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STARDUST NAVCAM CALIBRATED IMAGES OF 81P/WILD 2 - VERS 3.0 (PDS)

NSSDCA ID: PSSB-00655

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 Level 3 (RDR; calibrated) approach and encounter images taken by the Stardust Navigation Camera (NAVCAM) targeting comet 81P/Wild 2 (1978 A2). The impetus for this data set delivery is to apply the updated NAVCAM calibration procedure, developed during the later Stardust-NExT extended mission, to the approach and encounter images of the target body. Therefore, the images in this data set are only a subset of all images taken during approach and encounter; calibration images, and early approach images in which the target body cannot be detected, have been intentionally excluded.

Data Collection Periods

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 documents ``Log of Stardust NAVCAM Flight Images'', DOCUMENT/PIIMGLOG.LBL, and the detailed data set description, from earlier data sets containing these data, in CATALOG/OLD_DATASET_CAT.TXT. N.B. The NAVCAM data collection periods listed here have gaps between the stop time of one period and the start of the next; this is intentional and consistent with the NAVCAM data set in that no NAVCAM image data were taken between these periods. N.B. The NAVCAM data collection periods listed here overlap, but are defined differently than, the mission phases defined in the mission catalogs for this mission and for the extended mission (NExT). The following sections list the NAVCAM data collection periods. Approach -- 2003-11-13 to 2003-12-30 -- Images 528-1014 -----------------------------------------------------Approach imaging was mostly windowed, and comprised attempts to test and/or characterize various items: background stars; Wild 2 presence; auto-NAV pattern matching both through and off the periscope; different mirror angles and windowing sizes; optical navigation; exposure time response; a failed photometric calibration; assess camera decontamination. Exposure durations ranged from 1s to 20s, with most at 5s, 10s or 15s. Final approach -- 2003-12-30 to 2004-01-02 -- Images 1015-1122 -------------------------------------------------------------Final approach imaging during the last few days comprised windowed imaging at 5s and 10s exposure durations. Wild 2 Encounter -- 2004 JAN 02 -- Images 2005-2115 --------------------------------------------------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. Commanding constraints limited in the number of changes that could be made in the exposure time, so most were taken at settings of either 100ms or 10ms. Many of the longer exposures were saturated, but serve to bring out the many jets of gas and dust ejected by the comet. Scan mirror angles range from 1 to 176 The images with mirror angles over 170 degrees all exhibit a great deal of scattered light, probably from the sample return capsule. There are lesser amounts of scattered light in 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.

Instrument and Data Calibrations

Calibration sources ------------------The calibration data for NAVCAM were derived from pre-launch and in-flight testing; the calibration analysis and pipeline development were done during the Stardust-NExT extended mission. Re-calibration of the 5535 Annefrank and 81P Wild 2 subsets of prime mission data was performed in 2013/2014, which resulted in this data set. The NAVCAM was specified as an engineering instrument for the prime mission to Wild 2. Its main purpose was for navigation, calibration was done on a best-efforts basis, and late hardware deliveries severely hampered those efforts. For Stardust-NExT, imaging was a key part of the science goals, and review of existing data plus extensive in-flight calibration was done to characterize NAVCAM performance [KLAASENETAL2011B]. This data set includes documents (see /DOCUMENT/DOCINFO.TXT), references to published papers, and calibration files (see /CALIB/CALINFO.TXT) detailing the calibration of the NAVCAM instrument. Data calibration process -----------------------The data calibration pipeline comprised several steps: masking pixels outside any windows; quality checks (saturation); decompression of compressed data; bias estimation and subtraction; dark-current estimation and subtraction; signal-to-noise ratio calculation; flat-fielding to remove stable pixel-to-pixel variations; calculation of DN rate; conversion to radiance. The data calibration process does not remove coherent noise (CNoise) or Fixed-Pattern Noise (FPN) from the images. See below for a brief description of these effects. N.B. Coherent Noise (CNoise) ---------------------------Coherent Noise is usually only visible in underexposed, uncompressed images when viewed using extreme contrast enhancement, and appears as stripes of noisy dark and light pixels. The CNoise variation is about +/-5DN in the raw images [KLAASENETAL2011B]. N.B. Fixed-Pattern Noise (FPN) -----------------------------Fixed-Pattern Noise (FPN) is usually only noticeable in images where the NAVCAM has been on for more than ten hours. The rise in FPN is accompanied by an associated rise in CCD temperature. It occurs independent of the contamination level of the camera. But a peculiar aspect of the FPN is that even after long power-on times with elevated CCD temperatures, the FPN does not show up in dark frames, only in those that have had the shutter open to admit some level of external photons (even if only a low-level scattered light background). Investigation of the FPN during NExT showed that the FPN level also depends on the amount of background scattered light in an image. The Wild 2 approach images had scattered light levels of <100 DN and raw FPN amplitudes of 6-20 DN rms. But during the NExT approach to Tempel 1, much higher levels of scattered light were encountered, and the FPN amplitude increased to 25-45 DN rms even when the camera had been powered on for only a short time and the CCD temperature remained low. The FPN can be largely eliminated by successive frame differencing when identical frame pairs are acquired. No evidence of FPN is found in the Tempel 1 close encounter images, which were acquired using data compression, at short power-on time, and with minimal scattered light. No attempts to correct for FPN are included in the NAVCAM processing pipeline [KLAASENETAL2011B].

Data Product Type and Format Overview

NAVCAM data files provided in this archive are divided by activity (approach; encounter). The images in this data set are in FITS format with detached PDS labels. The Primary Data Unit (PDU) of each image file in this data set is a two-dimensional array of brightnesses as measured by the array of pixels in the NAVCAM CCD, and as viewed through the NAVCAM optics. The brightnesses in the PDU are the raw Data Numbers (DNs) from the NAVCAM Analog-to-Digital Converter (ADC) as it read the voltages in the CCD pixels. In some cases these DN data have been compressed via lookup table; see CALIB/NC_COMPR.LBL. Note that, to save on limited downlink at the time some image observations were made, only rectangular window subsets of the the full-frame of pixels were telemetered to Earth. This can be identified by WINDOW objects in PDS labels, and by the appearance of the image as gray/white rectangle against a rectangles against a black background in the BROWSE/ images. Note that BLS pixels are not avalable in these windowed images, which affects their calibration. Refer to the documentation and the calibration paper [KLAASENETAL2011B] for more detail about the effect windowing has on data calibration. The data calibration converts the raw Data Numbers (DNs) to radiance, typically, or to bias- and dark-subtracted DN for images with zero exposure duration. Extension Data Units (EDUs) of calibrated data contain maps of parameters associated with each PDU image pixel: a quality map; an uncertainty map; a signal-to-noise ratio map. The value of a quality map pixel indicates whether the corresponding pixel in the PDU could be calibrated or not, and if not, then why. The value of an uncertainty map pixel gives the calculated uncertainty for the corresponding pixel in the PDU. The value of a signal-to-noise ratio (SNR) map pixel gives the calculated SNR for the corresponding pixel in the PDU. Refer to the data labels and the calibration documentation for more details about the EDUs. Additional image-synoptic data such as CCD temperature, geometry and windowing parameters are stored in the image labels. In cases where only windows of the detector were stored and downlinked, the program filled the pixels in the image corresponding to the areas for which data had not been downlinked with raw zeroes. In such images WINDOW OBJECTs define the areas containing non-null data.

Parameters

The primary parameters in this data set are brightness images, two-dimensional arrays of brightnesses corresponding to the pixels in the NAVCAM CCD, and as viewed through the NAVCAM optics. The brightnesses are the calibrated Data Number (DN) values from the NAVCAM Analog-to-Digital Converter (ADC), converted to engineering units: radiance or bias- and dark-subtracted DN. Ancillary data include quality, uncertainty signal-to-noise ratio maps, and image-synoptic data such as CCD temperature and observational geometry.

Data Processing

The images in this data set were initially assembled at the Jet Propulsion Laboratory (JPL) from raw telemetry packets sent down by the spacecraft; attached preliminary PDS labels were populated with housekeeping values and computed geometry parameters from SPICE kernels. The JPL image files were then converted to FITS format, the data were calibrated and also stored in FITS format, and finally detached PDS labels for the FITS files were generated. The data calibration portion of the pipeline comprised several steps, summarized in Data Calibration above.

Ancillary Data

The geometry items included in the image PDS labels were computed using the SPICE kernels archived in the Stardust SPICE data set, SDU-C-SPICE-6-V1.0 [SEMENOVETAL2004B]; refer to that data set for details. Lockheed Martin Astronautics (LMS) provided image command logs, which were needed to calibrate the data; see /CALIB/CALINFO.TXT for details.

Reference Frames

The geometry items provided in the files are relative to the J2000 reference frame. Refer to the description of the geometry table columns in /INDEX/INDEX.TAB to see which parameters are defined in which frame. Epoch of Geometric Parameters: All geometric parameters provided in each image PDS label were computed at the epoch specified in the start time for that label.

Software

The images in this data set conform to the FITS standard, and have standard PDS image labels. They can be viewed by a number of PDS-provided and/or open-source and/or commercial programs. For this reason no data-set-specific software is provided with this data set.

Contact Information

For any questions regarding this archive, contact the Planetary Data System Small Bodies Node.

These data are available on-line from the Planetary Data System (PDS) at:

http://pdssbn.astro.umd.edu/holdings/sdu-c-navcam-3-rdr-wild2-v3.0/

Alternate Names

  • SDU-C-NAVCAM-3-RDR-WILD2-V3.0

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. Brian CarcichGeneral ContactCornell Universitycarcich@astro.cornell.edu
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