Data Set Information
DATA_SET_NAME MESSENGER MDIS EXPERIMENT DATA RECORD V1.0
DATA_SET_ID MESS-E/V/H-MDIS-2-EDR-RAWDATA-V1.0
NSSDC_DATA_SET_ID
DATA_SET_TERSE_DESCRIPTION
DATA_SET_DESCRIPTION Data Set Overview : The Mercury Dual Imaging System (MDIS) consists of two cameras, a Wide Angle Camera (WAC) and a Narrow Angle Camera (NAC), mounted on a common pivot platform. This dataset includes the Experiment Data Record (EDR) version of all available images acquired during the cruise phase to Mercury and includes post-launch checkout images, flyby images of Earth, Venus, Mercury, images acquired from Mercury orbit, and in-flight calibration images. 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 dataset also includes ancillary data files that tabulate the contents of the volume and documentation files. File names follow the structure: 'pcrnnnnnnnnnf', where p : product type : E for raw EDRs c : camera (W WAC or N NAC) r : spacecraft-clock-partition-number minus 1 [0, 1], associates with pre- or post-spacecraft-clock-reset nnnnnnnnn : Mission Elapsed Time (MET) counter taken from the image header (and same as original compressed filename from SSR). NOTE: this a spacecraft clock seconds counter, and the value in the filename corresponds to the LAST second of the exposure. f : Filter wheel position (A, B, C, D, E, F, G, H, I, J, K, L, U) for the WAC. It is M for the NAC, which has no filter wheel. It is U if the position is unknown. 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. Versions : The image content of EDRs has been unchanged to date. Beginning with MDIS PDS release 4 containing data through Mercury flyby 2, the labels were modified to include information on image statistics such as minimum, maximum, and mean values. Beginning in MDIS PDS release 5, which contains data through Mercury flyby 3, labeling of EDRs was updated in response to an MDIS flight software upload in August 2009. Several additional items of housekeeping were added to the headers of downlinked images, and propagated into the EDR labels. Beginning in MDIS PDS release 6, which contains data through the beginning of Mercury orbit, three additional items were added to EDR labels providing information on timing and image targeting. Parameters : MDIS observing scenarios are constructed using a set of key variables ('configurations') which include the following. All, with the exception of filter selection, are available for both the NAC and the WAC. Only the WAC has selectable filters. The imagers can only be used one at a time. Compression: MDIS images may optionally be compressed in a number of ways, including pixel binning on-chip, 12-8 bit compression, and FAST/Differential pulse code modulation (DCPM) lossless compression, all carried out in the instrument hardware; further pixel binning, subframing, wavelet compression, and jailbars, are all carried out in the spacecraft Main Processor (MP), either individually or in combinations. Pixel Binning: MDIS images can undergo 2x2 pixel binning in the focal plane hardware (also known as 'on-chip' binning), resulting in a 512 x 512 image. Images can also be compressed using the MP, either in addition to DPU binning, or instead of DPU binning. MP pixel binning options of 2x2, 4x4 or 8x8 are available. 12-8 bit compression: Images are read off the detector in 12-bit format. 12 bit images may converted to 8 bit images using lookup tables (LUTs) designed to preferentially retain information at low, medium, or high 12-bit DN values. Eight LUTs are available, and shared between the NAC and WAC. FAST/DPCM compression: All images are compressed losslessly using FAST/DCPM compression as they are read out of the DPU, to conserve recorder space. Once the data are written to the recorder, they can be uncompressed and recompressed more aggressively in the MP. Wavelet compression: Images may be integer wavelet transform- compressed in the MP, typically at 3:1 for color data and 4:1 for monochrome data, but any value from 1 to 32 can be used. Subframing: In order to manage downlink resources, up to 5 portions of the image can be selected in the MP and downlinked as subframes. The subframes are allowed to overlap. During EDR construction the subframes are all mosaicked into the original image. During orbital operations this option is not regularly used. Jailbars: Intended for data management during optical navigation, jailbars are selected columns of an image retained by the MP. Commanded column spacing values are not restricted and can be set to any integer value between 1 and 1024, but the spacing is fixed throughout the image. During EDR construction the jailbars are all mosaicked into the original image. During flight this option is not regularly used. Exposure Control: The exposure time of MDIS images can be set manually by command, or automatically by the software. In manual mode, exposure times from 1-989, 1001-1989, ..., to 9001-9989 ms are available. In autoexposure mode the exposure time of the next image is computed by the DPU software, and cannot exceed 989 ms in duration. If the time of the next image occurs before the calculation can be completed, and pixel binning or filter position change, then the algorithm compensates for predicted changes in scene brightness and filter transmission using an onboard data structure. Pointing: The MDIS imagers are mounted on a pivot platform, which is itself mounted to the MESSENGER spacecraft deck. The pivot platform is controlled by a stepper motor, which is controlled by the Data Processing Unit (DPU). The pivot platform can move in either direction. The total range of motion is 240 degrees, limited by mechanical 'hard' stops, and is further constrained by 'soft' stops applied by the software. The nominal pointing position for MDIS is defined as 0 degrees, aligned with the spacecraft +Z axis and the boresight for several other instruments. The range of the soft stops is set to 40 degrees in the spacecraft -Y direction (toward the MESSENGER sunshade) and +50 degrees in the +Y direction (away from the sunshade). The pivot position can be commanded in intervals of 0.01 degrees within this range. Filter selection: The WAC imager contains a 12 position filter wheel to provide spectral imaging over the spectral range of the CCD detector. Eleven spectral filters span the range from 395 to 1040 nm, while the twelfth position is a broadband filter for optical navigation. Processing : An MDIS image downlinked by the spacecraft unpacks into a succession of one or more compressed image subframes with binary headers containing housekeeping items that contain full status of the instrument hardware, including imager, software configuration, temperature, voltage, and current readings, pivot position, and a time stamp. The data in one EDR consists of a single reconstructed image frame, with the accompanying header translated into text format in the label. Each frame has dimensions of spatial samples in the form of detector columns and detector rows. Each frame is formatted into one file (suffix *.IMG), with an attached label. Each image has dimensions XX pixels in the sample dimension and YY pixels in the line dimension, where: XX (columns) : 1024/binning, where 1024 is the number of columns read off the detector, and binning is 2, 4, or 8 (the product of binning at the instrument level and by the MP). YY (rows) : 1024/binning, where 1024 is the number of columns read off the detector, and binning is consistent with the sample binning. Subframes are not retained as separate entities and are reassembled into their original coordinates in the image. Parts of the reconstructed image not included in the subframes are given a value of zero. Pixel binning and 12-to-8 bit compression are not inverted at the EDR level. The label contains all of the housekeeping information in their raw form. Selected ones are listed in duplicate in calbrated form (e.g. degrees Celsius, volts, or amperes. In addition, value-added information in the label describes image pointing and objectives. Files needed to calibrate the EDRs to radiance or I/F are found in the CALIB directory where their usage is described. There are up to six successive corrections applied: (1) removal of dark current; (2) removal of frame transfer smear; (3) correction for minor detector non-linearity; (4) 'flat field' correction for pixel-to-pixel response non-uniformity; (5) correction for detector responsivity and exposure time, yielding radiance; (6) division of radiance by the solar spectrum scaled by the square of solar distance, yielding I/F; and (7) optionally, application of an empirical correction for time variability of WAC responsivity. The calibration procedure is itself versioned. Version 1 was the prototype version produced internally to the MDIS team for validation. Version 2 was the first publicly released version of MDIS calibration, first released in MDIS PDS release 3 which contains data through Mercury flyby 1. In version 2 the calibrations of WAC filters 3 and 6 (C and F, at 480 and 433 nm) were adjusted from the version 1. Version 3 was first released in MDIS PDS release 4 containing data through Mercury flyby 2. In version 3 the accounting of CCD temperature in the responsivity correction was updated. Version 4 calibration was first released beginning in PDS release 9, which contains data from months 13-18 of Mercury orbit. In that update, four further corrections and additions to radiometric calibration are implemented. (a) A small error in calculation of the correction for frame transfer smear is corrected. (b) The flat-field correction for WAC filters 3 and 6 is updated. (c) The WAC temperature-dependent responsivity is updated again. (d) A new term 'Correct' is added to the calibration equation to correct for time variation in responsivity in all WAC filters beginning in May 2011. Version 5 CDRs are released in PDS release 15, the final end-of-mission data release which includes all data. In this update, three further corrections and additions to radiometric calibration are implemented. (a) The flat-field correction for several WAC filters in non-binned and binned states is updated, using the same approach of normalized median values of low-contrast field filling images acquired during orbit, except using more images less affected by compression than in version 4 CDRs. (b) The NAC and WAC temperature- dependent responsivity are updated based on further analysis of images acquired at differing temperatures. (c) The 'Correct' term in the calibration equation is updated based on extensive analysis of overlaps of images acquired at different times, and covers the entire period of Mercury orbit. Data : There is only one data type associated with this volume, the raw uncalibrated DNs. Ancillary Data : There are two types of ancillary data provided with this dataset: 1. The GEOMETRY directory contains the file GEOMINFO.TXT that points to and describes the function of each SPICE kernel relevant to MDIS. 2. The CALIB directory contains a summary of the processing required to convert raw data to units of radiance or I/F, as well as all of the matrices and coefficients needed. See CALINFO.TXT in that directory for more details. Coordinate System : The cartographic coordinate system used for the MDIS data products conforms to the J2000 celestial reference frame for star imaging, and the IAU planetocentric system with East longitudes being positive for planetary surfaces. The IAU2000 reference system for cartographic coordinates and rotational elements was used for computing latitude and longitude coordinates of planets. Media/Format : The MDIS archive is organized and stored in the directory structure described in the Mercury Dual Imaging System (MDIS) Experimental Data Record (EDR) Software Interface Specification (SIS). The contents of the archive, along with fiduciary checksums, are compressed into a single 'zip archive' file for transmittal to the PDS Imaging node. The zip archive preserves the directory structure internally so that when it is decompressed the original directory structure is recreated at the PDS Imaging node. The zip archive is transmitted to the PDS Imaging node via FTP to the URL specified by the node for receiving it.
DATA_SET_RELEASE_DATE 2015-10-09T00:00:00.000Z
START_TIME 2004-08-19T06:01:23.000Z
STOP_TIME 2015-04-30T11:07:43.000Z
MISSION_NAME MESSENGER
MISSION_START_DATE 2004-08-03T12:00:00.000Z
MISSION_STOP_DATE 2015-04-30T12:00:00.000Z
TARGET_NAME CAL TARGET
DARK SKY
OB STAR
SPACE
SPACECRAFT DECK
STARFIELD
2P/ENCKE 1 (1818 W1)
C/ISON (2012 S1)
M7
PLEIADES
EARTH
JUPITER
MARS
MERCURY
NEPTUNE
SATURN
URANUS
VENUS
MOON
ALIOTH
ARCTURUS
EPSILON CENTAURI
ETA CARINAE
FOMALHAUT
MIRZAM
RIGEL
SIRIUS
STAR
VEGA
TARGET_TYPE CALIBRATION
COMET
OPEN CLUSTER
PLANET
SATELLITE
STAR
INSTRUMENT_HOST_ID MESS
INSTRUMENT_NAME MERCURY DUAL IMAGING SYSTEM NARROW ANGLE CAMERA
INSTRUMENT_ID MDIS-NAC
INSTRUMENT_TYPE FRAMING CAMERA
NODE_NAME Imaging
ARCHIVE_STATUS ARCHIVED
CONFIDENCE_LEVEL_NOTE Confidence Level Overview : This is a raw data set. Known issues of concern are described below. Review : This archival data set was examined by a peer review panel prior to its acceptance by the Planetary Data System (PDS). The peer review was conducted in accordance with PDS procedures. Data Coverage and Quality : THE USER OF MDIS EXPERIMENT DATA RECORDS IS URGED TO EXAMINE THE DQI IN THE LABEL FOR POSSIBLE ISSUES OF DATA QUALITY, AND TO UNDERSTAND SOURCES OF UNCERTAINTY IN THE DATA LISTED BELOW. The 16-byte Data Quality Index or DQI is used to encode figures-of-merit into one parameter, including automated assessments of validity of the exposure time, presence of an excessive number of pixels at or approaching saturation, validity of the reported pivot position, validity of the reported filter wheel position, quality of spacecraft attitude knowledge from the MESSENGER star cameras, CCD temperature within range that supports nominal image calibration accuracy, and completeness of data within the commanded selection of subframes or full frame. A '1' in any of the fields of the data quality index indicates a condition that could adversely affect data quality. A value of '1' in bytes 0, 1, or 4 leads to a raw image or EDR not being calibratable. A value of '1' in byte 2 indicates that radiance values derived from the columns of the image experiencing saturation in any row are either invalid or strongly suspect. A value of '1' in byte 3 or 5 means that even if the image is radiometrically accurate, its reconstructed pointing is suspect. A value of '1' in byte 6 (CCD out of temperature range at which radiometric calibration is well-constrained) is a warning and does not necessarily indicate invalid data. Byte 0: Image source is CCD. 1 : Image source is test pattern as indicated by MESS:SOURCE:1:Test pattern or MESS:SOURCE:2:Inverted test pattern. 0 : Image source is CCD as indicated by MESS:SOURCE:0:CCD. Byte 1: Valid exposure time. 1 : Exposure time in ms as indicated by MESS:EXPOSURE equals 0 ms (during cruise) or is less than or equal to 2 ms (orbit). 0 : Exposure time in ms as indicated by MESS:EXPOSURE is greater than or equal to minimum valid value. Byte 2: Presence of an excessive number of pixels at or approaching saturation. As saturation is approached responsivity decreases, and signal becomes nonlinear with brightness for small sources. Saturation can be exceeded for very bright or large sources once pixel antiblooming is overwhelmed. The raw 12-bit DN level indicative of the onset of saturation varies between the two CCDs. In the WAC (MESS:IMAGER:0) it is approximately 3600; in the NAC (MESS:IMAGER:1) it is approximately 3400. If a LUT has been used to convert 12-bit to 8-bit DN, then an 8-bit DN value of 255 also indicates saturation. An 8-bit 255 is encountered before saturation of the 12-bit DN in the case of LUT 1. In autoexposure mode, the typical threshold for the allowable number of saturated pixels is 5 pixels. In manual exposure mode the number of saturated pixels is uncontrolled. 1 : There are > 5 pixels exceeding the DN indicating onset of saturation. 0 : There are < 5 pixels exceeding the DN indicating onset of saturation. Byte 3: Valid pivot position. 1 : Pivot position not valid, as indicated by pivot position validity flag MESS:PIV_PV:0:invalid. 0 : Pivot position valid as indicated by MESS:PIV_PV:1:valid. Byte 4: Filter wheel in position (WAC only; requires MESS:IMAGER:0, or else value of this byte : 0). 1 : Filter wheel not in position, as indicated by either of two conditions: (a) filter wheel position validity flag MESS:FW_PV:0:invalid, (b) an excessive difference between filter wheel resolver goal and actual position as given in table below. 0 : Filter wheel in position as indicated by an allowable difference between goal and position, and by MESS:FW_PV:1. Filter wheel encoder positions FILTER_NUMBER MESS:FW_GOAL Allowable (abs(MESS:FW_POS - MESS:FW_GOAL)) 1 17376 +/- 500 2 11976 +/- 500 3 6492 +/- 500 4 1108 +/- 500 5 61104 +/- 500 6 55684 +/- 500 7 50148 +/- 500 8 44760 +/- 500 9 39256 +/- 500 10 33796 +/- 500 11 28252 +/- 500 12 22852 +/- 500 Byte 5: Quality of spacecraft attitude knowledge. 1 : Spacecraft attitude knowledge is bad (MESS:ATT_FLAG is in the range 0-3). 0 : Spacecraft attitude knowledge is good (MESS:ATT_FLAG is in the range 5-7). Byte 6: CCD temperature range. 1 : CCD out of temperature range at which performance is well calibrated (MESS:CCD_TEMP is outside a range of between 1005 and 1130, which for the WAC is -45C to -11 C, and for the NAC is -48C to -14C). 0 : CCD within well calibrated temperature range (MESS:CCD_TEMP is within the stated range). Byte 7: Completeness of data within the commanded selection of subframes or full frame. Missing frames or portions of frames are indicated in an EDR with a value of 0 (this cannot be a valid data value). 1 : There are missing data (some pixels populated with 0). 0 : There are no missing data. Bytes 8-15: spare. In addition, the following caveats are applicable to radiances and map-projected products derived from the EDRs. (1) WAC CLEAR FILTER. Filter 2 on the wide-angle camera is broad- band and designed for star imaging. Even extremely short exposure times saturate on Mercury or other typical extended sources. Flat-field and responsivity corrections for WAC filter 2 are less accurate than in other filters. (2) NAC PSF. Due to mass constraints, the NAC aperture is smaller than what is required for diffraction-limited performance. The expected size of the Airy disk (approximately, the full-width at half-maximum of the point-spread function including only effects of diffraction) is > 2 pixels. In practice the PSF is further broadened by surface imperfections of optical elements and scatter centers on optical surfaces. (3) COMPRESSION ARTIFACTS. Wavelet compression applied to science images is lossy. At higher compression ratios, compression artifacts will degrade data precision over spatial scales comparable to or smaller than several pixels. The degradation can be greater proportionally to the image dynamic range of brightness, if the data are converted from 12 to 8 bits in such a way that a 1 DN error occupies a greater fraction of the digital dynamic range. Wavelet compression was used minimally prior to Mercury orbit. The initial configuration in Mercury orbit was to perform 12 to 8 bit conversion using LUT0 for the WAC and LUT2 for the NAC, with a wavelet compression ratio usually set to 4:1 for color, 8:1 for monochrome imaging, or lossless for star imaging. Initial images exhibited unexpectedly visible compression artifacts. Beginning 19 April 2011, LUT0 and LUT2 were replaced with LUT1 which better preserves image dynamic range. Beginning 19 May 2011, targeted color images began to be acquired with lossless compression. Beginning 31 May 2011, the wavelet compression ratio for color images was reduced to 3:1 for global mapping. Wavelet compression is bypassed for a variety of types of images depending on downlink bandwidth and availability of space on the spacecraft solid-state recorder. (4) FRAME TRANSFER SMEAR. At very short exposure times (<7 ms), the time for frame transfer is close to the total exposure so that the correction for frame transfer smear may leave perceptible artifacts. In October 2011 it was discovered that a software error in the image calibration pipeline leads to small filter- and exposure time-dependent systematic errors in correction of frame transfer smear. These are corrected beginning with version 4 CDRs. (5) TIME VARIATIONS IN MDIS ATTITUDE. The orientation of MDIS relative to the spacecraft reference frame was determined inflight using star calibrations to solve for WAC-NAC coalignment, the orientation of the pivot plane, and the origin of the reported pivot position within the plane. However these alignments can be affected by thermal state of the spacecraft, or by any other events that potentially shift the position of the MDIS base relative to the spacecraft star cameras that generate the attitude measurements. Mercury and Venus flybys are thermally benign. However in Mercury orbit there are thermal perturbations from which errors in reported MDIS attitude of up to 350 microradians might be expected. Regular star calibrations are conducted in orbit and indicate up to two or more types of time variation in attitude of an image relative to a given reproted pivot position: (a) A small number of abrupt shifts of WAC and NAC image alignment, near the time of Mercury orbit insertion and again in July 2012, several hundred microradians in magnitude. The first is being addressed in an updated frames kernel, and was first describe in a frames kernel released in conjunction with release 9. The frames kernel in conjunction with release 11 and the first release of DDRs also includes the shift of NAC image alignment near the time of orbit insertion. (b) In addition to abrupt shifts of WAC and NAC image orientation, subsequent gradual drift is observed in both from star calibration images. That shift and the abrupt one in July 2012 are addressed by modifications to the pivot C kernel that incorporate drift as part of the pivot attitude. Limited star calibrations that determine the relative alignments of the WAC and NAC during the orbital phase of the mission indicate that the two fields of view drift together, so a single modified pivot C kernel is used to describe the pointing for both cameras. (6) ARTIFACTS IN GROUND-DERIVED FLAT-FIELD CORRECTIONS. Two factors make the ground-derived flat-fields less than ideal. First, MDIS's structure generated reflections so that in the calibration chamber, the illuminated source created glint off MDIS blanketing that reflected off the chamber window, adding spatially non-uniform stray light to the measurements. Eliminating the backscattering off the chamber window required acquisition of flat-fields at ambient (room-temperature) conditions at which residuals from the dark current correction introduce artifacts. Ultimately the latter approach was chosen for ground derivations. Second, there are dust donuts (shadows behind out- of-focus dust on the CCD cover glass) in the WAC. The locations of some WAC dust donuts moved during launch, and other dust donuts disappeared. Application of the ground-derived flat-fields systematically under- or over-corrects the non-identical WAC dust donuts. The flat-fields were rederived inflight using images of the onboard calibration target, and validated with bland areas of Venus during Venus flyby 2. These images were taken with a cold CCD, eliminating residuals from dark current, and are thought to have WAC dust donuts in their 'final' post-launch position. The onboard target is uniform, but at the several-percent level shows evidence for low spatial frequency glint off the MDIS structure. Therefore to rederive the flat field for each WAC filter, an average of several images of the calibration target was divided by a median-filtered version of the same image, and multiplied by a median-filtered ground-derived flat field taken through the same filter with the same instrument binning. For the NAC the same procedure was used, with the inputs being Venus images. The following regions of different flat-fields were initially subject to errors tracing back to the ground calibration. In calibrated images, too-high values of the flat fields yielded too-low values of radiance or I/F, or vice versa. The most significant issues were in the upper rows of WAC filters 3 and 6. WAC filter 2: Flat-field values are place-holders only. WAC not-binned, all filters: In the last 25 columns on the right, values that are too high by 1-2%. WAC not-binned, filter 3: In the top 32 rows, long spatial frequency errors of up to a few percent, increasing to the top of the image. WAC not-binned, filter 6: In the top 145 rows, long spatial frequency errors of up to a few percent, increasing to the top of the image. WAC binned, all filters: In the last 15 columns on the right, values that are too high by 1-2%. WAC binned, filter 6: In the top 24 rows, long spatial frequency errors of up to a few percent, increasing to the top of the image. WAC, filter 5: A weak horizontal banding in the flat-field image probably originating from ground calibration. NAC binned: The most recent flat-field is ground-derived and flight measurements do not yield any improvement. For WAC filters 3 and 6 only, the flat-fields were updated one more time in Mercury orbit, to version 5. Approximately 400 images in each filter and with DPU binning on and off (4 data sets) were calibrated using the previous flat field version, and photometrically corrected to remove cross-scene illumination gradients. To produce updated flat-fields, the median of the 400 images was scaled to unity over the same central region as with ground-derived flat-fields. A final update to version 6 in the end-of-mission delivery 15 used a similar procedure, but more images for improved statistics, and excluded images with compression artifacts. An improved photometric correction reduced low spatial frequency errors. The following filter and binning combinations were upgraded to version 6: WAC filters 3 and 6, and binned images in WAC filters 3, 4, 5, 6, 7, 9, and 12. (7) RADIOMETRIC ACCURACY. The responsivities used to convert DNs to radiance are based on ground calibrations that were validated by comparison of MDIS with MASCS-VIRS measurements of Venus and Mercury. Except for 3 of the 12 filters, WAC radiances based on ground calibrations yielded a similar spectral shape with a 10-15% difference in absolute value. Filters 3 and 6 (at 480 and 433 nm) were systematically too low and too high respectively, so the responsivites used to calibrate them were adjusted empirically to improve correspondence with MASCS-VIRS. In addition WAC filter 2 (clear filter) radiances have not been validated. The relative accuracy of NAC and WAC filter 7 data (which correspond in central wavelength) were examined by comparing nearly simultaneous images taken during the Mercury 2 flyby and the NAC calibration was adjusted empirically to produce agreement. Both the NAC and WAC have CCDs whose responsivity to different wavelengths of light varies with CCD temperature. Inflight including Mercury orbit the CCD operating temperature range is typically -10C to -45C; however ground calibration measurements were acquired only at +23C, -31C, and -34C rendering the initial characterization of the temperature dependence inaccurate especially below -34C. M1 and M2 measurements of comparable surfaces, acquired at CCD temperatures of -34C to -43C, were used to improve the calibration of CCD temperature dependence over the lower end of the temperature range, and this correction was applied to version 3 CDRs. In version 4 CDRs, an additional update to responsivity improved temperature dependence over the full operating range. The correction was derived empirically by fitting as a function of CCD temperature the median values of images acquired at a wide range of temperatures but a narrow range of photometric geometries. In version 5 CDRs, a similar correction to responsivity used all Mercury images satisfying the illumination criteria. (8) SCATTERED LIGHT. In the NAC, scattered light from out-of-field sources is an issue. The geometry contributing most of the scatter is 1-2 fields-of-view sunward of the NAC boresight. For a very large, evenly illuminated source that overfills the field-of-view by a factor of several, ray-trace studies supported by testing during Venus flyby 2 suggest that 2-7% of the radiance measured in the field-of-view will have come from out-of-field sources. The spatial pattern of the scatter is variable, due to diffuse reflections off the internal instrument housing. The WAC is subject to scattered light originating from within the field-of-view or just outside it. In overexposed images, the source is evidently multiple reflections off of 13 optical surfaces (2 sides of each of 4 lenses, the spectral filter, and the CCD cover glass, as well as the CCD surface itself). The scatter becomes worse at longer wavelengths. Just off the limb of a large extended source near 1 field-of-view in size, like Venus or Mercury, measured radiance increases with wavelength from 2% to 7% of the value measured on the extended source. The value decreases with distance off the target more quickly at longer than at shorter wavelengths, but remains at 1% hundreds of pixels from the source. Conversely, light must be scattering from bright parts of an image to dark parts of an image. Averaged over sources tens of pixels in area, and away from abrupt brightness contrasts, scattered light affects shapes of spectra measured from WAC data at least at the 1-2% level, worse near brightness boundaries or for small, bright crater ejecta. The expected effect is enhanced brightness at >650 nm in dark areas, and decreased brightness at >650 nm in small bright areas. In the end-of-mission delivery 15, a forward model of the expected WAC scatter from a given scene was derived using optical design software modeling CCD structure and hardware, with magnitudes of scatter calibrated against flight measurements. The ray trace analysis reveal an in-scene component from light diffracted by the CCD and reflected by the CCD cover glass, and an out-of-scene component from light reflected off metallic surfaces alongside the CCD and back off the cover glass. CDRs with bright craters and other albedo non-uniformities located at geometries with the strongest scatter were identified, and excluded from the final version of color map products (MDRs, MD3s, and MP5s). (9) POINTING UNCERTAINTY. MDIS pivot C-kernels generated prior to September 2009 were based on counting steps of the pivot motor. There is periodicity at the scale of 3 degrees of pivot rotation in the relationship between pivot step size and physical angle, leading to reported pivot geometries being in error by up to 350 microradians in early versions of the pivot C kernel. Inflight tests during 2009 supported development of an alternative calculation of pivot angle using the pivot position resolver; this approach is included in MDIS pivot C kernels since MDIS release 5 (images through Mercury flyby 3). The uncertainties in pivot attitude in latter kernels are of the order of 35 microradians. Uncertainty in spacecraft pointing is by requirement less than 350 microradians, but unofficial estimates from the guidance and control team suggest a more typical value near 70 microradians. When convolved with error in pivot pointing, this results in the majority of error in knowledge of image pointing (exclusive of thermal distortion or time variations that can be calibrated out). An additional and generally larger source of error in map projection of MDIS images acquired during Mercury orbit may be uncertainty in spacecraft position. At low altitudes that source of error dominates, whereas at high altitudes pointing error dominates, such that in either case typical errors in map projection from the best reconstructed spacecraft position are expected to be smaller than 1 km. In the end-of-mission delivery 15 of SPICE files and map products, these errors are reduced by controlling images using c-smithed kernels and a global digital elevation model (DEM), both derived using a least-squares bundle adjustment of common features, measured as tie point coordinates in overlapping NAC and WAC-G filter images of Mercury at favorable solar incidence and emission angles. (10) OUT OF FOCUS PARTICULATES IN LONG-EXPOSURE IMAGES OF DEEP SPACE. A number of images collected by the MDIS wide-angle camera using long (longer than 5 s) exposure times contain artifacts that are most likely caused by glint from microscopic particles that are in close proximity to the spacecraft. Because the particles are out of focus they appear as extended sources. The length of these artifacts is most likely due to motion of the particles over the significantly large exposure time of the images. (11) ANOTHER FLAT-FIELD ARTIFACT. In Mercury orbit, it was discovered that the flat-field correction in the image column immediately adjacent to the 4 column wide dark strip at the edge of the CCD is made problematic by low raw DN levels. Therefore beginning with version 3 CDRs, in not-binned 1024x1024 pixel images, during the calibration process the leftmost 5 columns are assigned null values; in 512x512 binned images the leftmost 3 columns are assigned null values; and in 256x256 binned images the leftmost 2 columns are assigned null values. (12) TIME-VARIABLE WAC RESPONSIVITY. During Mercury orbit it was recognized that filter-dependent changes in WAC responsivity on the order of +/- 15% occurred over timescales as short as several days. Because those variations were not consistent from filter to filter, they led to spurious spectral features, which were particularly conspicuous near 750 nm. The cause(s) of these variations in responsivity are not known, but they could include transient radiation effects on the detector or electronics, aging of filters, periodic deposition and burn-off of contaminants on filters, or incorrect recording of exposure time. An initial empirical correction for images acquired in the first year of operations was developed at utilized in version 4 calibration. For version 5 calibration at end of mission, an updated correction covers the full duration of the orbital phase. Overlaps between color image sets in color mapping campaigns were used to derive a multiplicative correction factor for each filter and for each Earth day (2-3 orbits). It used all image data with a pixel scale >50 m and incidence angle (measured at the center pixel of the 750-nm image in the image set) of <80 degrees. The images were calibrated to I/F using the standard calibration with Correct(f,MET) set to unity, the latest version of temperature-dependence of detector responsivity, the latest correction for frame-transfer smear, and newly derived flat-field corrections. Images were then photometrically normalized to 30 degrees incidence angle, 0 degrees emission angle, and 30 degrees phase angle. Any portions of image sets lacking coverage in all filters, or with incidence angles >70 degrees or emission angles >30 degrees, were trimmed. Image sets were mapped to an equal-area (sinusoidal) projection at 4 km/pixel. All images acquired on each day were mosaicked for each filter, typically resulting in a fairly narrow north-south strip that had substantial overlap with surrounding days. With this dataset, multiplicative correction factors for each day were calculated through a weighted least-squares optimization that minimized the discrepancy between the median values for all spatial overlaps. The optimization was performed in two steps. First, a mosaic of data acquired before 22 May 2011 was held as constant, because this dataset was seen to be largely self-consistent. However, these data covered only a fraction of the planet, so a mosaic with greater coverage was created from images that were corrected in the first iteration with low residual values. In the second step, all data were allowed to vary in the simultaneous optimization, with the new mosaic held as reference. Correction factors were derived for days on which no data were included in our optimization process by interpolating between adjacent days. Filters with center wavelengths at 700, 950, and 1020 nm were not used for regional or global mapping and thus did not have enough overlap to derive correction factors with this method. Instead, their empirical correction was derived by comparing them with a synthetic global mosaic created by linear interpolation from images acquired with adjacent filters. An analysis of overlap among individual images shows that residual differences (which include errors from calibration, scattered light, and possible incomplete correction of photometric variation) average <2% for the majority of the planet. (13) TEMPERATURE-DEPENDENCE OF WAC and NAC FOCAL LENGTH. The star calibrations used to track the position of the MDIS pivot base, augmented by special calibrations using fields with high spatial densities of stars, sample much of the range of thermal environments experienced in orbit. Analyses of these data showed that focal length of each camera is correlated with temperature of the focal plane housing 'FOCAL_PLANE_TEMPERATURE', and that focal length varies over the cameras' operating temperature range by several parts in 10,000. This dependence is included in an updated delivery of the instrument kernel associated with PDS release 11. Limitations : None
CITATION_DESCRIPTION C. Hash, MESSENGER MDIS EXPERIMENT (EDR) DATA E/V/H V1.0, NASA Planetary Data System, 2008
ABSTRACT_TEXT Abstract : The Mercury Dual Imaging System (MDIS) consists of two cameras, a Wide Angle Camera (WAC) and a Narrow Angle Camera (NAC), mounted on a common pivot platform. This dataset includes the Experiment Data Record (EDR) version of all available images acquired during the cruise phase to Mercury and includes post- launch checkout images, flyby images of Earth, Venus, Mercury, and the Moon, images acquired from Mercury orbit, and inflight calibration images. In addition to the imagery, anciliary information (including calibration files needed to process the data further) is included.
PRODUCER_FULL_NAME CHRISTOPHER HASH
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  • Imaging Planetary Image Atlas
  • Mercury Orbital Data Explorer
  • Imaging Online Data Volumes
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