DATA_SET_DESCRIPTION |
Data Set Overview
=================
This dataset contains version 2.0 of raw calibration images acquired
by the High Resolution Visible CCD (HRIV) from 04 October 2007 through
28 November 2010 during the EPOCh, 103P/Hartley 2 Encounter, and cruise
phases of the EPOXI mission. This dataset supersedes version 1.0 which
contained raw calibration only through February 2010.
The purpose of these data are to monitor the HRIV CCD and improve its
calibration as needed. EPOXI calibration activities generally followed
those designed for the Deep Impact mission. For example standard
calibration targets continue to include the Moon, 16 Cyg A, Achernar,
Beta Hyi, Canopus, HD 60753, HD 79447, NGC 3114, NGC 7027, Vega, sky
frames, stim lamp frames, and dark frames. Additionally HD 80607 was
imaged in early November 2007 as a preliminary photometry check for
EPOCh exoplanet transit observations. The Deep Impact calibration
pipeline was the foundation for EPOXI until improvements were
implemented for the Hartley 2 encounter as described in the Hartley 2
calibration summary report located in the DOCUMENT directory. For a
detailed discussion of how the instruments were calibrated for EPOXI see
Klaasen, et al. (2011, in preparation) [KLAASENETAL2011]. The Deep
Impact instrument calibration is described by Klaasen, et al. (2008)
[KLAASENETAL2006] and Klaasen, et al. (2005) [KLAASENETAL2005].
A list of the calibration activities relevant to this dataset is
provided below and a description of each activity follows. The EPOXI
in-flight calibrations summary chart in the DOCUMENT directory provides
a quick-look at the activities.
------------------------------------------------------------------------
Phase and Exposure ID
Calibration Activity Obs Date/DOY Target Start Stop
---------------------------- -------------- -------- ------- -------
Cruise 1
Instrument Checkout 2007-10-04/277 Sky 1010000 1010015
EPOCh Photometry Test 2007-11-04/308 HD 80607 9300000 9400000
2007-11-05/309 HD 80607 9300000 9300000
2007-11-08/312 HD 80607 9500000 9500005
2007-11-09/313 HD 80607 9500005 9500006
HRIV Scattered Light Cal 2007-12-17/351 Moon 1000000 1000056
Dark 1000056 1000065
Lunar Calibration 2007-12-29/363 Moon 1000003 1000026
Dark 1000027 1000029
Standard Cruise Cal 2008-01-09/009 Beta Hyi 2000000 2000008
HD 79447 2000009 2000017
Achernar 2000018 2000025
Canopus 2000026 2001269
HD 60753 2002000 2002008
NGC 3114 2002009 2002020
Vega 2010000 2010012
16 Cyg A 2010013 2010021
NGC 7207 2010025 2010029
Dark 2010030 2010039
Stim Lamp 2010040 2010049
EPOCh
Darks/Stims for TransitObs 2008-01-22/022 Dark 9600000 9600001
*to*2008-08-31/244 Stim Lamp 9600002 9600003
Darks for Earth Obs #1 2008-03-19/079 Dark 1001235 1001241
Darks for Earth Obs #4 2008-05-29/150 Dark 1001235 1001241
Darks for Earth Obs #5 2008-06-05/157 Dark 1001235 1001241
Standard Cruise Cal 2008-06-23/175 Vega 2010003 2010015
16 Cyg A 2010016 2010024
NGC 7207 2010025 2010029
Dark 2010030 2010039
Stim Lamp 2010040 2010049
2008-06-24/176 Beta Hyi 2000000 2000008
HD 79447 2000009 2000017
Achernar 2000018 2000025
Canopus 2000026 2000038
Canopus 2001000 2001269
HD 60753 2002000 2002008
NGC 3114 2002009 2002018
2008-06-25/177 NGC 3114 2002019 2002020
Cruise 2
HRIV PSF Calibration 2008-12-17/352 Canopus 7100000 7100008
Achernar 7100009 7100017
Dark 7100018 7100018
Stim Lamp 7100019 7100020
EPOCh Earth N.Pole Darks 2009-03-28/087 Dark 1001235 1001241
Checkout after HRI Turnoff 2009-09-30/273 Sky 1010000 1010015
HRIV Mechanical Checkout 2009-10-01/274 Dark 1000001 1000007
EPOCh Earth S.Pole #2 Darks 2009-10-05/278 Dark 1001235 1001241
EPOCh Microlensing Obs 2009-10-08/281 Dark 9600000 9600001
Stim Lamp 9600002 9600003
EPOCh Mars Darks 2009-11-21/325 Dark 1001235 1001241
Standard Cruise Cal 2010-02-16/047 Beta Hyi 2000000 2000008
HD 79447 2000009 2000017
Achernar 2000018 2000025
Canopus 2000026 2001269
HD 60753 2002000 2002008
NGC 3114 2002009 2002020
Vega 2010003 2010015
16 Cyg A 2010016 2010024
NGC 7207 2010025 2010029
Dark 2010030 2010039
Stim Lamp 2010040 2010049
Hartley 2 Encounter
HRIV PSF Calibration 2010-09-03/246 Canopus 7100000 7100008
Dark 7100018 7100018
Stim Lamp 7100019 7100020
Standard Cruise Cal 2010-09-28/271 Vega 2010003 2010015
(pre-encounter) to 16 Cyg A 2010016 2010024
2010-09-29/272 NGC 7027 2010025 2010029
Dark 2010030 2010039
Stim Lamp 2010040 2010049
Beta Hyi 2000000 2000008
HD 79447 2000009 2000017
Achernar 2000018 2000025
Canopus 2000026 2001269
HD 60753 2002000 2002008
NGC 3114 2002009 2002020
Darks for Enctr E-18Hrs 2010-11-03/307 Dark various ExpIDs
to E+2Days *to*2010-11-06/310
Standard Cruise Cal 2010-11-27/331 Vega 2010003 2010015
(post-encounter) to 16 Cyg A 2010016 2010024
2010-11-28/332 NGC 7027 2010025 2010029
Dark 2010030 2010039
Stim Lamp 2010040 2010049
Beta Hyi 2000000 2000008
HD 79447 2000009 2000017
Achernar 2000018 2000025
Canopus 2000026 2001269
HD 60753 2002000 2002008
NGC 3114 2002009 2002020
------------------------------------------------------------------------
Instrument Checkout: On 4 October 2007, the three science instruments
were turned on for the first time in more than two years. Sky
frames acquired by the HRIV CCD confirmed the mechanical components
such as the shutter and filter wheel were functioning. The
instrument exhibited nominal behavior of background levels.
EPOCh Photometry Test: On 4-9 November 2007, EPOCh photometry tests
were performed. During these tests, the HRIV instrument observed a
bright (V=9) visual binary star (HD 80607) for 12 continuous hours
to check pointing and photometric stability. The observations were
taken using the 256-by-256 sub-array mode of the HRIV CCD. The
spacecraft successfully captured the star images and held them on
the sub-array for the full duration of the test. However the images
were unexpectedly offset by 59 microradian (12.2 arcsec). This was
due to stellar aberration, which was not included for stellar
observations during the Deep Impact mission. After correcting for
aberration, the spacecraft pointing was within specifications.
HRIV Scattered Light Calibration: On 17 December 2007, a
calibration for scattered light using Earth's moon was performed for
the HRIV instrument using long and short exposures. Many of these
images were obtained with the moon outside but near the field of
view of the HRIV CCD to allow analysis of the amount of light that
is scattered into the field of view from bright objects just outside
or within the field of view.
Lunar Calibration: On 29 December 2007 as the spacecraft approached
Earth, the three science instruments used the Moon as a target to
acquire data for recalibration purposes.
Standard Cruise Calibration: On 9 January 2008, the first of the
standard cruise calibrations for the three science instruments was
performed. The calibration sequence included observations of
several standard stars, both solar analogs and hot stars with few
absorption lines in their spectra for absolute calibration of all
instruments, a stellar cluster for checking geometric distortion in
the cameras, and a planetary nebula for checking the wavelength
calibration of the spectrometer. This sequence was designed such
that it could be rerun, with few if any changes, after completion of
the EPOCh observations and then again just before and just after the
observing program for comet 103P/Hartley 2.
EPOCh Darks/Stims for Transit Obs: Observations of EPOCh transiting
extrasolar planets were bracketed by sets of dark and internal
stimulator lamp frames to aid photometric analysis. These frames,
acquired from 22 January through 31 August 2008, are sub-frame
images (256x256 or 128x128 pixels) with exposure times of 50 or 100
milliseconds. Unique exposure IDs were assigned for these
EPOCh-specific calibrations: 9600000 and 9600001 for the darks and
9600002 and 9600003 for the stim lamp frames.
EPOCh Darks for Earth Obs: At the end of each Earth observation
period, a set of HRIV dark frames was acquired for calibration
purposes and is included in this dataset.
Standard Cruise Calibration: A second standard cruise calibration
was performed on 23-25 June 2008 for the HRIV and HRII instruments.
The sequence was very similar to that used for the calibration
performed on 9 January 2008, except the MRI instrument was turned
off because of thermal and telecommunication concerns.
HRIV PSF Calibration: On 17 December 2008, additional data were
acquired for improving point spread functions (PSF) for the HRIV
CCD. Analysis of early EPOXI calibration data and EPOCh stellar
observations indicated the PSFs had changed since mid-2005 when the
functions were last generated for Deep Impact.
EPOCh Earth North Pole Darks: On 27-28 March 2009, EPOCh acquired a
full set of observations (24 hours) of Earth at high northern
latitudes using the HRIV CCD and the IR spectrometer. At the end of
this campaign, HRIV darks were acquired for calibration purposes and
included in this dataset.
Checkout after HRI Turnoff & HRIV Mechanical Checkout: Before
repeating the Earth South Pole observation of 27-28 September 2009
that was prematurely aborted when HRI turned off, a standard imaging
checkout of the HRII, HRIV, and MRI instruments was performed after
HRI was powered up on 30 September 2009. HRIV took images of the
sky. A mechanical checkout of HRIV was performed on 01 October:
a miniature HRIV Earth observing sequence of darks that cycled
through the filters 21 times was run to check for 5-V telemetry
channel noise. The results of the tests were satisfactory.
EPOCh Earth South Pole #2 Darks: On 04-05 October 2009, EPOCh
acquired a full set of observations (24 hours) of Earth at high
southern latitudes using the HRIV CCD and the IR spectrometer. At
the end of this campaign, HRIV darks were acquired for calibration
purposes and included in this dataset.
EPOCh Microlensing Obs: On 05-08 October 2009, HRIV spent four days
imaging the known exoplanet microlensing event MOA-2009-BLG-266,
where the foreground lensing star and its exoplanet bend light of
the source star. EPOCh reused the sequence from the HRIV exoplanet
transit observations in 2008 to obtain these data. At the end of
this observing period, a set of HRIV dark and internal stimulator
lamp frames was acquired to aid analysis. These EPOCh-specific
calibrations are sub-frame images (256x256 or 128x128 pixels) with
exposure times of 50 or 100 milliseconds and are assigned unique
exposure IDs: 9600000 and 9600001 for the darks; 9600002 and
9600003 for the stim lamp frames.
EPOCh Mars Darks: On 20-21 November 2009, EPOCh observed Mars for
24 hours with the HRIV CCD and the IR spectrometer. At the end of
this campaign, HRIV darks were acquired for calibration purposes
and included in this dataset.
Standard Cruise Calibration: A full, standard cruise calibration
for HRII, HRIV, and MRI was completed on 16 February 2010. The
sequence was very similar to that used for the standard cruise
calibrations in June 2008.
HRIV PSF Calibration: On 03 September 2010, the HRIV CCD imaged
Canopus to monitor the point spread functions (PSF) for that
instrument. Analysis of HRIV calibration data since 2005 and
EPOCh stellar observations in 2008 indicate the PSFs can change
over time.
Pre-Encounter Standard Cruise Calibration: A full, standard cruise
calibration for HRII, HRIV, and MRI was performed on 28-29
September 2010. The sequence was very similar to that used
earlier in 2010.
Darks for E-18 Hours to E+2 Days: The imaging sequence that was
executed from 03 to 06 November 2010 for the flyby of Hartley 2
included HRIV dark frames for background and stripe removal
analyses.
Post-Encounter Standard Cruise Calibration: A full post-encounter
standard cruise calibration for HRII, HRIV, and MRI was performed
on 27-28 November 2010. The sequence was nearly identical to the
pre-encounter calibration performed in September.
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-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 EPOXI datasets, the
science data products, and defines keywords in the PDS labels.
HARTLEY2_CAL_PIPELINE_SUMM.PDF
- The EPOXI Hartley 2 Calibration Pipeline Summary provides an
overview the calibration pipeline as of June 2011 used for
processing data acquired during the Hartley 2 Encounter.
EPOXI_INFLIGHT_CAL_SUMMARY.PDF
- The EPOXI In-Flight Calibrations Summary provides an overview of
the instrument calibrations performed during the entire EPOXI
mission.
INSTRUMENTS_HAMPTON.PDF
- The Deep Impact instruments paper by Hampton, et al. (2005)
[HAMPTONETAL2005] provides very detailed descriptions of the
instruments.
HRIV_2_EPOXI_CALIBRATIONS.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 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 calibration purposes:
DIF-E-HRIV-2-EPOXI-EARTH-V1.0
DIF-E-HRIV-3/4-EPOXI-EARTH-V1.0
- Raw and calibrated HRIV Earth observations
DIF-M-HRIV-2-EPOXI-MARS-V1.0
DIF-M-HRIV-3/4-EPOXI-MARS-V1.0
- Raw and calibrated HRIV Mars observations
DIF-C-HRIV-2-EPOXI-HARTLEY2-V1.0
DIF-C-HRIV-3/4-EPOXI-HARTLEY2-V1.0
- Raw and calibrated HRIV comet Hartley 2 observations
DI-C-HRII/HRIV/MRI/ITS-6-DOC-SET-V3.0
- Deep Impact and EPOXI documentation set including a draft of the
Deep Impact instrument calibration paper by Klaasen, et al. (2008)
[KLAASENETAL2006]
DIF-C/E/X-SPICE-6-V1.0
- EPOXI SPICE kernels
DIF-CAL-HRII/HRIV/MRI-6-EPOXI-TEMPS-V2.0
- 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
DIF-CAL-HRIV-2-9P-CRUISE-V1.0
DIF-CAL-HRIV-2-9P-ENCOUNTER-V1.0
- Deep Impact raw HRIV calibrations datasets from 2005
DIF-CAL-HRII/HRIV-2-GROUND-TV2-V1.0
DIF-CAL-HRII/HRIV/MRI-2-GROUND-TV4-V1.0
- Deep Impact raw HRIV pre-launch calibrations from 2002 and 2003
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 EPOXI SIS document. 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.
Each FITS file is accompanied by a detached PDS data label. The
EPOXI SIS document provides definitions for the keywords found in
a PDS data label. Many values in a 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-V3.0.
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 1000000, the
first FITS file name would be HV07122918_1000000_001.FIT and the last
would be HV07122918_1000000_032.FIT.
Image Compression
-----------------
For some HRIV calibration frames the raw data numbers were
compressed on board the flyby spacecraft by use of a lookup table
then downlinked, processed, and archived in the same format. A
compressed image is identified by the value 'COMPRESSED' in the
COMPRESSED_IMAGE_VALUE keyword in the data labels or the COMPRESS
keyword in the FITS headers. 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 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
----------
Raw image data are in units of raw data numbers.
Target Name and Description
---------------------------
The TARGET_NAME keyword in the data labels is set to the intended
target, 'CALIBRATION', for each observation in this dataset. The
TARGET_DESC keyword provides the name of the specific calibration
target, such as 'DARK' or 'VEGA'.
Imaging Modes
-------------
A summary of the imaging modes is provided here. For more
information see the EPOXI SIS and EPOXI Hartley 2 Calibration
Pipeline Summary documents, Hampton, et al. (2005) [HAMPTONETAL2005]
and Klaasen, et al. (2011) [KLAASENETAL2011].
X-Size Y-Size
Mode Name (pix) (pix) Comments
---- ------ ------ ------ ---------------------------------------
1 FF 1024 1024 Full frame, shuttered
2 SF1 512 512 Sub-frame, shuttered
3 SF2S 256 256 Sub-frame, shuttered
4 SF2NS 256 256 Sub-frame, not shuttered
5 SF3S 128 128 Sub-frame, shuttered
6 SF3NS 128 128 Sub-frame, not shuttered
7 SF4O 64 64 Sub-frame, not shuttered
8 SF4NO 64 64 Sub-frame, not shuttered, no overclocks
9 FFD 1024 1024 Full-frame diagnostic, 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.
Filters
-------
A summary of the MRI filters is provided here. For more information
see the EPOXI SIS and EPOXI Hartley 2 Calibration Pipeline Summary
documents, Hampton, et al. (2005) [HAMPTONETAL2005] and Klaasen, et
al. (2011) [KLAASENETAL2011].
Filter Center Width
# Name (nm) (nm) Comments
- ---------- ----- ----- -------------------------------
1 CLEAR1 650 >700 Not band limited
2 BLUE 450 100
3 GREEN 550 100
4 VIOLET 350 100 Shortpass coating
5 IR 950 100 Longpass
6 CLEAR6 650 >700 Not band limited
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.
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. NAIF used these kernels to produce the
EPOXI SPICE dataset, 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.
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 FITS files in this dataset were reviewed internally by the EPOXI
project and were used extensively by the science teams to improve
the calibration of instrument.
Review
======
This dataset, Version 2.0, was peer reviewed and certified for
scientific use on 15 August 2011. It supersedes Version 1.0
which contained data only from October 2007 through February 2010.
Data Coverage and Quality
=========================
There are no unexpected gaps in this dataset. All calibration
observations received on the ground were processed and included in this
dataset.
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 flyby spacecraft clock SPICE kernels (SCLK) used to convert to
UTC and to calculate geometry-related parameters for this dataset
have a known accuracy of no better than 0.5 seconds. However the
latest SCLK (science version 84) applied to the Hartley 2 encounter
data is good to within 0.01 seconds for converting the spacecraft
timestamps to ephemeris time for observations acquired around
closest approach. Please note that the SCLK (version 65) used to
compute UTC values and geometry for calibration data acquired from
January 2009 through July 2010 has known discontinuities of up to a
second. Those discontinuities have been corrected in the latest
SCLK, science version 84, applied to Hartley data.
The mission operations team has figured out how to correct raw clock
correlation data for the Deep Impact flyby spacecraft to allow
timing fits that are accurate to well under the sub-second level as
evidenced by the 0.01-second accuracy around the time the Hartley 2
encounter. The EPOXI project plans to use this method to generate a
complete and highly accurate set of UTC correlations for the flyby
spacecraft since the launch, resulting 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/. The EPOXI
project will provide more precise times for archived data as time
and funding permit.
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. (2008) [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 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].
Out-of-Frame Target
-------------------
Some lunar calibration frames in this dataset do not contain the
moon because of deviations in pointing. However the intended target
is specified by the TARGET_DESC 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 EPOXI SIS document.
|