| DATA_SET_DESCRIPTION |
Data Set Overview
=================
This data set set contains raw 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 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_2_EPOXI_GJ436.TAB
HRIV_2_EPOXI_HATP4.TAB
HRIV_2_EPOXI_HATP7.TAB
HRIV_2_EPOXI_TRES2.TAB
HRIV_2_EPOXI_TRES3.TAB
HRIV_2_EPOXI_WASP3.TAB
HRIV_2_EPOXI_XO2.TAB
HRIV_2_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 based on the
transiting planet observations 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-3-EPOXI-EXOPLANETS-V1.0
- Calibrated 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 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 Deep Impact instrument
calibration document and the EPOXI SIS document included in this data
set. The SDC did not apply any type of correction or decompression
algorithm to the raw data.
Data
====
FITS Images and PDS Labels
--------------------------
Each raw HRIV 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 of eight bit flags to indicate the quality of
each pixel in the primary image. The data label provides a short
description of each bit. 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 or the Deep Impact
instrument calibration 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.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), 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 32
frames were commanded for a scan with an exposure ID of 1000001, the
first FITS file name would be HV08051200_9200003_000.FIT and the
last would be HV08051200_9200003_032.FIT.
Image Compression
-----------------
Although raw data numbers for HRIV frames could be compressed on
board the flyby spacecraft by use of a lookup table then downlinked,
processed, and archived in the same format, EPOCh exoplanet transit
images acquired during the time period covered by this data set were
never compressed. Therefore the COMPRESSED_IMAGE_VALUE keyword in
the data labels is always set to 'UNCOMPRESSED'. 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
----------
Raw image data have units of raw data numbers.
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.
|
| CONFIDENCE_LEVEL_NOTE |
Confidence Level Overview
=========================
The data files in this data set were reviewed internally by the EPOXI
project.
Review
======
This data set is archived at the PDS Small Bodies Node (SBN) and the
Multi-Mission Archive at STScI (MAST). It passed a peer review held
by SBN on 23 July 2009; MAST personnel participated.
Data Coverage and Quality
=========================
There are no unexpected gaps in this data set. All Earth 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
===========
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.
Out-of-Frame Target
-------------------
Some exoplanet transit images in this data set do not contain the
star because of deviations in pointing. However the intended target
is specified by the TARGET_NAME keyword in the data labels.
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 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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