PDS_VERSION_ID = PDS3 RECORD_TYPE = FIXED_LENGTH RECORD_BYTES = 80 OBJECT = TEXT PUBLICATION_DATE = 2014-07-01 NOTE = "Data set overview from data set SDU-A-NAVCAM-2-EDR-ANNEFRANK-V1.0 provided with this newer version of those data. " END_OBJECT = TEXT END Preface ======= The Stardust prime mission Wild 2 NAVCAM data have been re-delivered using the improved calibration developed as part of the Stardust-NExT mission. The data set description in the data set catalog for the new data sets SDU-C-NAVCAM-2-EDR-WILD2-V3.0 SDU-C-NAVCAM-3-RDR-WILD2-V3.0 SDU-A-NAVCAM-2-EDR-ANNEFRANK-V3.0 SDU-A-NAVCAM-3-RDR-ANNEFRANK-V1.0 has been re-written and mostly shortened. This document contains the data set overview from the old data set description; it has more detail of the imaging sequences. Note that the old description has been copied verbatim, and has not been corrected for typos or grammar. Note also that the old document describes more imaging (e.g. calibrations; sequences where the target was not in the downlinked data) than the new data set, which only contains data that capture, or nearly so, asteroid Annefrank Refer to the original data set for that additional imaging. The old data set overview follows the next two lines. ======================================================================== 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. 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.