| DATA_SET_DESCRIPTION |
Data Set Overview
=================
This data set contains the calibrated images of comet 81P/Wild 2
taken by the Navigation Camera (NAVCAM) during the Stardust
spacecraft's approach to and encounter with the comet, and the post
encounter observations that were used to aid in the calibration
process. The raw data versions of these (and other) images are
included in the NAVCAM raw data archive
(SDU-C-NAVCAM-2-EDR-WILD2-V2.0). This version has also been
corrected for the exposure time errors that were found after the
initial submission of the data (see the file SHUTTER_CORRECTION.ASC
in the documents directory for more information).
These images have been divided into two groups. The 'NAVIGATION'
directory contains the images of the comet obtained as part of
navigation sequences. These are not full-frame images, but
contain sub-frame windows of the navigation stars and the comet.
The 'ENCOUNTER' directory contains the images obtained during the
encounter with the comet and are full-frame views.
Every image provided in this data set was taken as a part of a
particular imaging sequence, each of which is described in this
section by the NAVCAM Science Lead, Dr. Raymond L. Newburn, Jr.
For the complete list of images and their parameters, refer to
the data set's index table, INDEX/INDEX.TAB. For additional
notes on individual images also consult with the document ``Log
of Stardust NAVCAM Flight Images', DOCUMENT/PIIMGLOG.PDF,
provided with this data set.
Image Sequence Information
---------------------------
Each sequence is listed here, with information about any
problems that were encountered during the exposure, readout or
downlinking of the data. Additional information about other
sequences is available in the catalog files for the raw NAVCAM
data set (SDU-C-NAVCAM-2-EDR-WILD2-V2.0).
2003 DEC 29 05:00:00 -- Images 979-996
--------------------------------------
In the series of images 979 through 996, only 983 and 984 are
missing packets. All images were acquired at a scan mirror
angle of 21.87 degrees. Images 979 through 982 and 985 through
987 were given 20 second exposures, while 988 through 996
received 10 second exposures. All complete images had three 301
x 301 pixel windows.
2003 DEC 30 05:27:15 -- Images 997-1014
---------------------------------------
Images 997 through 999 were missing the first packet, thereby
ruining each entire image. In this hectic period, no attempt
was made to reconstruct them. Image 1000 consisted of one
normal 301 x 301 pixel window and two 1 x 1 pixel windows. The
image of Wild2 was in one of the latter. Images 1001 through
1014 were normal, each having three 301 x 301 windows. The
first five good frames received 20 second exposures, and the
last nine were all 10 second exposures. These, of course, were
again taken for optical navigation purposes.
2003 DEC 30 22:35:15 -- Images 1015-1032
----------------------------------------
The second set of December 30 images probably included 1015
through 1021, which are missing. I say this because there are
only two images with 20 second exposures instead of the usual
nine. The two 20 second images are 1022 and 1023. These are
followed by 1024 through 1032, which are 10 second exposures.
All were taken at a scan mirror angle of 21.00 degrees and with
three 301 x 301 pixel windows.
2003 DEC 31 07:00:00 -- Images 1033-1050
----------------------------------------
Images 1033 through 1050 constituted the first set of December
31 exposures. All were taken at a scan mirror angle of 20.77
degrees and with three 301 x 301 pixel windows. The first nine
were 20 second exposures and the second nine were 10 second
exposures.
2003 DEC 31 20:18:00 -- Images 1051-1058
----------------------------------------
The second set of December 31 images consisted of eight images
numbered 1051 through 1058. The first four were 15 second
exposures, the second four 10 seconds. All utilized a scan
mirror of 24.49 degrees. Each consisted of three windows of 291
x 291 pixels.
2004 JAN 01 00:18:00 -- Images 1059-1066
----------------------------------------
The first set of January 1 images, 1059 through 1066, were
taken at a scan mirror angle of 24.49 degrees. All were
windowed images using three 291 x 291 pixels with a scan mirror
angle of 24.49 degrees. The first four were 15 second
exposures, the second four, 10 second exposures.
2004 JAN 01 05:18:00 -- Images 1067-1074
----------------------------------------
The second set of January 1 images, 1067 through 1074, were
also taken at a scan mirror angle of 24.49 degrees. All were
windowed images using three 291 x 291 pixels with a scan mirror
angle of 24.49 degrees. The first four were 15 second
exposures, the second four, 10 second exposures.
2004 JAN 01 08:18:00 -- Images 1075-1082
----------------------------------------
The third set of January 1 images, 1075 through 1082, once
again were taken at a scan mirror angle of 24.49 degrees. All
were windowed images using three 291 x 291 pixels with a scan
mirror angle of 24.49 degrees. The first four were 15 second
exposures, the second four, 10 second exposures.
2004 JAN 01 12:18:00 -- Images 1083-1090
----------------------------------------
The fourth set of January 1 images, 1083 through 1090, once
again were taken at a scan mirror angle of 24.49 degrees. All
were windowed images using three 291 x 291 pixels with a scan
mirror angle of 24.49 degrees. The first four were 15 second
exposures, the second four, 10 second exposures.
2004 JAN 01 16:18:00 -- Images 1091-1098
----------------------------------------
The fifth set of January 1 images, 1091 through 1098, once
again were taken at a scan mirror angle of 24.49 degrees. All
were windowed images using three 291 x 291 pixels with a scan
mirror angle of 24.49 degrees. The first four were 15 second
exposures, the second four, 10 second exposures.
2004 JAN 01 21:18:44 -- Images 1099-1106
----------------------------------------
The sixth set of January 1 images, 1099 through 1106, were
taken at a scan mirror angle of 24.49 degrees. All were
windowed images using three 291 x 291 pixel windows. Image 1099
was missing. Image 1100 had two windows, but only the image
containing the comet was complete. Images through 1102 were 15
second exposures, the last four were 10 second exposures.
2004 JAN 02 05:18:44 -- Images 1107-1122
----------------------------------------
Images 1107 through 1115, one complete set, are missing. The
final set of optical navigation images, numbered 1116 through
1122, were acquired using three 291 x 291 pixel windows and a
scan mirror angle of 24.49 degrees. The first three utilized 15
second exposures and the final four 10 second exposures.
2004 JAN 02 18:54:28 -- Images 2000-2115 -- Wild 2 Encounter Set
----------------------------------------------------------------
Images 2005 through 2115 are the 81P/Wild 2 encounter set.
There are 72 complete images in this set, the missing numbers
having been used only to establish the auto-tracking on the
nucleus. There was sufficient memory only for the 72 images. It
is the nature of the shutter that alternate exposures are 1.65
milliseconds shorter than the set exposure time. The even
numbered images are the shorter ones taken during the
encounter. We were quite limited in the number of changes that
could be made in the exposure time, so most were taken at
settings of either 100 milliseconds (actually 98.35
milliseconds) or 10 milliseconds. Many of the longer exposures
were saturated, but these serve very well to bring out the many
jets of gas and dust ejected by the comet. Scan mirror angles
ranged from 1.37 degrees for image 2005 through 176.05 degrees
for image 2115. The images with the mirror at more than 170
degrees all exhibit a great deal of scattered light, probably
from the sample return capsule. There are lesser amounts of
scattered light on images back to about 160 degrees. There is a
problem with images taken near 0 degrees as well, from light
scattered from the launch vehicle adapter ring which actually
occludes a bit of the periscope.
2004 JAN 13 02:24:28 -- Images 2116-2130
----------------------------------------
Images 2116 through 2130 were taken to attempt to calibrate all
of the images, to check the post-encounter state of the
periscope, and to check the calibration lamp, which appeared to
have failed pre-encounter. Images 2116 and 2129 were bias
frames. Images 2117 through 2125 constituted the very limited
calibration set. Images 2126 through 2128 were for the
periscope check, and 2130 was the calibration lamp check. The
calibration images all were taken at scan mirror angles near 24
degrees, four with 1 second exposures, four with 5 second
exposures and one with a 15 second exposure. All were
uncompressed full frame images. The periscope checks were done
at an angle of -0.10 degrees and were full of scattered light.
This was expected to some extent, but the very mottled
appearance of the images indicated that one or both mirrors of
the periscope were thoroughly sandblasted by the passage
through the Wild 2 coma. The check of the calibration lamp
unfortunately was done with a 1 second exposure rather than 20
milliseconds. It was obviously working just fine, since every
pixel in the frame was saturated.
Processing and Calibration
===========================
The images in this data set were created by the DMAPKTDECOM
program developed by Applied Coherent Technology Corp, Herndon,
Virginia and operated by the Stardust Data Management and Archive
Team at JPL, Pasadena, California. This program assembled images
from raw telemetry packets sent down by the spacecraft and
populated the images labels with housekeeping values, decommutated
the binary image headers, and computed geometry parameters using
SPICE kernels. This program did not apply correction of any kind
to the image data.
In the cases when only certain sections of the detector were
downlinked, the program filled the pixels in the image
corresponding to the areas for which data had not been downlinked
with hex null values (i.e., zeroes). In such images window
objects define the areas containing non-null data.
Radiometric Calibration
-----------------------
The images have been radiometrically calibrated to give
radiance in units of W m**-2 sR**-1. Processing can be
summarized as consisting of six steps:
1) Undo square-root compression of raw data numbers.
2) Subtract offset estimated from averaging baseline.
stabilization pixel values in prefix/suffix of the Image lines
from all pixels in the image.
3) Divide by estimate of relative sensitivity of each pixel
compared to image center pixels.
4) Scale from data number to radiance based on exposure
time, distance from Sun, and absolute calibration
coefficient derived from analysis of post-encounter star
images. The conversion coefficient used to convert to
radiance is 1.525 x 10^{-6} W/m^2/sr/(DN/s).
5) Geometrically transpose image data (not including line prefix
and suffix regions), resulting in a right-reading
(unmirrored) view of the sky from the spacecraft such as
would have been obtained by a conventional camera without
periscope.
6) Update values of geometric parameters to correspond to
the transposed image geometry.
An extensive discussion about the calibration procedures can
be found in the NAVCAM_CALIBRATION.PDF and the
SBN_CALIBRATION_COMMENTS.ASC documents in the DOCUMENT
directory.
Data
====
The images in this data set have been converted by the SBN from
the standard PDS format to FITS format with detached PDS labels.
Relevant information in the PDS label has been placed into the
FITS header, though due to differing conventions, the keywords
may have changed. The description field in the FITS header
should make it clear what the keyword represents.
Camera Description
------------------
The STARDUST camera has an angular resolution of
59 microrad/pixel (12 arcsec/pixel) and a focal length of 202 mm
at an f-ratio of about f/3.5. Early in the flight the filter
wheel failed, possibly due to a failed power supply. Fortunately
it failed on the filter with the largest throughput, but the
broad bandpass of that filter caused images taken through it to
have significant chromatic aberration, which resulted in an
image resolution of about 2.5 pixels at FWHM (full width at half
maximum) when observing a point source such as a star. (The high
resolution filter, intended to be used for near encounter
imaging, would have resulted in resolution exceeding a half
pixel.) Without any image processing, the 2.5 pixel resolution
resulted in a best linear resolution at closest approach of
about 20 m/pixel.
After launch, it was discovered that the camera optics had been
contaminated by an unknown substance that significantly reduced
the quality of the images, and affected the photometric
properties of the image. Tests showed that heating the camera
before critical observations reduced this contamination and
improved the quality of the images, though some effects still
remained. Because the contamination varies with time, a plan
was developed to obtain calibration images immediately before
and after the encounter with comet Wild 2, to improve the
possibility of being able to calibrate the images
photometrically. Unfortunately, problems with the camera and
calibration lamp meant that the only useful calibration
sequences were obtained 11 days after the encounter. Further
details regarding the calibrations are discussed in the
NAVCAM_CALIBRATION document in the DOCUMENT directory.
The camera has a 1024x1024 array as the active portion of the
CCD, though the first and last columns are uncalibratable, and
thus were removed during processing. The resulting images have
been trimmed to an array size of 1022x1024.
Windowed Images
---------------
The IMAGE size parameter in the image label reflects the size
of the detector, however in the navigation images, data from
only certain sections of the detector were downlinked. In
these cases the pixels in the image corresponding to the areas
for which data had not been downlinked are filled with hex
null values (i.e., zeroes). WINDOW objects define the areas
containing non-null data.
Compression Modes
-----------------
The NAVCAM images can be either 8-bit or 12-bit data. The 12-bit
data is commonly referred to as 'uncompressed data', while the
8-bit is referred to as 'compressed data'. This compression is
accomplished by a 12-bit to 8-bit square-root look-up-table
compression method, which is implemented in the hardware of the
camera electronics. This compression is lossy and the estimate of
the 12-bit image can be recovered using the look-up table
mentioned in Appendix 3 of the Calibration Document.
In uncompressed mode with 12-bit data, the pixels are expressed
in two bytes, as 16 bits per pixel. The upper nibble of the most
significant byte is always zero for these images. In compressed
mode with 8-bit data, the pixels are expressed in a single byte.
Exposure Durations
------------------
The double-bladed shutter utilized by the camera has a delay in its
slide mechanism that introduces an offset in the actual exposure
time, compared to the commanded exposure time. During the
recalibration of the NAVCAM as part of the Stardust NExT mission, it
was determined that the original information about the exposure time
offsets and the forward/reverse shutter parity were incorrect. In
this version (2.0) the images have been corrected to account for the
updated information. See the file SHUTTER_CORRECTION.ASC in the
documents directory for more information.
Target Name in the Image Labels
-------------------------------
The target name in the image labels was set only for the images
where the target is either seen in the image or computed to be
with the camera field of view. For all other images the target
name was set to ``N/A'.
Consequently the label geometry items pertaining to the target
-- spacecraft-target position, velocity and distance, pixel
scales, and phase angle -- are only supplied for the images where
target name is not ``N/A' and were computed for that specified
target.
Noise in the Images
-------------------
If the images are stretched to the limit, regular wide
vertical stripes appear in every image (orignally at the
level of 1 to 2 dn above background). These appear to be the
product of coherent noise somewhere in the data stream or from
a power supply. Their source is unknown.
Ancillary Data
==============
The geometry items included in the image labels were computed
using the following SPICE kernels archived in the Stardust SPICE
data set, SDU-C-SPICE-6-V1.0:
Kernel Type File Name
------------ ---------------------
LSK naif0008.tls
PCK pck00007.tpc
PCK sdu_wild2_v01.tpc
SCLK sdu_sclkscet_00141.tsc
FK sdu_v20.tf
IK sdu_navcam_v22.ti
SPK sdu_l_2003_w2.bsp
SPK sdu_l_2004.bsp
SPK sdu_w2_opnav.bsp
CK (s/c) sdu_sc_rec_2003_w2_v2.bc
CK (s/c) sdu_sc_rec_2004_v2.bc
CK (s/c) sdu_sc_rec_w2_opnav.bc
CK (camera) sdu_nc_rec_v2.bc
Coordinate System
=================
Geometric Parameter Reference Frame
-----------------------------------
Earth Mean Equator and Vernal Equinox of J2000 is the inertial
reference system used to specify observational geometry items
provided in the image labels. Geometric parameters are based on
best available SPICE data at time of image creation.
Epoch of Geometric Parameters
-----------------------------
All geometric parameters provided in the image labels were
computed at the epoch specified in the START_TIME label field.
Image Orientation
------------------
In the original version of the Stardust datasets, there were
inconsistencies in the image orientation and the definition of
the TWIST_ANGLE. In this version of the images, the orientation
has been corrected so that, when displayed in the manner defined
by the LINE_DISPLAY_DIRECTION and the SAMPLE_DISPLAY_DIRECTION,
the images will have the correct orientation as seen on the sky.
Similarly, the TWIST_ANGLE (and corresponding
NORTH_CELESTIAL_CLOCK_ANGLE) have been corrected so that they
follow the PDS definition. The values in the keywords
RETICLE_POINT_RA and RETICLE_POINT_DECLINATION have also been
adjusted to reflect the updated orientation of the images.
Additional keywords have been added to the FITS header to better
define the geometry, including the Celestial and Ecliptic North
clock angles and the projected sunward vector clock angle.
In this version, all geometric parameters have been recomputed
using the most recent SPICE kernels (listed above)
Contact Information
===================
For any questions regarding the data in this archive,
contact the SBN:
Tony L. Farnham
Phone: +1 (301) 405-3856
Electronic mail address: farnham@astro.umd.edu
Department of Astronomy
University of Maryland
College Park, MD 20742
USA
or Stardust Data Management and Archive Team (SDMA):
Charles H. Acton, Jr.
Phone: +1 (818) 354-3869
Electronic mail address: Charles.Acton@jpl.nasa.gov
Boris V. Semenov
Phone: +1 (818) 354-8136
Electronic mail address: Boris.Semenov@jpl.nasa.gov
MAIL STOP 301-125L
Jet Propulsion Laboratory
California Institute of Technology
4800 Oak Grove Drive
Pasadena, CA, 91109-8099
USA
Author
------
The descriptions in this file were written by Tony Farnham, and
include relevant information that was originally archived with
the PDS archive of NAVCAM raw images.
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