Dataset overview : Framing Camera 2 is one of two identical units flying on Dawn spacecraft. This dataset includes the Reduced Data Record (RDR) version of all available images acquired during the Vesta Encounter (Approach, Survey, Transfer to HAMO, HAMO, Transfer to LAMO, LAMO, Transfer to HAMO2, HAMO-2, and Transfer to Ceres). In addition to the imagery, anciliary information is stored within the image headers, as well as a detailed account of all the reference files used for the calibration. Calibration files needed for further processing are archived as a separate data set. The images in this dataset have been radiometrically calibrated by means of fully reversible procedures, transforming the value of the pixels from digital units to applicable engineering units without modification of the geometry. For more information on the contents and organization of the volume set refer to the aareadme.txt file located in the root directory of the data volumes. Parameters : Compression: FC images may optionally be compressed by wavelet transformation. Wavelet compression: Images may be integer or floating point wavelet transform-compressed in the DPU. The integer approach produces lossless compression, while the floating point can induce distortions of +/-1 DN. The encoding of the wavelets ensures that any length of the data starting at the beginning can be decoded, thus allowing to compress further by discarding the trailing part. Windowing: In order to manage downlink resources, up to 10 portions of the image can be selected in the DPU and downlinked as subframes. The subframes are allowed to overlap. During EDR construction each subframe is stored separately and also mosaicked into the original image. Exposure Control: The exposure time of FC images is set by command for each image. Pointing: The Framing Cameras are fix mounted onto the Dawn Instrument deck, with the nominal alignment of the optical axis with the +Z axis of the spacecraft. Deviations from the nominal are documented in the calibration data. Filter selection: The FC has 8 filters at its disposal, numbered from F1 to F8. F1 is a broadband (clear) filter, sensitive in the wavelength interval from 400 to 1100 nm peaking at 700 nm. The narrow-band color filters are of the interference type, and have wavelengths ranging from 430 to 980 nm. Processing : An FC image downlinked by the spacecraft unpacks into a succession of one or more compressed image subframes with an attached PDS headers containing housekeeping items that include full status of the instrument hardware, software configuration, temperature, voltage and current readings. The data in one EDR consists of one or more reconstructed image frames, with the accompanying header translated into text format in the label. The dimensions and location (pixel coordinates) of each frame are included for proper further processing. Each frame is formatted into one file (suffix *.IMG), with an attached label. Subframes are retained as separate entities and also are reassembled into their original coordinates in the 'summary' image. Parts of the reconstructed image not included in the subframes are given a value of zero. The label contains all relevant housekeeping information in calibrated form (e.g. degrees Celsius, volts, or amperes). The calibration process is divided into several steps that aim to remove the effect of different source of noise: conversion from 16-bit integer to 32-bit floating point, read-out bias, dark current, read-out smear, flat field non-uniformity, correction of under-sensitive pixels, exposure normalization, radiometric calibration. For more detailed informatin on the calibration process, please refer to [SCHROEDER&GUTIER2011].

Exposure times : Due to an idiosyncracy of the proximity electronics, that takes approximately 193 milliseconds from the commanding of the image acquisition to the actual start of the exposure, the images report the commanding time and the end of exposure. This results in a difference of 193 milliseconds between the usual calculation of exposure time (STOP_TIME - START_TIME) and the reported EXPOSURE_DURATION, which is always shorter than the difference of the other two. Data Coverage : The following table provides a list of major image losses and anomalies which affect FC science data coverage. --------------------------------------------------------------------- Start Phase/Obs Date Description --------------------------------------------------------------------- VSA/OpNav #11 11/179 All images from were lost due to a spacecraft safing. VSA/RC3 11/205 16 images were 0 ms exposures and are not included in the L-1B data set. VSA/RC3B 11/205 16 images were 0 ms exposures and are not included in the L-1B data set. VSH/Cycle 1/ 11/276 104 images were tranmitted in raw format due Orbit 9 to an overflow in the number of images in camera memory. VSH/Cycle 1/ 11/277 68 images were tranmitted in raw format due Orbit 10 to an overflow in the number of images in camera memory. VSH/Cycle 2/ 11/278 All FC images from orbit 1 were lost due to a Orbit 1 delay in uploading science command sequences. VTL/OpNav #2 Observation cancelled as the spacecraft was on the night side of Vesta at the scheduled time. VTL/OpNav #3 Observation cancelled as the spacecraft was on the night side of Vesta at the scheduled time. VSL/Cycle 1 11/347 131 images lost due to a spacecraft safing. VSL/Cycle 6 12/014 All images lost due a spacecraft safing. VSL/Cycle 11 12/049 40 images lost due to a spacecraft safing. VSL/Cycle 13 12/063 252 images lost due to a spacecraft safing. --------------------------------------------------------------------- Data Quality : The following table contains a list of known corrupted images. These images have been omitted from the RDR data set. --------------------------------------------------------------------- Corrupted Images --------------------------------------------------------------------- Phase/Obs Image # Notes --------------------------------------------------------------------- VSA/MoonSrch 0002889 image double exposed VH2/Cyc3/Orb9 0028659