| DATA_SET_DESCRIPTION |
Data Set Overview
=================
This dataset contains calibrated clear-filter images of comet
C/Garradd (2009 P1) acquired by the High Resolution Visible CCD (HRIV)
from 20 February through 09 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). Observations were obtained with the HRIV CCD and
the Medium Resolution Visible CCD (MRI, archived separately) during a
series of windows from 20 February through 09 April 2012 as the comet
receded from 1.74 AU to 2.11 AU from the Sun and at a distance of
1.88 AU to 1.30 AU from the spacecraft.
Hourly sequences were obtained between 20 February and 07 March and
again between 06-09 April with a few gaps for spacecraft maneuvers and
data downloads. During these windows, HRIV sequences consisted of a
single Clear image, while each MRI sequence consisted of 13 broad- and
narrowband images: 3 Clear, 3 CN, 2 C2, 2 OH and 2 Green Continuum.
Images were unbinned 256x256-pixel subframe sections with the comet
near the center of the frame.
Modified visible imaging sequences were interspersed with the 1.05- to
4.8-micron infrared spectral imaging scans (archived separately)
obtained March 26-27 and April 2-3. During these windows, 3 HRIV Clear
images were obtained every hour, while the MRI sequences consisted of 1
Clear and 1 CN image sampled every 15 minutes.
Initial results based on the visible-wavelength imaging are presented
in Farnham, et al. (2014) [FARNHAMETAL2014]. See the 'Related Data
Sets' section below for a list of separately archived visible- and
infrared-wavelength data.
In the course of the observations of comet Garradd, a massive solar
coronal ejection event occurred on 06 March 2012. The effects of this
event were seen in the data obtained on 07 March, with cosmic rays
rapidly increasing until they eventually overwhelmed the detector.
This was the last day of the collection period, so the images had
returned to normal by the next collection period on 25 March 2012.
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.
HRIV_3_4_EPOXI_GARRADD.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-HRIV-2-EPOXI-GARRADD-V1.0
- Raw 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-HRII-2-EPOXI-GARRADD-V1.0
DIF-C-HRII-3/4-EPOXI-GARRADD-V1.0
- Raw and calibrated HRII infrared spectral images of comet Garradd
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 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 final version of the pipeline for HRIV processing,
dated January 2013, was used. (This version is identical to the one
used in June 2011 to process comet Hartley 2 data.) 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 each CCD image, 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. A RADREV image can
be converted to unitless I-over-F by multiplying by the value
assigned to the DATA_TO_IOVERF_MULTIPLIER keyword in the PDS
label. Alternatively, a RADREV image can be converted from
radiance units to calibrated data numbers by multiplying by the
value assigned to the DATA_TO_DN_MULTIPLIER in the PDS label.
- 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. A RAD image can be converted
to unitless I-over-F by multiplying by the value assigned to
the DATA_TO_IOVERF_MULTIPLIER keyword in the PDS label.
Alternatively, a RAD image can be converted from radiance units to
calibrated data numbers by multiplying by the value assigned to
the DATA_TO_DN_MULTIPLIER in the PDS label (though interpolated
pixels will not be real data). Please note that values in the
overclock rows and columns bordering the active CCD area are
set to 0 in the RAD product.
The calibration pipeline performed the following processes, in the
order listed, on the raw HRIV FITS data to produce the RADREV and
RAD products found in this data set (the process uses the image
mode and filter to select the appropriate set of calibration files):
- Decompression of compressed images (all images in this dataset
were never compressed, so this step was bypassed)
- Correction for bias
- Subtraction of a dark frame
- Removal of horizontal, instrumental striping
- Removal of electronic cross-talk
- Application of a normalized flat field
- Removal of CCD transfer smear
- Conversion of data numbers to units of radiance for an absolute,
radiometric calibration that is reversible (RADREV)
- 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
- Calculation of multiplicative factors to convert a RADREV or RAD
image to I-over-F
- The RAD stream has a potential step for deconvolving HRIV images
to correct for the out-of-focus condition for the HRI telescope
but this step was *not* performed
As part of the calibration process, the pipeline updated the
pixel-by-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 (missing pixels were identified when the raw FITS files
were created).
The pipeline also created a FITS image extension to capture the
signal-to-noise ratio map and another extension to capture the values
used to remove horizontal striping. 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 image is stored as FITS. The primary data unit
contains the two-dimensional CCD image which is followed by two
image extensions that are two-dimensional pixel-by-pixel maps
providing additional information about the CCD 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 a signal-to-noise ratio for
each pixel in the primary image.
- The third extension contains the two columns of DN values that
were subtracted from every non-overclock column in the left
and right halves of the primary image array by the stripe
removal process.
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
HVyymmddhh_eeeeeee_nnn_rr.LBL or FIT where 'HV' identifies the HRIV
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 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 6
frames were commanded for a scan with an exposure ID of 4010000, the
first FITS file name would be HV12022019_4010000_001_RR.FIT and the
last would be HV12022019_4010000_006_RR.FIT.
Image Compression
-----------------
All data products in this dataset are uncompressed. Specifically
all raw CCD 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.
Instrument Alignment
--------------------
For a comparison of the field of view and the relative boresight
alignment of HRIV to the Medium Resolution Instrument Visible CCD
(MRI) 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
----------
The calibrated RADREV and RAD 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
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
-------
One HRIV filter was used for all images in this dataset:
Filter Center Width
# Name (nm) (nm) Comments
- ---------- ----- ----- -------------------------------
1 CLEAR1 650 >700 Not band limited
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 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.
|
| 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/.
HRI Telescope Focus
-------------------
Images of stars acquired early during the Deep Impact mission in
2005 indicated the HRI telescope was out of focus. In-flight
bakeouts during late February and early March 2005 reduced the
defocus from about 1.0 cm to about 0.6 cm, resulting in a decrease
in the width of stars from about 12 pixels to 9 pixels. For more
detail, please see the Deep Impact instrument calibration paper by
Klaasen, et al. (2008) [KLAASENETAL2006], the Deep Impact image
restoration paper by Lindler, et al. (2007) [LINDLERETAL2007], and
the EPOXI Comet Hartley 2 image restoration paper by Lindler, et al.
(2013) [LINDLERETAL2013].
Please note the PSF files included in this dataset are the initial
set produced by the science team for inclusion in the calibration
pipeline; the PSFs are very non-circular due the focus problem, and
the pixelization can lead to offsets of the center in one direction
or another. Deconvolution with a PSF can be very difficult and one
should fully understand the nature of the PSFs and the nature of
the instrument before trying to use any PSF. For more information,
see Barry, et al., (2010) [BARRYETAL2010], Lindler, et al. (2007)
[LINDLERETAL2007], and Lindler, et al. (2013) [LINDLERETAL2013].
The HRIV images in this dataset were not deconvolved.
CCD Horizontal Gap
------------------
Calibration analysis combining Deep Impact and early EPOXI data
determined the two halves of the HRIV 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 calibrated
FITS images. Since the values in these pixels vary dramatically,
it is recommended that the values of the EPOXI:MINIMUM and EPOXI: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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