PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM OBJECT = TEXT PUBLICATION_DATE = 2016-02-14 NOTE = "Description of the CALIB directory contents for the MESSENGER MDIS CDR PDS Archive" END_OBJECT = TEXT END CALIB Directory Contents The CALIB directory contains files needed to reduce raw MDIS images (EDRs) to units of radiance or I/F. Files needed for major corrections are arranged into subdirectories by correction type. The calibration approach was revised and a new term added to the equation in November 2012 in order to address effects of time variability of WAC responsivity beginning in May 2011. CALINFO.TXT - The file you are reading. It contains descriptions of calibration files and directions for their application to MDIS EDRs. /LUT_INVERT - This directory contains the inverse lookup table required for inverting 8-bit images into their original 12-bit format. There are two files: MDISLUTINV_0.TAB - This file contains 8-bit values, and the 12-bit values to which they correspond. There is one set of 12-bit values for each of the eight available lookup tables in the instrument. MDISLUTINV_0.TAB - The label that describes the preceding file. Version history: Version 0 is based on testing using Venus 2 flyby images. /DARK_MODEL - This directory contains tables of coefficients needed to model the dark level in the NAC or WAC, with on-chip pixel binning turned on or not. There are tables of coefficients with the nomenclature: MDIScam_bining_DARKMODEL_v.TAB where: cam = camera, NAC or WAC bining = binning, NOTBIN or BINNED v = version number, 0-9,a-z Detached labels describing the tables have the nomenclature: MDIScam_bining_DARKMODEL_v.LBL Version history: Version 0 is based on ground calibration measurements. /FLAT - This directory contains flat-field images which correct for response variations from pixel to pixel and across the CCD. There separate files for each of the 12 WAC filters and for the NAC, with on-chip pixel binning turned on or not. Flat-field files for the WAC have the nomenclature: MDISWAC_bining_FLAT_FILnn_v.FIT where: bining = binning, NOTBIN or BINNED nn = filter number, 1-12 v = version number, 0-9,a-z Detached labels describing the images have the nomenclature: MDISWAC_bining_FLAT_FILnn_v.LBL. Flat-field files for the NAC have the nomenclature: MDISNAC_bining_FLAT_v.FIT where: bining = binning, NOTBIN or BINNED v = version number, 0-9,a-z and detached labels describing them have the nomenclature: MDISNAC_bining_FLAT_v.LBL. Version history: Version 2 is the latest version based on ground calibration measurements. There are systematic artifacts in the flat fields due to unremoved dark current. In addition, the locations of some dust donuts has changed since ground calibration leading to under - or over-correction of the donuts' flat-field effects. Version 4 addresses both shortcomings by using flight measurements of the onboard calibration target or Venus to improve the high spatial frequency characteristics of the flat fields. Version 4 is available for both cameras and binning states with the exception of WAC filter 2 and NAC binned; for these cases, version 2 is the latest. Version 5 is only available for WAC filters 3 and 6 and includes an update based on the median of hundreds of photometrically normalized images of Mercury. Version 6 is available for WAC filters 3 and 6 not-binned, and filters 3, 4, 5, 6, 7, 9, 12 binned, updated based on the median of thousands of photometrically normalized images of Mercury. /RESPONSIVITY - This directory contains tables of coefficients used to convert corrected DN to units of radiance. There are separate tables for the WAC and NAC, with on-chip pixel binning turned on or not. The tables have the nomenclature: MDIScam_bining_RESP_v.TAB where: cam = camera, NAC or WAC bining = binning, NOTBIN or BINNED v = version number, 0-9,a-z Each of the WAC tables has records for all of the filters. Detached labels describing the tables have the nomenclature: MDIScam_bining_RESP_v.LBL. Version history: For the WAC, version 0 is based on ground calibration measurements, version 1 includes updates based on improved characterization of filter bandpasses, and version 2 includes validation from the first Mercury flyby. A significant error in units discovered in versions 0 and 1 is corrected in version 2. Version 3 (for filters 3 and 6 only) includes small updates based on comparison with MASC-VIRS data. Version 4 updates the correction for CCD temperature, using a quadratic instead of linear function. Version 5 includes a further update for CCD temperature using coefficients that improve the fit and are based on a larger number of points. Version 6 includes the final end-of-mission correction for CCD temperature using all orbital data. For the NAC, version 0 is based on ground calibration measurements. Version 1 combines updates based on improved characterization of the bandpass, validation from the first Mercury flyby, and correction of the previously mentioned units error. Version 2 updates the correction for CCD temperature, using a quadratic instead of linear function. Version 3 includes small updates based on comparison with WAC data in filter 7, which has the same central wavelength as the NAC. Version 4 introduces a CCD temperature correction paralleling that of the G filter (filter 7, 750 nm) in the WAC whose bandpass is included within the 50-nm wide NAC bandpass. /CORRECT - This directory contains tables of coefficients used to correct time variations in WAC responsivity beginning in May 2011. There is only a table for the WAC having the nomenclature: MDISWAC_CORRECT_v.TAB where: v = version number, 0-9,a-z Each of the WAC tables has columns for all of the filters, and records relative responsivity as a function of time. The correction is set to unity prior to 24 May 2011, and values greater, less than or equal to unity afterward. The correction at any instant should be that listed for the immediately preceding time. Detached labels describing the tables have the nomenclature: MDISWAC_CORRECT_v.LBL. Version history: For the WAC, version 1 is based on analysis of median values of calibrated WAC images of intermediate terrain taken within a narrow range of photometric angles. Version 4 appends corrections for temporal variations in the 3 filters used in 3-color mappings (6, 7, and 9, or F, G, and I). Version 5 updates corrections for all filters based on analysis of filter overlaps as a function of time. /SOLAR - This directory contains tables of solar irradiance used to convert radiance to units of I/F. There are separate tables for the WAC and NAC. The tables have the nomenclature: MDIScam_SOLAR_v.TAB where: cam = camera, NAC or WAC v = version number, 0-9,a-z The WAC table has records for all of the filters. Detached labels describing the tables have the nomenclature: MDIScam_SOLAR_v.LBL. Version history: Version 0 is based on ground characterization of the filter bandpasses and the solar spectrum of [KURUCZETAL1997]. /SUPPORT - This directory contains characterizations of the instrument that are not part of the calibration process per se, but were used to derive the calibration files. MDISLUTFWD_0.TAB contains the onboard forward lookup tables used optionally to convert 12-bit to 8-bit images. MDISLUTFWD_0.LBL describes that file. Bandpasses for each WAC filter and for the NAC are given in tables having the nomenclature MDISBPWa.TAB where: a = A through M for different filters: DETECTOR, FILTER #, NAME, LETTER IN FILENAME WAC, 1, 700 BP 5 , A WAC, 2, 700 BP 600, B WAC, 3, 480 BP 10 , C WAC, 4, 560 BP 5 , D WAC, 5, 630 BP 5 , E WAC, 6, 430 BP 40 , F WAC, 7, 750 BP 5 , G WAC, 8, 950 BP 7 , H WAC, 9, 1000 BP 15 , I WAC, 10, 900 BP 5 , J WAC, 11, 1020 BP 40 , K WAC, 12, 830 BP 5 , L NAC, N/A, N/A , M USAGE ===== Raw units are of DN converted to the physical units of radiance or I/F, following the calibration equation: L(x,y,f,T,t,b) = Lin[DN(x,y,f,T,t,b) - Dk(x,y,T,t,b) - Sm(x,y,t,b)] / {Flat(x,y,f,b) * t * Resp(f,b,T)} where: L(x,y,f,T,t,b) is radiance in units of W / (m**-2 microns**-1 sr**-1), measured by the pixel in column x, row y, through filter f, at CCD temperature T and exposure time t, for binning mode b, DN(x,y,f,T,t,T) is the raw DN measured by the pixel in column x, row y, through filter f, at CCD temperature T and exposure time t, for binning mode b, Dk(x,y,T,t,b) is the dark level in a given pixel, derived from a model based on exposure time and CCD temperature, Sm(x,y,t,b) is the scene-dependent frame transfer smear for the pixel, Lin is a function that corrects small nonlinearity of detector response, Flat(x,y,f,b) is the non-uniformity or 'flat-field' correction, Resp(f,b,T) is the responsivity, relating dark-, flat-, and smear-corrected DN per unit exposure time to radiance, and t is the exposure time. The above equation assumes that data are in the native 12-bit format in which they were read off the CCD, and that onboard application of 12-to-8 bit lookup tables (LUTs) has been inverted. This correction is done step-wise using the calibration tables and images in this directory as follows. (1) Inversion of 12 to 8 bit Compression ======================================== 8-to-12 bit inversion of DN values is required when the value of MESS:COMP12_8 is 1 (when the data are 8-bit). There are 8 inverse lookup tables (LUTs). The table to use is indicated by the value of MESS:COMP_ALG from 0 through 7. An 8-bit value (in a row of the table) is inverted by replacing it with the 12-bit value in the column corresponding to a particular LUT. The inversion tables are given in the product MDISLUTINV_0. (2) Subtraction of modeled dark level ===================================== Two methods are used for subtraction of the dark level, one for exposure times >1 sec and one for exposure times <1 sec. In the method for exposure times >1 sec, the DN values are valid columns in the masked 'dark strip' at the edge of the CCD are fitted linearly as a function of line in the image. Then the fits are subtracted from the pixels in each line of the CCD. In the method for exposure times <1 sec, a model is used which accounts for variations in column (sample) in the image, which the previous method cannot do. However if the model were applied to longer exposures, it would yield larger residuals than the previous method. There are four separate models of dark level (dark current plus electronics bias), for the MDIS-WAC and MDIS-NAC (as indicated by the keyword INSTRUMENT_ID), and for each camera, without pixel binning turned on (MESS:FPU_BIN = 0) or with pixel binning turned on (MESS:FPU_BIN = 1). The models estimates the dark level Dk(x,y,t,T) as a function of column position x, row position y, exposure time t (as indicated by the keyword MESS:EXPOSURE or EXPOSURE_DURATION), and CCD temperature T (as indicated by the keyword MESS:CCD_TEMP): Dk(x,y,t,T) = C(T) + D(T) + [E(T) + F(T) * t] * y + {O(T) + P(T) * t + [Q(T) + S(T) * t] * y} * x Variables C(T), D(T), E(T), F(T), O(T), P(T), Q(T), and S(T) are all third-order functions of CCD temperature, for example: C(T) = H0 + H1 * T + H2 * T**2 + H3 * T**3 In all cases x or y is in the range 0-1023 for a not-binned image (as indicated by the keyword MESS:FPU_BIN = 0) or 0-511 for a binned image (as indicated by the keyword MESS:FPU_BIN = 1). t is in units of milliseconds, and T is in UNCALIBRATED raw counts of CCD temperature. For each pixel in column x and row y of an image, application of the correction is: DN_dark(x,y,t,T) = DN(x,y,t,T) - Dk(x,y,t,T) where DN(x,y,t,T) is DN in 12-bit format, Dk(x,y,t,T) is the predicted DN level from the dark modeldark model, and DN_dark(x,y,t,T) is dark-corrected DN. The eight sets of coefficients for the WAC not-binned, WAC binned, NAC not-binned, and NAC binned dark models are given in the products MDISWAC_NOTBIN_DARKMODEL_0, MDISWAC_BINNED_DARKMODEL_0, MDISNAC_NOTBIN_DARKMODEL_0, and MDISNAC_BINNED_DARKMODEL_0 respectively. (3) Frame Transfer Smear Correction =================================== Accumulation of signal continues during the finite duration of frame transfer induces a streak or frame-transfer smear in the wake of an illuminated object in the field of view, parallel to the direction of frame transfer. This smear is approximated as: Sm(x,y,t,b,f) = SUMM(1,y-1) { t2/t * [DN_dark(x,y,t,b) - Sm(x,y,t,b,f)] / Flat(x,y,b,f)} where Sm(x,y,t,b,f) is the smear in column x and row y at exposure time t in binning mode b and filter f, Dk_dark (x,y,t,b) is dark-corrected DN in column x and row y at exposure time t and temperature T in binning mode b, Flat(x,y,b,f) is the flat-field correction in column x and row y in binning mode b and filter f, t is exposure time in milliseconds, and t2 is the time for frame transfer (about 3.4 ms) divided by the number of lines in the image in the direction of frame transfer, 1024 for full-frame images (when MESS:FPU_BIN = 0) or 512 for binned images (when MESS:FPU_BIN = 1). For each pixel in column x and row y of an image, application of the correction is : DN_dark_smear(x,y,t,b,f) = DN_dark(x,y,t,b,f) - Sm(x,y,t,b,f) where DN_dark_smear(x,y,t,b,f) is dark- and smear- corrected DN, DN_dark(x,y,t,b,f) is dark-corrected DN, and Sm(x,y,t,b,f) is the smear calculated as shown above. (4) Correction for CCD non-linearity ==================================== To remove effects of nonlinearity in WAC image data, the following corrections should be applied after correction of dark current, bias, and smear. For DN_dark_smear > 1 DN_lin = DN_dark_smear/[0.008760 * Ln(DN_dark_smear) + 0.936321] For DN_dark_smear <= 1 DN_lin = DN_dark_smear/0.936321 To remove effects of nonlinearity in NAC image data, the following procedure should be applied after correction of dark current, bias, and smear. For DN_dark_smear > 1 DN_lin = DN_dark_smear/[0.011844 * Ln(DN_dark_smear) + 0.912031] For DN_dark_smear <= 1 DN_lin = DN_dark_smear/0.912031 where DN_dark_smear is the input dark- and smear-corrected DN, and DN_lin is linearized dark- and smear-corrected DN. (5) Flat-field correction ========================= The flat field correction removes pixel to pixel differences in detector responsivity, so that the responsivity coefficients can be expressed as scalars for each filter. There is a separate flat-field image for MDIS-WAC and MDIS-NAC (as indicated by the keyword INSTRUMENT_ID), without pixel binning turned on (MESS:FPU_BIN = 0) or with pixel binning turned on (MESS:FPU_BIN = 1), for each separate filter (as indicated by the keyword FILTER_NUMBER). All of the files are in the 'FLAT' directory. For each pixel in column x and row y of an image, application of the correction is DN_flat(x,y,f,b) = DN_lin(x,y,f,b) / Flat(x,y,f,b) where DN_flat(x,y,f,b) is flat-fielded, linearized, dark- and smear-corrected DN, DN_lin(x,y,f,b) is linearized dark- and smear-corrected DN, and Flat(x,y,f,b) is the value in the appropriate flat-field image. (6) Conversion from DNs to radiance =================================== The value that relates corrected DN's measured per unit time to radiance is the responsivity. Responsivity is modeled as a function of which camera is being used (MDIS-WAC or MDIS-NAC as indicated by the keyword INSTRUMENT_ID), its binning state (as indicated by the keyword MESS:FPU_BIN), and in the case of the WAC the filter number (as indicated by the keyword FILTER_NUMBER). The coefficients used to calculate the responsivity are in the 'RESPONSIVITY' directory. For a camera, binning state, and filter, the responsivity is calculated from uncalibrated CCD temperature (as indicated by the keyword MESS:CCD_TEMP) as follows: Resp(f,T,b) = R(f,t=-30.3C,b) * [correction_offset(f,b) + T(CCD) * correction_coef1(f,b) + T(CCD)^2 * correction_coef2(f,b)] where Resp(f,T,b) is responsivity in filter f at CCD temperature T in binning state b, T(CCD) is raw CCD temperature in units of DNs, R(f,t=-30.3C,b) is responsivity in filter f in binning state b at CCD temperature of 1060 DN (-30.3C), correction_offset(f,b) is camera- and filter-dependent temperature correction offset for filter f and binning state b, correction_coef1(f,b) is the camera- and filter-dependent temperature correction first-order coefficient for filter f and binning state b. correction_coef2(f,b) is the camera- and filter-dependent temperature correction second-order coefficient for filter f and binning state b. To apply responsivity to obtain radiance L, the expression is L(f) = DN_flat(f) / (t * Resp(f,T,b)) where L is radiance in units of W / (m**2 microns**1 sr**1), DN_flat is dark-, smear-, linearity-, and flat field-corrected DN, t is the exposure time in seconds, and Resp(f,T,b) is the responsivity in filter f at CCD temperature T and binning state b. Images calibrated to units of radiance have a data type in the file name denoted as 'RA'. (7) Conversion from radiance to I/F =================================== To convert from radiance to I/F (also known as radiance factor, the ratio of measured radiance to that which would be measured from a white perfectly Lambertian surface), the following expression should be applied: I_over_F(f, MET) = L(f) / Correct(f,MET)* pi * (SOLAR_DISTANCE/149597870.691)**2 / F(f) where Correct(f,MET) is the responsivity correction in filter f at time MET, which is a real number not equal to unity in WAC images where the correction is applied, or unity in other images, L(f) is calibrated radiance calculated as described above for some filter f, SOLAR_DISTANCE is that value for distance of the target object from the center of the sun in kilometers (as indicated by the keyword SOLAR_DISTANCE) 149597870.691 is the number of kilometers in 1 AU F(f) is effective average solar irradiance sampled under the filter bandpass. The effective average solar radiance for each camera and bandpass is given in the SOLAR directory. Images calibrated to units of I/F have a data type in the file name denoted as 'IF' if the Correct term is applied. Alternatively the Correct term may be omitted in which case the data type is denoted as 'IU'. (8) Treatment of special pixels =============================== Two types of pixels cannot be validly calibrated to either radiance or I/F: (a) Pixels under the dark mask at the edge of the detector do not measure light from the scene, yet deviation of their calibrated value from zero is a valuable measure of calibration residuals. The average calibrated value under the dark mask is reported in the label as DARK_STRIP_MEAN, but the actual pixel values are replaced by the value indicated in the label for CORE_NULL. (b) Saturated pixels do not have a known correspondence to scene radiance. The pixel values in saturated pixels are replaced by the value indicated in the label for CORE_HIGH_INSTR_SATURATION.