| DATA_SET_DESCRIPTION |
Data Set Overview
=================
This data set contains raw pre-encounter and encounter images
taken by the Stardust Navigation Camera during the encounter
with asteroid Annefrank. This is version 2.0 of the dataset, in
which the IMG formatted images have been converted to FITS
format, and a number of corrections, described below, have been
applied. This version supersedes version 1.0.
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.
2002-09-03: Image Sequence #32 (Images 301-324)
-----------------------------------------------
In preparation for engineering readiness tests utilizing the
asteroid #5535 Annefrank, as STARDUST once more approached the
Sun and Earth sufficiently to begin limited imaging, a first
test was made of the new pattern matching and windowing
software. Coincident with this, a series of geometric
calibrations was attempted, since those of June 2001 were not
totally successful. In addition a calibration lamp image and
four full frame fields were acquired at zero and thirty
degrees, one of the latter compressed. These were intended as a
modestly comprehensive check for contamination, for scattered
light, and for compression. It was found that there had been a
small amount of recontamination in the 10 months since the
previous image. This was most obvious in the calibration lamp
image. Star images remained sharp, with the same point spread
as earlier, but with a very shallow skirt of scattered light.
The pattern matching and windowing failed at 14 of the 19
angles. At larger scan mirror angles there was a problem with
increasing scattered light. The windows used were only 21x21
pixels, and it became clear that somewhat larger windows were
necessary and that there were still geometric calibration
problems. The contamination on the periscope was found to be
significantly reduced compared to that of two years earlier,
perhaps due to some evaporation of the condensate into the
vacuum of space.
2002-10-09: Image Sequence #33 (Images 325-345)
-----------------------------------------------
This series of images again was intended as a test of pattern
matching and windowing and to supply some geometric calibration
of the system. The camera was brought above freezing for 60
hours and then allowed to cool back to normal operating
temperatures in an effort to remove contamination before
initiating these exposures. The series consisted of 20 pattern
matching and windowing tests, ten each at 47.8 and 64.0 degree
scan mirror settings, each image consisting of four 41x41 pixel
windows, and one full frame image at a scan mirror setting of
15 degrees to check on the effect of a split field (partially
on and partially off the periscope). Half of the 47.8 degree
and all of the 64.0 degree tests were successful in locking up
on the desired pattern of stars, but the target stars still
were not well centered. These were engineering tests and led to
significant improvement in the software and to a better
understanding of spacecraft behavior and capabilities, but the
images provided little useful data for any sort of photometric
calibration following the fourth heating cycle that preceded
this series. The split image indicated that it should be
possible to use the periscope on Wild 2 approach as always
intended. Good geometric calibration of the periscope still
remains to be carried out, and the periscope was not used for
the Annefrank encounter. In the absence of any dust hazard, it
was not necessary to keep the spacecraft oriented along the
velocity vector, so Annefrank tracking utilized mirror angles
from 17.7 to 111.3 degrees.
2002-10-31: Image Sequence #34 (Images 346-350)
-----------------------------------------------
One of the goals of the engineering readiness tests on
Annefrank was to exercise the optical navigation team and to
attempt to improve flyby accuracy using optical data. The
approach to Annefrank was from a phase angle of 150 degrees,
unfortunately, which meant the asteroid would be poorly
illuminated and be very faint. There were no asteroid data
available for phase angles larger than 100 degrees. It was
assumed that Annefrank would be about 1.5 magnitudes fainter
than the nearly linear decrease of about 0.03 mag/deg that is
common to asteroids at smaller phase angles. It seemed that we
would have a fair chance of detection 38 hours before
encounter, which time was used for this first attempt. Five
images were obtained using three 151x151 pixel windows and
exposures of 1, 1, 2, 5, and 5 seconds. As we later found out,
the asteroid was much fainter than expected and the spacecraft
drift during the exposures (smear rate) much larger than we had
previously experienced. Further the camera pointing was not as
accurate as we had expected (the geometric calibration was not
yet solid). The use of new controller software, inadequate
settle times after attitude changes, and a larger moment of
inertia with the aerogel grid open were all suggested as
reasons for the drift and pointing problems. The cause is still
being investigated. This is why these tests were run, to make
sure something like this doesn't happen to us on Wild 2. The
bottom line is that the asteroid was not found in these five
E-38 hour images.
2002-10-31: Image Sequence #35 (Images 351-355)
-----------------------------------------------
A second set of approach images was acquired at E-32 hours. The
same windows and exposure times were used as for the previous
set at E-38 hours. The problems were much the same, as were the
results. Annefrank was not found.
2002-10-31: Image Sequence #36 (Images 356-360)
-----------------------------------------------
A third set of images was acquired at E-26 hours. The same
windows and exposure times were used as for the previous sets
taken at E-38 and E-32 hours. The problems were much the same,
as were the results. Annefrank was not found.
2002-11-01: Image Sequence #37 (Images 361-365)
-----------------------------------------------
Given the experience of the first three sets of approach
images, the navigators decided to increase the window size to
181x181 pixels and make all of the exposures 5 seconds for this
set at E-18 hours. The image smear can only be described as
horrendous. This doesn't matter for measurement purposes, IF
the target can be found. The asteroid was still several
magnitudes too faint for detection when smeared over some 20
pixels, and it was not located.
2002-11-01: Image Sequence #38 (Images 366-370)
-----------------------------------------------
A final set of approach images was attempted at E-12 hours.
This time all of the available communication bandwidth was
given to one window in one image (#368), making it 701x701
pixels. The other four images were given 3x3 pixel windows and
were retained only to avoid having to reprogram and transmit
too much last minute new command software. The smear was still
large (21.7 pixels) and the asteroid still was not located.
2002-11-01: Image Sequence #39 (Images 371-407)
-----------------------------------------------
Twenty-five minutes before the closest approach, images were
acquired to attempt autotracking. Pointing was based upon radio
navigation of the spacecraft and the best ephemeris for the
asteroid supplied by JPL's celestial mechanics specialists. By
this time the phase angle was down to 130 degrees and the range
was only 11,415 km. Annefrank appeared in the first image,
though far from centered. The navigators chose an exposure of
65 ms to make sure they were going ``deep enough,'' so the
images were well exposed. After the first few images, only
every third image was transmitted to the ground, the others
being used only to initiate autotrack. After 15 minutes, at a
range of 5434 km, exposure was reduced to 25 ms. In all, 15 of
37 images taken with 65 ms exposure were received on Earth. Of
these, the first two or three were partially on the periscope,
and three show a large amount of smear, but several are of
scientific use. Autotracking was initiated shortly before
reducing the exposure, and image 410 and all subsequent
Annefrank images are well centered in their frames.
2002-11-01: Image Sequence #40 (Images 410-445)
-----------------------------------------------
Exposure times on Annefrank were reduced to 25 ms beginning
with image 410 at a range of 5088 km and a phase angle of 113
degrees. Images beginning with #420 started to show saturation.
This was predicted, but these images were being taken to test
the autotracking rather than for scientific purposes, and
autotrack works perfectly well with saturated images. The
images soon reached 80% saturation, so images 420 through 445
are of limited scientific use. Every image was transmitted to
the ground beginning with #426, a total of 26 images with 25 ms
exposure. Twenty-two of these have some to nearly total
saturation.
2002-11-01: Image Sequence #41 (Images 446-476)
-----------------------------------------------
Beginning with image #446, exposure time was reduced to 5 ms.
In fact the characteristics of the shutter are such that
alternate images are given exposures shorter by 1.5 ms than the
set value, so in fact all even numbered images have an exposure
of 3.5 ms and odd numbered ones 5 ms. It was intended that
these images be of scientific as well as engineering use. If
Annefrank had not been acquired by this time, there was little
hope of acquiring it, so there was no need to saturate the
images. The subsequent images (through image 476) taken at
phase angles from 71.0 to 47.2 degrees constitute the best
images for scientific use. During this period the range fell
from 3133 km to 3078.5 km and increased back to 3162 km, so
there is minimal change to scale.
Processing
==========
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 were used to define the areas containing non-null data.
The images were converted from PDS format into FITS format at
the PDS Small Bodies Node. For the images that originally
consisted of window objects, the window data has been embedded
into full-frame images, with regions that were not downlinked
set to zero. During the conversion process, the overscan pixels
on both sides of the images were recovered from the IMG
files. (For consistency, the overscan regions were inserted into
the windowed images, though their pixel values are set to zero.)
Also, the orientation set up so that the images are properly
oriented when displayed as defined by the LINE_DISPLAY_DIRECTION
and SAMPLE_DISPLAY_DIRECTION keywords, and the twist angle was
corrected to conform to the PDS definition.
Data
====
The images in this data set were originally produced in standard
PDS format, and then converted to FITS format. Each file
includes a detached PDS label. The label also describes the
circumstances surrounding the collection of the calibration
image.
Camera Description
------------------
The camera has a 1024x1024 array as the active portion of the
CCD. The images that are stored on this volume, however, contain
more than just the active portion of the CCD. Each line contains
a sync pattern, a line counter, 12 baseline stabilization pixels,
the 1024 pixels from the active portion of the CCD, and finally 8
over-clock pixels used to measure the quantum efficiency. The
number of rows for each image is always 1024, no matter what
compression mode is used, but the number of columns for each
image depends on the compression mode used.
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. Both the
image and histogram portions of the data file require different
amounts of storage space, dependent on the compression mode used.
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.
During the conversion to FITS format, all of the images were
stored as 16-bit integers. A record of whether the original
data were compressed or not is retained in the
COMPRESSION_TYPE keyword in the PDS labels (COMPRESS in the
FITS header). A value of '8_BIT' indicates the 8-bit data, and
a value of 'NONE' denotes uncompressed images.
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. In
version 1.0 of this dataset, an attempt was made to correct
this problem, however, 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 ``CALIBRATION FIELD''.
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 ``CALIBRATION FIELD'' and were computed
for that specified target.
Windowed Images
---------------
The IMAGE size parameter in the image label reflects the size
of the detector, however in some cases 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).
Noise in the Images
-------------------
If the images are stretched to the limit, regular wide
horizontal stripes appear in every image 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_annefrank_v01.tpc
SCLK sdu_sclkscet_00141.tsc
FK sdu_v20.tf
IK sdu_navcam_v22.ti
SPK sdu_l_2002.bsp
SPK sdu_w2_opnav.bsp
CK (s/c) sdu_sc_rec_2002_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 and Geometry
------------------------------
In version 1.0 of this data set, the image orientation was was
flipped from what would be seen by an actual observer. In
addition, the TWIST_ANGLE that was given did not correspond to
the definition in the PDS Data Dictionary. Both of these
inconsistencies have been corrected in this version of the data.
When displayed as defined in the SAMPLE_DISPLAY_DIRECTION and
LINE_DISPLAY_DIRECTION keywords, the image would appear as it
would to an in situ observer, and the TWIST_ANGLE corresponds to
the definition given in the PDS Data Dictionary. 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)
Software
========
The images in this data set are in standard FITS format and,
therefore, can be viewed by any standardized FITS reader. For
this reason no special software is provided with this data set.
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
|
.png)
.png)
