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-V1.0).  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.  This version contains an error in the exposure times of some of the images (see 'Exposure Duration' comments below). This error has been corrected in Version 2.0 of the dataset. The calibrations have also been corrected, to account for the actual exposure time.  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-V1.0).  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.TXT 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 ------------------ NOTE: Below it states that the exposure times have been adjusted so that the correct times are given in the labels. This is incorrect. The labels in this version of the data still contain the downlinked times. The exposure times and calibrations have been corrected in Version 2.0 of this dataset (SDU-C-NAVCAM-3-RDR-WILD2-V2.0).  The double-bladed shutter utilized by the camera has the property that in one direction the exposures are 1.65 ms shorter than in the other. Therefore a setting of 5 ms, which is the shortest possible, results in alternate 5 and 3.35 ms exposures, those at 25 ms, alternate 25 and 23.35 ms exposures, and so on. Occasionally bias frames, which do not require shutter action, are transmitted to Earth. This changes the ``parity.''  While not always the case, for Wild 2 image sequences the even numbered images have the shorter exposures. The downlinked exposures for these images have been adjusted and the correct exposure duration values are provided in the labels.  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 horizontal 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 naif0007.tls PCK pck00007.tpc SCLK sdu_sclkscet_00105.tsc FK sdu_v17.tf IK sdu_navcam_v21.ti SPK sdu_l_2003_w2.bsp SPK sdu_w2_opnav.bsp CK (s/c) sdu_sc_rec_2003_w2_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 archive of raw NAVCAM data, there is a discussion of the ambiguities and 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.  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 includes relevant information that was originally archived with the PDS archive of NAVCAM raw images.

Calibration Quality :  The absolute calibration of the Wild 2 images is relatively crude due to the problems associated with the contaminated optics in the navigation camera. The data presented here represent the best attempt at producing an absolute calibration, using the standard star measurements obtained eleven days after the comet encounter. Further details regarding the calibration can be found in the calibration discussions in the DOCUMENTS directory.  Data Coverage :  The images contained in this archive comprise all of the Stardust images of comet Wild 2 (including the navigation frames and encounter data) as well as the calibration images that were obtained after the encounter. Some of the images in the navigation sequences did not contain the comet and these images have not been included in this dataset.  Review :  The data were peer reviewed on 30 April 2007.