Dataset overview  :  Framing Camera 2 is one of two identical units flying on Dawn  spacecraft. This dataset includes the Experiment Data Record (EDR)  version of all available images acquired during the Mars Flyby phase.  In addition to the imagery, ancillary information is stored within the  image headers. Calibration files needed for further processing are  archived as a separate data set. This EDR dataset is the primary record  of image data as it was received on Earth. The images in this dataset  have not been processed in any way other than by decompressing,  extracting header information, and generating and attaching PDS labels.   This data set includes individual dark and calibration lamp images.  These images are the inputs to the composite calibration files which  are needed for further processing of the raw images. The composite  calibration files are archived as a separate data set.   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.   Darks are acquired in a different mode than normal images, leading to  different cropping and therefore different image file structure.  Darks are acquired as full 1092x1056 pixel images, which are cropped  due to operational considerations to include 1088x1056 (Frame 0) and  4x1056 (Frame 6) allowing the complete reconstruction of the original  full frame. In contrast, normal images are cropped into several  smaller sections (the most prominent of which is the 1024x1024 active  area), and therefore have several smaller frames.   For further details on the layout of the data and the possible  combinations of frames, please refer to the Software Interface  Specification.   The label contains all relevant housekeeping information in calibrated  form (e.g. degrees Celsius, volts, or amperes).

Exposure times  :  Due to an idiosyncrasy 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  :  Dawn approached Mars from the dark side and most of the scheduled  observations were planned for after closest approach (C/A). However,  the spacecraft entered SAFE MODE very shortly after C/A and nearly  all of the planned images were never acquired. Most of the approach  images were still in the camera and were lost when the power was shut  off by the spacecraft. Only about 40 total images were returned and  most of these were acquired on the dark side of the terminator.  The images from the lit side of Mars acquired near C/A were expected  to be highly smeared. Some of these images were intentionally  overexposed while others have the exposure time set to zero as an  engineering activity.