| DATA_SET_DESCRIPTION |
Data Set Overview
=================
This data set contains calibrated narrow band filter images
(350-950 nm) images of Mars acquired by the Deep Impact High
Resolution Visible CCD (HRIV) for the EPOCh project during the second
cruise phase of the EPOXI mission. One set of observations was
acquired on 20-21 November 2009 to characterize Mars as an analog for
extrasolar planets. The observing period lasted approximately 24
hours. HRIV images were acquired once per hour with the filters
centered on 350, 750 and 950 nm, whereas the 450-, 550-, 650-, and
850-nm data were taken every 15 minutes. ersion 2.0 includes the
application of a horizontal destriping process, revised electronic
crosstalk calibration files, and a corrected spacecraft clock
algorithm to remove a known systematic error at the subsecond level
in the conversion of the spacecraft times to UTCs.
Required Reading
---------------
The documents detailed below are essential for the understanding and
interpretation of this data set. Although a copy of each document is
provided in the DOCUMENT/ directory of this data set, 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 the data set, 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.
EPOCH_MARS_OBS.PDF
- This document describes of the EPOCh Mars observations
although most of the information is captured in this data set
catalog file you are reading.
EPOCH_MARS_SEQ_2009.PDF
- This document provides pointing and sequencing information
for the EPOCh Mars observations in 2009, including descriptions
of the HRII scans of Mars (scan direction, rate, etc.).
EPOCH_OVERVIEW.PDF
- This presentation provides an overview of the EPOCh phase of
the EPOXI mission.
HRIV_3_4_EPOXI_MARS.TAB
- This ASCII table provides image parameters such as the mid-obs
Julian date, exposure time, filter, mission activity type, and
description or purpose for each observation (i.e., data product)
in this data set. This file is very useful for determining which
data files to work with.
Publications of the scientific results from the Mars observations
in this data set include Crow, et al. (2011) [CROWETAL2011].
Related Data Sets
-----------------
The following PDS data sets are related to this one and may be useful
for research:
DIF-M-HRIV-2-EPOXI-MARS-V1.0
- Raw HRIV Mars observations
DIF-CAL-HRIV-2-EPOXI-CALIBRATIONS-V1.0
- Raw HRIV dark frames taken at the end of each set of Mars
observations in this data set
DIF-M-HRII-2-EPOXI-MARS-V1.0
DIF-M-HRII-3/4-EPOXI-MARS-V2.0
- Raw and calibrated 1.05- to 4.8-micron HRI IR spectra of Mars,
covering the same observing period as this data set
DIF-M-MRI-2-EPOXI-MARS-V1.0
DIF-M-MRI-3/4-EPOXI-MARS-V2.0
- Raw and calibrated MRI visible CCD images of Mars at
750 nm, serving as context for the IR spectra and covering
the same observing period as this data set
DIF-C/E/X-SPICE-6-V1.0
- EPOXI SPICE kernels
DIF-CAL-HRII/HRIV/MRI-6-EPOXI-TEMPS-V1.0
- 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
DI-C-HRII/HRIV/MRI/ITS-6-DOC-SET-V4.0
- Deep Impact and EPOXI documentation set including a draft of the
Deep Impact instrument calibration paper by Klaasen, et al. (2008)
[KLAASENETAL2006]
Processing
==========
The calibrated two-dimensional FITS CCD 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 final version of the pipeline, dated December
2012, was used. Known limitations and deficiencies of the pipeline
are discussed in the EPOXI Calibration Pipeline Summary document
in this dataset or 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 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 (reversible)
- Correction for bias (reversible)
- Subtraction of a dark frame (reversible)
- Removal of horizontal, instrumental striping (reversible)
- Removal of electronic cross-talk (reversible)
- Application of a normalized flat field (reversible)
- Removal of CCD transfer smear (reversible)
- 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 of eight, bit flags to
describe the quality of each pixel in the primary image.
The PDS data label defines the purpose of each bit.
- 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; since destriping may remove scattered light
from very bright sources, the EPOXI SIS document describes
how to 'restripe' an image if needed.
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 8
frames were commanded for a scan with an exposure ID of 1000001, the
first FITS file name would be HV08060416_1000001_001_RR.FIT and the
last would be HV08060416_1000001_001_RR.FIT.
Image Compression
-----------------
All calibrated data products are uncompressed. If an associated raw
data product 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 8-bit lookup tables to
decompress the raw image. See the EPOXI SIS and EPOXI Hartley 2
Calibration Pipeline Summary documents as well as Klaasen, et al.
(2008) [KLAASENETAL2006] for more information.
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) increasing to the right in the display window and the
slowest-varying axis (lines) 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 of 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 section 5.1 of Klaasen, et al. (2013) [KLAASENETAL2011].
Parameters
==========
Data Units
----------
The calibrated RADREV and RAD image data have units of radiance,
W/(m**2 steradian micron).
Imaging Modes
-------------
One image mode was used for all HRIV Mars observations:
X-Size Y-Size
Mode Name (pix) (pix) Comments
---- ------ ------ ------ ---------------------------------------
2 SF1 512 512 Sub-frame, shuttered
All modes are unbinned. Most image modes have a set of bias
overclock rows and columns, located around the edges of the image
array. All overclock pixels were excluded from the calculation of
the values for MINIMUM, MAXIMUM, MEDIAN, and STANDARD_DEVIATION in
the data labels. These overclock areas described in the Deep
Impact instruments document and the Deep Impact instrument
calibration document included with this dataset.
Filters
-------
A summary of the HRIV filters used for the Earth observations is
provided below. For more information about the filters, see the
Deep Impact instruments document. For the effective center
wavelengths and the corresponding full-width-half-max values see
Klaasen, et al. (2013) [KLAASENETAL2011].
Filter Center Width
# Name (nm) (nm) Comments
- ---------- ----- ----- -------------------------------
2 BLUE 450 100
3 GREEN 550 100
4 VIOLET 350 100 Shortpass coating
5 IR 950 100 Longpass
7 RED 750 100
8 NIR 850 100
9 ORANGE 650 100
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.
For Earth observations, sub-spacecraft and sub-solar longitude and
latitude coordinates (planetocentric, body-fixed rotating) are
provided, when available, in the data labels by
SUB_SPACECRAFT_LONGITUDE, SUB_SPACECRAFT_LATITUDE,
SUB_SOLAR_LONGITUDE, and SUB_SOLAR_LATITUDE.
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 for certain
calibration targets.
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.
Ancillary Data
==============
The 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, unless specified otherwise (e.g,
SUB_SPACECRAFT_LONGITUDE).
Software
========
The observations in this data set 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
data set.
|
| CONFIDENCE_LEVEL_NOTE |
Confidence Level Overview
=========================
The data files in this data set were reviewed internally by the EPOXI
project.
Review
======
This dataset was peer reviewed and certified for scientific use on
05 March 2013.
Data Coverage and Quality
=========================
There are no unexpected gaps in this data set. All Mars observations
received on the ground were processed 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 EPOXI SIS document.
Limitations
===========
Timing
------
An improved operations spacecraft clock SPICE kernel,
DIF_SCLKSCET.00119.TSC, was used to convert to UTC and to calculate
geometry-related parameters for this dataset. Therefore observation
times for these calibrated images are improved and differ at the
sub-second level when compared to the archived raw images.
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 that will retroactively change
correlation for **all** Deep Impact and EPOXI data. When this
kernel is available, it will be added to the SPICE data sets for
the two missions and posted on the NAIF/SPICE web site at
http://naif.jpl.nasa.gov/naif/. If time and funding permit, the
EPOXI project will provide more precise times for archived data.
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
details, please see the Deep Impact instrument calibration paper by
Klaasen, et al. (2006) [KLAASENETAL2006] and the Deep Impact image
restoration paper by Lindler, et al. (2007) [LINDLERETAL2007].
CCD Horizontal Gap
------------------
The two central rows of the CCD are physically 1/6-pixel narrower and
collect only 5/6 of the charge of a normal row (Klaasen, et al., 2008
[KLAASENETAL2006] and Klaasen, et al., 2013 [KLAASENETAL2011]).
However, the data pipeline reconstructs images with uniform row
spacing, which introduces a 1/3-pixel extension at the center of the
raw and calibrated image arrays. Thus for two features on either side
of the midpoint line outside of the two central rows, the vertical
component of the true angular separation between those features is
one-third of a pixel less than their measured separation in the
reconstructed image. As for all geometric distortions, correction of
this 1/3-pixel extension will require resampling of the image and an
attendant loss in spatial resolution. The data pipeline process does
not perform this correction in order to preserve the best spatial
resolution. However, it does correct for the 1/6 decrease of signal
in the two central rows by the flat-field division so that the pixels
in those two rows have the correct scene radiance in the calibrated
images. Thus, the surface brightness measurement is preserved
anywhere in the geometrically distorted but calibrated images. Point
source or disk-integrated photometric measurements using aperture
photometry that includes these central rows will be slightly distorted
unless special adjustments are made, such as subtracting 1/6-pixel
worth of signal to the two central rows and adjusting for the
geometric distortion in the 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 are included in the calibrated
FITS images. Since the values in these pixels vary dramatically,
it is recommended that the values of the MINIMUM and 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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