| 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.
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-V3.0, 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.
CALIBRATION_PAPER_DRAFT.PDF
- The Deep Impact instrument calibration paper by Klaasen, et al.
(2008) [KLAASENETAL2006] describes how the instruments were
calibrated for Deep Impact and similarly for EPOXI and explains
the calibration process used for both missions. The published
version should be available online in the Review of Scientific
Instruments by the American Institute of Physics. The EPOXI
archive provides only an incomplete draft.
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, 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.
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-V1.0
- 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-V1.0
- 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
DI-C-HRII/HRIV/MRI/ITS-6-DOC-SET-V3.0
- Deep Impact and EPOXI documentation set
DIF-C/E/X-SPICE-6-V1.0
- EPOXI SPICE kernels, available at end of EPOXI mission
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
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. 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).
The calibration pipeline performed the following processes on the raw
HRIV FITS data to produce the RADREV and RAD products found in this
data set:
- Decompression of compressed images (Mars images were not
compressed)
- Correction for bias
- Subtraction of a dark frame
- 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 for a signal-to-noise
ratio map. The calibration steps and files applied to each raw image
are listed in the PROCESSING_HISTORY_TEXT keyword in the PDS data
label. For a detailed discussion of the calibration pipeline and the
resulting data, see the Deep Impact instrument calibration document
and the EPOXI SIS document.
Data
====
FITS Images and PDS Labels
--------------------------
Each calibrated HRIV 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.
For more information about the FITS primary image and the extensions,
refer to the Deep Impact instrument calibration document or 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.
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 data products in this data set are uncompressed. If the
associated raw data products was compressed on board the flyby
spacecraft (and thus received on the ground and archived as
compressed) then the calibration pipeline uses one of four 8-bit
lookup tables to decompress the raw image. However, the Mars
images acquired acquired during the time period covered by this data
set were never compressed. For more information about this topic,
see the image compression section of the Deep Impact instrument
calibration documents.
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 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
Deep Impact instrument calibration document. Also the EPOXI SIS
has a brief discussion of this topic.
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 relative boresight alignments section of the Deep
Impact instrument calibration document.
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. For a thorough description of the imaging
modes, please see the Deep Impact instruments document or the Deep
Impact instrument calibration document. Also the EPOXI SIS has a
brief discussion of this topic.
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.
Filters
-------
A list of the characteristics of the filters used for the HRIV Mars
observations is provided below. For more information about the
filters, see the Deep Impact instruments document or the Deep Impact
instrument calibration document. Also the EPOXI SIS has a brief
discussion of this topic.
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
Mars-based observer should have been able to see an event recorded
by the instrument.
For Mars 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 UTC and geometry values included in the data labels and FITS headers
were computed using the best available SPICE kernels at the time the
products were generated. The final, best SPICE kernels will be posted
on the NAIF/SPICE web site at http://naif.jpl.nasa.gov/naif/ and
archived in the PDS data set DIF-C/E/X-SPICE-6-V1.0 after the end
of the EPOXI mission.
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
======
The PDS Small Bodies Node held a peer review for this data set on
29 September 2010. The reviewers certified this data set for
scientific use and archiving on 29 September 2010.
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
------
The flyby spacecraft clock SPICE kernel (SCLK) used to compute UTC
values in the data labels and FITS headers has known discontinuities
of up to a second. If time and funding permit, the EPOXI project
will provide more precise times after the mission ends.
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
------------------
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 biased during
integration so that the centers of the two halves are apparently
1/6 pixel closer to the center, and the two boundary rows show a
decrease in sensitivity of 1/6. Reconstructed image files space
all lines evenly, so the true image is erroneously vertically
pushed apart by 1/3 pixel at its center in these reconstructions.
When making science measurements from HRIV images, one must
therefore be very careful to properly account for the two flaws
introduced by the apparently narrow central lines on the CCD - a
geometric error that separates the image by an extra 1/3 pixel at
the horizontal quadrant boundary, and 2) insertion of extra total
radiance into calibrated images due to the flat-field correction,
which corrects for an apparent radiance deficit in the two central
rows because of the smaller number of photons actually incident on
those rows.
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 Deep Impact instrument
calibration document or the EPOXI SIS document.
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