nasa.gov/naif/.
Predict Geometry
----------------
Many ISON frames have predicted geometry because the observation
times fell within gaps of the relevant reconstructed pointing kernel,
DIF_SC_110301_130627_V2.BC.
CCD Horizontal Gap
------------------
Calibration analysis combining Deep Impact and early EPOXI data
determined the two halves of the MRI CCD - the boundary being the
two horizontal central lines 511 and 512 (zero based) - while
physically consistent across the boundary, are 1/6 of a pixel
smaller vertically than a normal row. Therefore, reconstructed
images, which have uniform row spacing, have a 1/3-pixel extension
introduced at the center of the array. Thus for two features on
either side of the midpoint line, the vertical component of the
actual angular separation between those features is one-third of a
pixel less than their measured difference in vertical pixels in the
image. As for all geometric distortions, correction of this
distortion will require resampling of the image and an attendant
loss in spatial resolution. The standard pipeline process does
not perform this correction so as to preserve the best spatial
resolution.
The two 1/6-pixel narrower central rows collect only 5/6 of the
charge of a normal row. This effect is corrected by the flat-field
division for calibrated science images so that the pixels in these
rows have the correct scene radiance assigned to them. However,
point-source or disk-integrated photometric measurements using
aperture photometry areas that include these central rows will be
slightly distorted unless special adjustments are made. For
example, the aperture photometry process for comet 9P/Tempel 1 added
an extra 1/6-pixel worth of signal to the to the pixels in each of
these two rows in the reconstructed, calibrated images as described
in Appendix A of Belton, et al., (2011) [BELTONETAL2011].
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 EPOXI:DERIVED_MINIMUM and
EPOXI:DERIVED_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 EPOXI SIS
document.
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