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STARDUST NAVCAM IMAGES OF ANNEFRANK - Version 2.0 (PDS)

NSSDCA ID: PSSB-00184

Availability: Archived at NSSDC, accessible from elsewhere

Description

This description was generated automatically using input from the Planetary Data System. 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

Alternate Names

  • SDU-A-NAVCAM-2-EDR-ANNEFRANK-V2.0

Discipline

  • Planetary Science: Small Bodies

Additional Information

Spacecraft

Experiments

Questions and comments about this data collection can be directed to: Dr. David R. Williams

 

Personnel

NameRoleOriginal AffiliationE-mail
Dr. Ray L. Newburn, Jr.Data ProviderNASA Jet Propulsion Laboratorynewburn@scn1.jpl.nasa.gov
Dr. Ray L. Newburn, Jr.General ContactNASA Jet Propulsion Laboratorynewburn@scn1.jpl.nasa.gov
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