DATA_SET_DESCRIPTION |
Data Set Overview
=================
This data set set contains calibrated radiance images of eight known
transiting extrasolar planetary systems (hot Jupiters) acquired by the
Deep Impact High Resolution Visible CCD (HRIV) during the EPOCh phase of
the EPOXI mission. From 22 January through 31 August 2008, EPOCh took
advantage of the permanent on-orbit defocus of the HRI telescope by
using the HRIV CCD to collect over 172,000 usable, photometric-quality,
visible light images of these exoplanet systems: HAT-P-4, HAT-P-7, GJ
436, TrES-2, TrES-3, XO-2, XO-3, and WASP-3. Time series of continuous
50-second integrations in a subframe mode of 128x128 or 256x256 pixels
with the clear #6 optical filter (350 to 1000 nanometers) were used to
observe each system for about three weeks, typically covering five or
more transits as well as secondary eclipses; an exception is XO-3 which
was observed only briefly before the spacecraft unexpectedly entered
safe mode. For most observations the 128x128-pixel subarray was used.
The larger subarray of 256x256 pixels was commanded during some transit
and secondary eclipse periods to ensure that pointing jitter did not
cause the star to fall beyond the edges of the subarray. The transiting
planet systems were observed in the integrated light of the planet and
star; no spatially resolved image of the planet was possible. The
out-of-focus HRIV telescope defocuses the images to about 10 pixels or
4 arcseconds at full-width half-max and introduces visible structure.
The calibrated HRIV images in this data set have been bias-subtracted
with removal of electronic crosstalk and transfer smear and nominally
flat-fielded, using calibration exposures of an integrating sphere made
on the ground before launch. However, the CCD detector had changed over
time while in space, and the ground-based flat fields were not adequate
for precision stellar photometry. Therefore the EPOCh team used an
independent bootstrap procedure, fitting aperture photometry derived
from the calibration images in this data set to a 2-D spline, to define
corrections to the flat-field calibration.
The following table chronologically lists the EPOCh transiting exoplanet
observations. For most targets, preview imaging was performed to
determine if the pointing bias needed to be modified for that target
series.
Target Start Date/DOY Stop Date/DOY Comments
------- -------------- -------------- --------------------------
HAT-P-4 2008-01-22/022 2008-02-12/043
XO-3 2008-02-12/043 2008-02-17/048 S/C entered safe mode
TRES-3 2008-03-06/066 2008-03-08/068
XO-2 2008-03-09/069 2008-03-11/071 Preview for pointing bias
TRES-3 2008-03-11/071 2008-03-18/078
XO-2 2008-03-20/080 2008-03-28/088
GJ 436 2008-05-04/125 2008-05-27/148
TRES-2 2008-06-28/180 2008-06-29/181 Preview for pointing bias
HAT-P-4 2008-06-29/181 2008-07-08/190
TRES-2 2008-07-08/190 2008-07-17/199
WASP-3 2008-07-17/199 2008-07-19/201 Preview for pointing bias
TRES-2 2008-07-20/202 2008-07-30/212
WASP-3 2008-07-30/212 2008-08-08/221
HAT-P-7 2008-08-08/221 2008-08-10/223 Preview for pointing bias
WASP-3 2008-08-10/223 2008-08-16/229
HAT-P-7 2008-08-16/229 2008-08-31/244
The time series for each target was typically bracketed by a set of
dark and internal stimulator lamp frames to monitor changes in the
CCD detector and to aid transit photometry. These data are archived
separately in the raw EPOXI calibrations data set,
DIF-CAL-HRIV-2-EPOXI-CALIBRATIONS-V1.0.
The general characteristics of the observed planetary systems as
described by Ballard, et al. (2009) [BALLARDETAL2009] is provided
here:
Stellar #Transits
Target V_mag Observed Points of Interest
------- ----- --------- -------------------------------------------
HAT-P-4 11.22 10 Low density planet, large radius for
its mass
XO-3 9.91 1 Eccentric orbit, second planet suspected
TrES-3 12.40 7 Short period (31 hours), reflected light
target
XO-2 11.18 3 Fainter component in wide visual binary,
metal rich
GJ 436 10.67 8 Eccentric orbit, unseen planet suspected,
star is M-dwarf
TrES-2 11.41 7 Kepler target, additional planets possible
WASP-3 10.64 8 Strongly heated, reflected light and
visible thermal emission possible
HAT-P-7 10.50 8 Kepler target, even more strongly heated
than WASP-3
Required Reading
---------------
The following documents are essential for the understanding and
interpretation of this data set. Please note the most recent
version of these documents, including other formats such as ASCII
text, can be found in the Deep Impact and EPOXI documentation data set,
DI-C-HRII-HRIV-MRI-ITS-6-DOC-SET-V2.0.
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_OVERVIEW.PDF
- This presentation provides an overview of the EPOCh phase of
the EPOXI mission.
EPOCH_TRANSIT_OBS.PDF
- This document describes of the EPOCh stellar transit observations
although most of the information is captured in this data set
catalog file you are reading.
EPOCH_TRANSIT_OBS_SCLK2BJD.PDF
- This report describes the calibration of spacecraft clock timing
and reduction to Barycentric Dynamic Time Julian Date for EPOCh
observations of transiting extrasolar planets.
HRIV_3_EPOXI_GJ436.TAB
HRIV_3_EPOXI_HATP4.TAB
HRIV_3_EPOXI_HATP7.TAB
HRIV_3_EPOXI_TRES2.TAB
HRIV_3_EPOXI_TRES3.TAB
HRIV_3_EPOXI_WASP3.TAB
HRIV_3_EPOXI_XO2.TAB
HRIV_3_EPOXI_XO3.TAB
- These ASCII tables provide 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.
Publications of the preliminary photometry results derived from the
transiting planet observations in this data set include Ballard, et
al. (2009) [BALLARDETAL2009B] and Christiansen, et al. (2009)
[CHRISTIANSENETAL2009].
Related Data Sets
-----------------
The following PDS data sets are related to this one and may be useful
for research:
DIF-E-HRIV-2-EPOXI-EXOPLANETS-V1.0
- Raw HRIV extrasolar planet transit observations
DIF-CAL-HRIV-2-EPOXI-CALIBRATIONS-V1.0
- Raw HRIV dark frames (exposure IDs 9600000 and 9600001) and
internal stimulator lamp images (exposure IDs 9600002 and 9600003)
acquired to monitor changes in the CCD detector for EPOCh transit
photometry purposes
DI-C-HRII-HRIV-MRI-ITS-6-DOC-SET-V2.0
- Deep Impact and EPOXI documentation 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
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 generated a calibrated
uncleaned radiance data product 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 although I-over-F data were not used by the EPOXI team.
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.
The calibration pipeline performed the following processes on the raw
HRIV FITS data to produce the RADREV products found in this data set:
- Decompression of compressed images (Earth 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)
- Calibration 'cleaning' processes such as interpolating over bad
and missing pixels, despiking (i.e., cosmic ray removal), and
denoising is never performed for RADREV data
- Calculation of multiplicative factors to convert a RADREV image
to I-over-F or to original DN
- Deconvolution to correct for the out-of-focus for the HRI telescope
was *not* performed by the calibration because this potential
process is part on the RAD stream used to clean RADREV products.
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, RADREV (reversibly
calibrated, 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 32
frames were commanded for a scan with an exposure ID of 1000001, the
first FITS file name would be HV08051200_9200003_000_RR.FIT and the
last would be HV08051200_9200003_032_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 EPOCh
exoplanet transit 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.
Parameters
==========
Data Units
----------
The calibrated RADREV image data have units of radiance,
W/(m**2 steradian micron).
Imaging Modes
-------------
Two HRIV image modes were used for the EPOCh transiting planet
observations:
X-Size Y-Size
Mode Name (pix) (pix) Comments
---- ------ ------ ------ -------------------------------
3 SF2S 256 256 Sub-frame, shuttered
5 SF3S 128 128 Sub-frame, shuttered
All modes are unbinned. For most observations the 128x128 mode was
used. The larger subarray of 256x256 pixels was commanded during
some transit and secondary eclipse periods and for preview imaging
to ensure that pointing jitter did not cause the star to fall beyond
the edges of the subarray. 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
-------
One HRIV image mode was used for the EPOCh transiting planet
observations:
Filter Center Width
# Name (nm) (nm) Comments
- ---------- ----- ----- -------------------------------
6 CLEAR6 650 >700 Not band limited
For more information about the filter, see the Deep Impact
instruments document or the Deep Impact instrument calibration
document. Also the EPOXI SIS has a brief discussion of this topic.
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.
It is important to note that the spacecraft clock is affected by a
systematic drift relative to ground clocks due to the changing
thermal environment of the spacecraft. The EPOCh team calibrated
the spacecraft clock versus ground clocks, and the corrected times
were computed for each transiting system (giving Julian Date and
Barycentric Julian Date by image file name). However to aid
transit timing analysis, the computation was implemented in the
data pipeline, and the EPOCh team verified that the resulting
values were consistent with theirs. Thus the Barycentric Dynamic
Time Julian Date (BJD) for the mid-point of an observation when
light reaches the solar system barycenter is provided by the
KPKSSBJT keyword in the FITS header of each data product. For more
information about calculating the BJD, please see the document
EPOCH_TRANSIT_OBS_SCLK2BJD.PDF. The pipeline also computed the
Barycentric Dynamic Time at the mid-point of the observation at the
spacecraft; it is provided as a Julian Date in the FITS header
keyword OBSMIDJT. The project elected to omit these two
barycentric-related values from the PDS labels.
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. For
example, some HAT-P-4 observations have inaccurate values for
boresight RIGHT_ASCENSION and DECLINATION because only predicted
pointing information, and not final reconstructed pointing, was
available to the data pipeline. 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
that were calculated for the time when the light arrived at the
target and the earth-observer-to-target values that were calculated
for the time when the light left the target.
The flyby spacecraft clock SPICE kernels (SCLK) used to convert to
UTC and to calculate geometry-related parameters for this data set
have a known accuracy of no better than 0.5 seconds. However as
this data set was being produced, the mission operations team
figured out how to correct raw clock correlation data for the
flyby spacecraft to allow timing fits that are accurate to at
least the sub-second level. The project plans to generate a
complete, corrected set of 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 data sets for the two missions and posted on the NAIF/SPICE
web site at http://naif.jpl.nasa.gov/naif/.
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. NAIF used these kernels to produce the
EPOXI SPICE data set, DIF-C/E/X-SPICE-6-V1.0.
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.
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