Instrument Overview
===================


The Mercury Dual Imaging System (MDIS) on the MESSENGER spacecraftprovides critical measurements tracing Mercury's origin and evolution.
MDIS consists of a monochrome narrow-angle camera (NAC) and amultispectral wide-angle camera (WAC). The NAC is a 1.5 degree field-of-view (FOV) off-axis reflector, coaligned with the WAC, a four-elementrefractor with a 10.5 degree FOV and 12-color filter wheel. The focalplane electronics of each camera are identical and use a 1024x1024 Atmel(Thomson) TH7888A charge-coupled device (CCD) detector. Only one cameraoperates at a time, allowing them to share a common set of controlelectronics. The NAC and the WAC are mounted on a pivoting platform thatprovides a 90 degree field of regard, extending 40 degrees sunward and 50degrees anti-sunward from the spacecraft +Z-axis, the boresight directionof most of MESSENGER's instruments. Onboard data compression providescapabilities for pixel-binning, remapping of 12-bit data into 8 bits, andlossless or lossy compression.


Scientific Objectives
=====================


The MESSENGER spacecraft made three flybys of Mercury, in January2008, October 2008, and September 2009, during which regions unexplored byMariner 10 were imaged by MDIS. In March 2011, the spacecraft began theorbital phase of its mission, which including the Primary Mission and twoExtended Missions continues through spacecraft impact in March or April2015.

The primary imaging objectives during the flybys were:
(1) acquisition of near-global NAC coverage at about 500 m/pixel,(2) 11-color multispectral WAC mapping at about 2 km/pixel, and(3) high-resolution WAC and NAC imaging of select regions.

During the initial 1-year segment of theorbital phase of the mission,the MDIS observing shifted to acquisition of additional datasets:
(4) a nadir-looking monochrome (750-nm) global mosaic at moderate solar incidence angles (55-75 degrees) and 250 m/pixel or better sampling;(5) a 20-degree off-nadir monochrome mosaic to complement the nadir-looking mosaic for stereo;(6) repeated monochrome mosaics of the south polar regions to identify regions which remain permanently shadowed, using the WAC 750-nm filter one solar day and the NAC the next solar day;(7) WAC multispectral mapping at 1 km/pixel in 8 colors at low incidence angles;(8) high-resolution (20-50 m/pixel) NAC image strips across features representative of major geologic units and structures;(9) 3-color WAC imaging of select regions at low incidence angles, commonly at pixel scales near 400 m/pixel;(10) WAC color photometry to support derivation of a photometric function for the 8-color map, repeatedly covering representative areas near the Rembrandt and Beethoven basins at a wide variety of incidence, emission, and phase angles, initially using the same 8-color filter set as for the global 8-color map;(11) 3 weekly sets of 2x1 frame WAC 750-nm image mosaics showing the entire limb of Mercury, to help define the low-order global shape model for Mercury; and(12) images of star fields to calibrate NAC and WAC focal length and geometric distortion, the orientation of the cameras on the pivot, the pivot angle, and the orientation of the base of the pivot and how it changes with time as a result of thermaldistortions.

For the extended missions, several additional data sets were acquired:
(13) a nadir-looking monochrome (750-nm) 'albedo' global mosaic at low incidence angles and 250 m/pixel or better sampling;(14) a 20-degree off-nadir monochrome mosaic to complement the nadir-looking low-incidence angle mosaic for stereo with fewer gaps due to shadows in areas with steep topography;(15) a nadir-looking monochrome (750-nm) 'high incidence from the west' global mosaic with incidence angles near 80 degrees and solar azimuth to the west, at 250 m/pixel or better sampling;(16) a nadir-looking monochrome (750-nm) 'high incidence from the east' global mosaic with incidence angles near 80 degrees and solar azimuth to the east, at 250 m/pixel or better sampling;(17) a nadir-looking 3-color mapping of northern and equatorial latitudes at an improved spatial sampling compared to the 8-color map, about 400 m/pixel;(18) a WAC 750-nm monochrome mosaic of the south polar regions to identify regions which remain permanently shadowed;(19) NAC 'ride-along' imaging of the northern hemisphere to acquire a broad sampling of different terrains at <= 20 m/pixel;(20) repeated WAC color imaging of high southern latitudes to track time variations in radiometric calibration;(21) off-nadir WAC 5-color mapping of the northern plains at a near- constant low phase angle near 30 degrees;(20) WAC clear-filter and color imaging of permanently shadowed craters, to determine the spatial distribution of low- and high-reflectance deposits of frozen volatiles;(21) 11-color targets using the full set of WAC spectral filters covering regions of interest for their spectral variations;(22) 3-color WAC targets at 2 to 5 phase angles within single orbits or groups of two orbits, to measure spatial differences in photometric properties; and(23) movies using the WAC 750-nm filter pointed into the ram direction to capture a view of flying over Mercury's surfaceat a low altitude.

For monochrome global maps, data are acquired with the NAC for southernlatitudes when spacecraft altitude is high, and with the WAC at loweraltitudes over the northern hemisphere. This two-camera strategy resultsin near-uniform global coverage with an average spatial resolutionof about 200 m/pixel. Off-nadir mosaics for stereo were acquired undernearly identical lighting geometries to the nadir map to facilitateautomated stereo matching.


Subsystems
==========


MDIS consists of two cameras, a monochrome narrow-angle camera (NAC) and amultispectral wide-angle camera (WAC), coaligned on a common pivotplatform. The passively cooled detectors in each camera are thermallytied to the complex thermal system. This allows the detectors to bemaintained within their operating temperature of -10C, except during thehottest portion of the orbit at Mercury. The pivot platform provides anadded degree of freedom to point the dual cameras with minimal impact on thespacecraft. MDIS is mounted in MESSENGER's payload adaptor ring and facesin the nadir direction in Mercury orbit along the spacecraft +z axis, withthe pivot aligned in the y-z plane where -y is toward the sun.

Specifications of the two cameras are given below:
Narrow Angle Wide AngleField of view 1.5 x 1.5 degrees 10.5 x 10.5 degreesPivot range Both -40 deg (toward Sun) to +50 deg (away from Sun)Exposure time 1 ms to 9989 ms 1 ms to 9989 msFrame transfer time 3.4 ms 3.4 msImage readout time 1 s 1 sSpectral filters 1 filter 12 filtersSpectral range 725-783 nm 395-1040 nmFocal length 550 mm 78 mmCollecting area 462 mm**2 48 mm**2Detector Both TH7888A CCD, 1024x1024 14-micron pixelsIFOV 25 microrad 179 microradPixel FOV 5.1 m at 200-km altitude 35.8m at 200-km altitudeQuantization Both 12 bits/pixelCompression Both have options of lossless, lossy wavelength, 12-to-8 bits, and pixel binning
The full MDIS instrument includes the pivoting dual camera system as wellas two redundant external Data Processing Units (DPUs). The dual cameraassembly without the DPUs is simply referred to as 'MDIS'. Only one DPUmay be active at a time, and due to thermal constraints only one camerawill operate at a time; however, observations with the two cameras can beinterleaved at 5-second intervals. A separate electronics assemblyaccommodates switching between the various modes of operating with theredundant DPUs. The pivot platform has a large range of motion (240degrees) to allow the cameras to be 'tucked away' to protect the opticsfrom contamination. The pivot motor drive-train provides precisionrotation over the 90-degree operational range of motion about thespacecraft +Z axis.

A spectral calibration target is mounted on the inside of the payloadadapter ring. Early in the MESSENGER mission it was possible to tilt thespacecraft in order to provide solar illumination on the calibrationtarget. The large range of motion of the pivot assembly enabled eithercamera to point at the target permitting an absolute inflight radiometriccalibration and flat-field measurement.


Optical Design
==============


The WAC consists of a 4-element refractive telescope having a focal lengthof 78 mm and a collecting area of 48 mm**2. The detector located at thefocal plane is an Atmel (Thomson) TH7888A frame-transfer CCD with a1024x1024 format and 14-micron pitch detector elements that provide a 179-microrad pixel (instantaneous) field-of-view (IFOV). A 12-position filterwheel provides color imaging over the spectral range of the CCD detector.
Eleven spectral filters spanning the range from 395 to 1040 nm are definedto cover wavelengths diagnostic of different potential surface materials.
The twelfth position is a broadband filter for optical navigation. Thefilters are arranged on the filter wheel in such a way as to providecomplementary bandpasses (e.g., for 3-color imaging, 5-color imaging,etc.) in nearby positions.

The NAC is an off-axis reflective telescope with a 550-mm focal length anda collecting area of 462 mm**2. The NAC focal plane is identical to theWAC's, providing a 25-microrad IFOV. The NAC has a single medium-bandfilter (50 nm wide), centered at 750 nm to match to the corresponding WACfilter for monochrome imaging.

One of several impacts of the thermal environment on calibration accuracyis the relative responsivity of the CCDs at wavelengths longer than about700 nm. Response at longer wavelengths increases strongly withtemperature. If data are acquired over a large temperature range,inaccuracies in correction for temperature-dependent responseintroduce systematic errors in spectral properties at 850-1000 nm thatultimately could lead to false mineralogic interpretations. To protect theMDIS CCDs from wide temperature swings, incoming thermal infraredradiation (IR) is rejected in the optics by heat-rejection filters on thefirst optic of each camera. In the WAC, this rejection is accomplishedusing a short-pass filter as the outer optic; for the NAC the bandpassfilter has a specially designed heat-rejection coating on its firstsurface.

WAC Design
==========

Telescope Design. The WAC consists of a refractive telescope, dictated bythe required wide FOV and short focal length. The design approach was toselect the simplest lens design that gives acceptable image quality overthe field; however, an important constraint on the design is the limitedselection of glasses because of the radiation environment. The designrequirements were achieved by starting from a simple Cooke triplet andsplitting the central negative element. The resulting design, a Dogmar,gives good image quality although not over the full 395-1040 nm spectralrange. The uncorrected axial chromatic aberration was reduced by varyingthe thickness of each filter.

The WAC is focused at infinity with a 78-mm focal length to spread the10.5 degree FOV across the 14.3-mm detector. The 14-micron square pixelsof the CCD provide an instantaneous FOV (IFOV) of 179 microrad. The fieldcurvature of the WAC produces a small focus variation across the field,but it has a negligible effect on the final modulation transfer function(MTF). Optical distortion has a value of 0.06 percent at the corner ofthe field, less than a pixel. The design is diffraction-limited overthe entire field, and MTF degradation is dominated by the pixel size.

Spectral Filter Design. A 12-position multispectral filter wheel providescolor imaging over the spectral range of the CCD detector (395-1040 nm).
Eleven spectral filters are defined to cover wavelengths diagnostic ofcommon silicate minerals and glasses and have full-width half maximum(FWHM) bandwidths from 5-40 nm. A broadband clear filter is includedfor optical navigation imaging of stars.

Each filter consists of two or three pieces of glass using a radiation-resistant substrate in combination with a long-pass filter.
Two filters required an additional layer to achieve the desiredpassband. The variation in filter thickness used to remove residualchromatic aberration results in a small variation in the focal length ofthe camera between filters. The extreme filters give a focal length ofabout 78 mm at 480 nm and about 78.5 mm at 1020 nm respectively. Thisdifference results in a variation in the image scale of 0.7 percent.

By positioning the filter in front of the detector, the size of thefilters is minimized. However, the incident angle of the beam on thefilter varies with field angle, causing a shift in the spectral bandpassof the interference filters across the field. The effect of the incidentangle of the converging rays on the filter is much more serious thanthe spatial variations across the surface of the filter. With a maximumangle of 7.6 deg. at the corner of the field, spectral shifts of about 3 nmare expected.

NAC Design
==========

The primary purpose of the NAC is for high-resolution imaging of Mercury.
Because of the very bright optical signal from the planet, a largecollecting area is not required for sensitivity. The reflective design hasa long focal length that required folding the optical path in order to fitin the available volume. The monochromatic design has a single medium-band filter centered at 750 nm with a FWHM of 50 nm. The centerwavelength was chosen to match a corresponding WAC filter tofacilitate switching between cameras for global mapping.

The NAC has a 1.5-degree FOV that is spread across the 14.3-mm detector,requiring a focal length of 550 mm. The NAC has a 25-mm aperture,resulting in an F/22 system. An off-axis Ritchey-Chretien design wasselected over a Cassegrain in order to avoid the centralobscuration of the secondary mirror. The ellipsoidal primary mirror andhyperboloidal secondary mirrors are gold-coated aluminum. In this design,the image plane is tilted at an angle of approximately 9 degrees tolocate the optimal image quality.

A bandpass filter is the first optical component of the assembly anddefines the spectral range of the instrument. A specially designedinterference coating serves as a heat-rejection filter to minimizeadmittance of IR radiation from Mercury that would heat the CCD duringimaging and increase background levels.

Due to mass limitations the aperture is less thanrequired for diffraction limited performance, so the Airy disk isapproximately 2 pixels across. Low distortion of the image was animportant design specification for the NAC. At the edge of the FOV thisamounts to 1.28 pixels. The all-aluminum assembly of the NAC makes itinsensitive to thermal distortions over the operational temperature rangeof the instrument.

Filters Bandpasses
==================

The WAC camera utilizes a twelve position filter wheel with band passesbetween 415 and 1020 nm. The NAC is a broadband imager with a bandpasscentered near 750 nm.

Filter Filter Center BandpassNumber letter wavelength width in file (nm) (nm) name1 A 698.8 5.3
2 B 700 600.0
3 C 479.9 10.1
4 D 558.9 5.8
5 E 628.8 5.5
6 F 433.2 18.1
7 G 748.7 5.1
8 H 947.0 6.2
9 I 996.2 14.3
10 J 898.8 5.1
11 K 1012.6 33.3 0
12 L 828.4 5.2
NAC M 747.7 52.6


Pivot Mechanism
===============


The MDIS pivot platform is controlled by a stepping motor whose phases arecontrolled directly by the DPU software to move the platform. The phasepattern can be adjusted by software to move the platform forwards orbackwards. The pivot platform's range of motion is mechanicallyconstrained by hard stops. The range of motion is further constrained bysoft stops applied by the software. The total range of motion of MDIS isabout 240 degrees, limited by hard mechanical stops in the pivot motor.
The hard stops are fixed at -185 degrees and 55 degrees from thespacecraft +z axis. The pivot motor drive-train provides precisionrotation over the 90 degrees operational range of motion 40 degreessunward and 50 degrees antisunward of the spacecraft +z axis.

The MDIS pivot actuator is capable of accurately stepping in intervals of0.01 degrees (about 150 microrad) per step. Pointing knowledge isdetermined by first 'homing' the instrument, which is accomplished bydriving the actuator into one of the mechanical hard stops for a period oftime sufficient to ensure the orientation of the instrument if it had beenpreviously stopped at the opposite extreme of travel. Once the locationof the pivot actuator is known, the flight software retains this knowledgeand subsequent pointing commands are achieved by counting pulses (steps)to the motor.


Thermo-mechanical Design
========================


The MESSENGER spacecraft is protected from direct solar illumination bythe spacecraft sunshade. The thermal environment of the instruments islargely benign with the exception of short intervals during the 12-hourorbital period when Mercury's illuminated surface subtends a large solidangle. The passive thermal design of MDIS buffers temperature changes toprevent the instrument from exceeding the CCD's high operationaltemperature limit (-10C to limit the noise contribution from darkcurrent) except for occasional short periods. Survival heaters ensurethat cold operational limits are not violated.

The DPU is directly coupled to the spacecraft deck and blanketed withmulti-layer insulation (MLI). The -40 C to +50 C operating temperature ofthe DPU follows the deck temperature. The temperature of the imagerinterface board and the low-voltage power supply board are measuredinternal to the DPU and reported in the instrument housekeeping data. TheMDIS temperature sensors are mounted at key locations on the instrumentand are reported in image headers. The CCD temperature is the mostimportant temperature for calibration.

Thermal Design of the MDIS Platform Assembly. Each camera has its ownthermal control system, and each CCD is mounted to a heat sink that boltsonto a bracket that is thermally isolated from the focal plane unit (FPU).
Titanium spacers minimize heat conduction from the FPU to the CCD whilemaintaining focus over the operating temperatures of the instrument.
A thermal link made of made of multiple sheets of thin aluminum foilties the CCD to a phase-change material (wax pack). A thermal strap,made of copper braid secured to the top of the wax pack using aconductive epoxy, thermally ties the wax pack to the evaporatorportion of a stainless steel, butane-charged diode heat pipe.
Each camera has its own wax pack and heat pipe assembly.
However, the heat pipes for both cameras are connected to a commonradiator assembly made of three beryllium plates mounted to the instrumentwith titanium spokes.

The heat pipes, fabricated by Swales Aerospace, are made from 9.5-mm-diameter stainless steel tubing filled with about 5 g of butane. Duringnormal operation, heat flowing from the CCD and through the wax packis absorbed at the evaporator. Liquid butane is evaporated and thegaseous butane flows down the center of the tube toward the radiator to becondensed on the cold walls of a condenser. An internal stainless steelwire mesh acts as a wick to transport the liquid from the condenser backto the evaporator, continuing the fluid loop.

A separate pipe, the liquid trap, is attached to the evaporator sectionthat allows the heat pipe to act as a diode, i.e. allowing heat to flowonly in one direction. A small tube connects the liquid trap to theevaporator. The diode action occurs when the condenser/radiators becomeswarmer than the evaporator and the butane liquid in the condenservaporizes. The now gaseous butane flows down the pipe and condenses ontothe walls of the relatively cool (-10C) evaporator and liquid trap.
Since there is no wick in the small tube between the liquid trap and theevaporator, the liquid cannot flow out of the trap, back to theevaporator, and up to the condenser for re-evaporation. As the radiatorplates continue to heat up, more liquid is vaporized and flows down to theliquid trap area. This process continues until most of the liquid islocated in the liquid trap and no more liquid can flow back up the pipe tothe condenser. In this state, the heat pipe is effectively shut off andthere is no longer any heat transfer between the evaporator and condenser.

Once the radiators cool back down and the condenser again drops below theevaporator temperature, the heat pipe will start back up and conduct heatlike a regular heat pipe.

On orbit at Mercury, when the spacecraft is away from the hot planet, theMDIS radiator plates do not see the planet and are only exposed to deepspace. In this condition, the radiator plates become very cold (-77C)and the diode heat pipes operate as normal heat pipes. The gas internal tothe pipe moves freely along the length of the pipe, transporting heat fromthe wax pack out to space. The heat from the CCD flows from the CCDthrough the thermal link, through the wax pack and thermal strap, to theevaporator of the heat-pipe. As the liquid in the pipe evaporates, heatflows along the heat pipe to the condensers and finally to the radiatorpanels. When the paraffin in the wax pack is all frozen, the wax pack,thermal link, and CCD continue to cool below the freezing point ofthe paraffin.

As the spacecraft starts to approach the planet, the radiator plates beginto absorb heat from the hot planet. When the temperature of the condenserrises above the temperature of the evaporator, most of the heat-conductinggas in the pipe condenses in the liquid trap, effectively shutting off theheat pipe. Heat that is still flowing from the CCD will now be absorbedinto the wax. The paraffin warms up to its melting point and remainsat that temperature until all 240 g of paraffin has melted. The amount ofwax was selected so that all the wax would not completely melt during thehours surrounding periapsis. The melting wax clamps the temperature ofthe CCD at warm operating temperature limit (-10C). As the spacecraftmoves farther away from the planet, the MDIS radiator and condensertemperatures fall below the evaporator temperature and allow the heatpipe to operate normally again. With the condenser temperature below theevaporator temperature, heat flows from wax pack to the radiators andbegins refreezing the paraffin. The partially melted wax pack remainsat its melting (freezing) point until all the paraffin solidifies. Atthat point, the wax pack temperature continues to drop down to -45Cwhere heaters and thermostats control the minimum temperature.

Other than the CCD-to-heat pipe heat path, the majority of the cameracomponents are thermally connected to the main platform. Heaters on theplatform prevent the camera optics and FPU electronics from dropping below-35C. There are no radiators associated with the platform. The camerasand platform are isolated from the environment by MLI blankets and donot overheat due to the relatively small amount of heat dissipated in theelectronics.


Detectors
=========


The CCD used in both the NAC and WAC is an Atmel (formerly Thomson-CSF)TH7888A with 14-micron square pixels and antiblooming control, that usesframe transfer to obtain electronic exposure control. The active imageforms an array 1024x1024 pixels (14.3x14.3 mm) in size in whichoptical energy is accumulated. At the end of the exposure period, theaccumulated charge in each pixel is transferred in about 3.4 ms tothe masked-off 1024x1024 memory zone. A tantalum radiation shield notonly protects the CCD from ionizing radiation but also blocksillumination to the memory zone. The combined image and memory zones'1024x2048 pixel array has two on-chip output amplifiers, thoughonly one amplifier is used in the MDIS design. Manual and autoexposurecontrol from 1 ms to 9989 ms permit imaging over a broad range ofintensities.

The CCD provides 4 specially shielded columns to provide an accuratemeasure of the dark level in a full resolution image (1024x1024). In thecase of 2x2 binning, the corresponding columns are two wide but shiftedout of the shielded area of the CCD.

The antiblooming feature of the CCD causes a scene-dependent nonlinearityat high DN levels. Small sources do not typically exceed 3500 DN, but onlarger sources where antiblooming is overwhelmed, higher DN values canoccur.


Electronics
===========


The main electronics systems of MDIS include the DPUs, the DPU InterfaceSwitching Electronics (DISE box), and the FPU camera electronics.

Focal Plane Electronics. The detector electronics for both the WAC and NACare identical. Each CCD is bonded to a camera-specific mounting bracket(heat-sink) prior to assembly into the FPU electronics. The NAC heat-sinkis tilted 9 degrees to match the optimal orientation of the NAC focalplane, whereas the WAC heat-sink has no tilt. Due to thermal, power, andoperational constraints, only one camera operates at a time. The framerate of each FPU is fixed at 1 Hz; however, the frame rate to thespacecraft is not fixed but cannot exceed 1 Hz.

The focal plane electronics are reduced in size to save mass. A type ofconstruction of the printed circuit boards known as rigid-flex enabled theelectronics to be folded into a small space. The clock drivers use anintegrated circuit (IC) designed to drive the gates of power metal oxidesemiconductor field effect transistors (MOSFETs) and are almost ideal todrive the capacitive gates of a CCD. The TH7888A has a relatively highinternal gain of 6 microV/e. This value is large enough to drive thecorrelated double sampler analog-to-digital converter (ADC) integratedcircuit directly while meeting the system noise requirements. The outputis a 12-bit digital word for each CCD pixel value. A Field-ProgrammableGate Array (FPGA) is used to provide clocks for the CCD and ADC derivedfrom an input clock from the DPU. The DPU also provides a coded exposuretime signal for the camera, as the autoexposure algorithm is in the DPU.
The FPGA also formats the data for transfer back to the DPU. Theinterface between cameras and DPU uses low-voltage differential signalingfor low noise and to tolerate differences in grounding between the units.

DISE. The MDIS instrument contains six major electrical subsystems: twoFPUs, a filter wheel motor, a filter wheel motor resolver, a platformpivot motor, and a platform pivot motor resolver. The pivot motorcontains redundant windings with one set going to each DPU. The remainingsystems must be controllable by either of the two redundant DPUs.
Selection between these units is provided by the DISE box.

The DISE box switches between the DPU and MDIS electronics whileminimizing the connections through the rotary twist-capsule feedthrough.
There are two switching functions: one selects which DPU is master and theother selects which camera will be active. After the DPU asserts itselfas master, it then applies power to the camera of interest. Each DPUprovides separate power lines for each camera. The DISE board uses thesepower connections to power its internal circuitry that selects the correctcamera as well as powering the camera of interest. This selection is donein a master/slave configuration with the NAC having priority. If power issupplied to both cameras at the same time, signals from the NAC will betransmitted. This arrangement ensures that the master DPU communicateswith the intended camera.

In addition to delivering each camera with the 15 V provided by the masterDPU, the DISE box linearly regulates a 6-V power supply. This regulationis done in the DISE box to limit power dissipation and reduce mass in thecameras. To ensure good thermal conduction of these components so thatthey can safely dissipate their heat, they are mounted directly to thelower corners of the DISE box frame. The DISE box switches control linesusing the low-voltage differential signaling (LVDS) protocol to and fromeach camera. Each camera has a single LVDS interface, and each DPU has asingle LVDS interface. Switching between them is done with cold sparingLVDS receivers and cold sparing complementary metal oxide semiconductor(CMOS) multiplexers.

Data Processing Unit. The DPU electronics provides two distinct services:it acts as a communications router and interface between all theinstruments and the spacecraft, and it provides complete support for theMDIS camera electronics. Two separate sets of electronics boards arephysically stacked and packaged together.

The Event Processing Unit (EPU) board provides data processing capabilityand creates the telemetry data packets passed (through the DPU) to thespacecraft. Its design is based on the Intersil (formerly Harris) RTX201016-bit processor. In addition to the RTX2010, the EPU board contains 64kB of fuse-link programmable read-only memory (PROM) for boot code, 256 kBof electrically erasable PROM (EEPROM), and 256 kB of SRAM. An ActelRT54SX32S FPGA provides all the logic necessary to interconnect theprocessor with the stacking connector data bus. The EPU also receives(via the DPU) a once per second timing pulse from the spacecraft. Thissynchronization pulse is a key part of the overall timekeeping design tomaintain <1-ms end-to-end timing error between the mission operationscenter and the payload.

The Low Voltage Power Supply (LVPS) board converts spacecraft primarypower, ranging from 20 V to 36 V, to isolated secondary power. The LVPSboard utilizes an in-rush limiter, a VPT electromagnetic interference(EMI) 461 filter, and two VPT hybrid dual output converters (one for +/-5V and the other for +/-12 V). These converters provide better than1 percent load and line regulation for nominal operation. Two radiation-hardened opto-field effect transistor (FET) relays were used to switch upto 1 A of primary power to off-board loads. An analog monitoring circuit,comprised of a 14-bit ADC, instrumentation amplifiers, a 16-1 multiplexer,and an Actel FPGA provided 16 input analog channels, which were used tomonitor four voltages and six currents on the board, and five analogvoltages from off the board; the remaining channel measured boardtemperature.

Communication between each DPU and the instruments (other than MDIS) takesplace through a Universal Asynchronous Receiver/Transmitter (UART) hubboard with eight 38.4-kbaud, RS-422, full duplex UART channels. Eachchannel has a separate first circuit interface for maximum faulttolerance. Independent SRAM-based receive buffers indexed by FPGA-basedregisters are used to offload byte-by-byte operations from the DPUsoftware.

Communications between each DPU and the spacecraft is accomplished throughMIL-STD-1553 buses. A 1553 Interface Board has a design is based on theUTMC SUMMIT UT69151DXE protocol/transceiver chip and also includes anActel RT14100A FPGA and 64 KB of SRAM. Each DPU is a remote terminal onthe spacecraft 1553 bus. All payload commands and telemetry, except MDISimage data, are communicated via the 1553 bus.

All image acquisition control and hardware image processing occurs in theImager Interface Board. It utilizes two Actel RT54SX72S FPGAs, 2 MB ofSRAM, and a number of LVDS receiver and transmitter circuits tocommunicate both with the FPUs and the spacecraft high-speedIntegrated Electronics Moduloe (IEM) interface card.
The Imager Interface Board contains two FPGA designs: an imager-interface FPGA and an RTX-bus FPGA. Image data are continuously receivedfrom the active FPU and buffered in SRAM. If the DPU receives a commandto record an image, the image data are transferred to either of the tworedundant IEMs on the spacecraft. The main function of the imager-interface FPGA is to read images from the FPU and send them to the IEM.
The image data read from the focal plane are received from a serial LVDSinterface at 16 MHz. A full 1024x1024 image requires approximately 850 msto be received in the DPU. The data are sent at 4 MHz to the IEM,limiting the image frame rate. 512x512 on-chip binned images may be sentto the IEM at 1 Hz; however, full-frame images can be sent only at 0.25Hz. This interface limitation to the IEM requires the imager-interfaceFPGA to buffer the images in SRAM. The main functions of the RTX-bus FPGAare to store a 1024-bit image header and the FPU command words generatedby software running on the RTX processor in the DPU. This FPGA is alsocapable of generating a 32-bin histogram created from the pixel valuesproduced by the FPU. This histogram is made available to the RTXprocessor and is integral to the MDIS autoexposure algorithm.

The MDIS Imager Power Board provides switched +15 V to the NAC and WACelectronics. A separate MOSFET-switched output is used for each NAC andWAC FPU electronics.

Each of the pivot and filter wheel actuators has a dedicated MotorController Board. Although the characteristics of the two actuators aredifferent, the board designs are effectively identical. Each boardutilizes an EMI filter, an Actel A1020 FPGA, and a bipolar UDS2998 motor-driver to generate the appropriately phased motor drive waveforms. Theangular position of each actuator is measured via an AD2S80A motorresolver readout hybrid.


Software
========


The APL-developed MESSENGER instruments share common software, aninstrument domain library. The library was created in 1994 to serve theNEAR instruments. It is currently used in MESSENGER as well as theCompact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrumenton Mars Reconnaissance Orbiter and the LOng Range Reconnaissance Imager(LORRI) and Pluto Energetic Particle Spectrometer Science Investigation(PEPSSI) instruments on New Horizons.

The common software runs as a layer below the application-specificsoftware. The first layer provides standard services, for example,command handling and telemetry packet queuing. Another layer below theservice layer is a host abstraction layer. This layer abstracts theinput/output (I/O) interfaces used by the instrument to communicate withthe spacecraft. The common software includes a boot program. Its mostfrequent use is to boot application code by copying it from EEPROM to RAMand then start the application. However, the boot program also hascommands for loading memory from the ground, dumping memory back to theground, and copying memory to and from EEPROM. These features can be usedto upload new instrument programs. The boot program is stored in non-volatile ROM. In the event that an instrument's program in EEPROM getscorrupted, the boot program will allow the instrument software to bereloaded.

The common command software can store and run sequences of commands,called macros. Macros can loop and call other macros. Up to 64 macroscan be executing at any time. The common software's telemetry serviceprovides formatting and queuing of telemetry packets. The softwaregathers the packet's data and constructs a Consultative Committee forSpace Data Systems (CCSDS) header with the appropriate flags, sequencenumber, and other information. The packet is queued for transmission tothe spacecraft. The common software also includes status/housekeepingpacket generation, time management, voltage and current monitoring, andI2C bus management services.

MDIS-specific software controls the imager hardware, exposure time, andmechanisms. Commands are accepted from the spacecraft and forwarded tothe appropriate instrument. Telemetry is collected from the instrumentand forwarded to the spacecraft.

Imager Control. Only one of the MDIS cameras, either the NAC or WAC, canbe used at any given time. The MDIS software selects the camera to usebased on user command. Images are collected in three stages: expose theimage in the FPU, read the image from the FPU to the DPU, and finallytransmit it from the DPU to the spacecraft. Different processing stepsare applied to the image during each of these stages. The steps includeexposure, optional 2x2 binning, optional lossy 12-to-8-bit compression,and optional lossless FAST compression.

Exposure Control. The exposure time of images can be set manually bycommand or automatically by the software. In manual mode, a full 1 ms to9989 ms range of exposure times is available. In automatic mode, theexposure time of the next image is computed by the software but limited tobetween 1 ms and 989 ms. This computation has two distinct steps.
The first step computes a new exposure time based on the brightness of aprevious image. The second step anticipates filter wheel motions andadjusts the computed exposure time accordingly.

During the read stage of the image pipeline, the hardware generates ahistogram of the image. The histogram is analyzed by the software todetermine if the image is overexposed or underexposed. First, thehistogram is scaled by a factor of four if it comes from a 2x2 binnedimage. If the brightest histogram value exceeds a saturation threshold(typically 3500 DN), the image is considered overexposed and the exposuretime is scaled back. Otherwise the image is considered underexposed.
Histogram values are accumulated starting from the brightest bin downtowards the dimmest bin, until a commandable, allowed number of pixelsexceeding a target brightness is reached. The brightness value thatcauses the sum to exceed the allowed value is the actual image brightness.
The exposure time is scaled by the ratio of the commanded targetbrightness minus a background brightness (dark current) to the actualbrightness minus the background brightness. The algorithm usesuploadable parameters for the saturation threshold, overexposure fallback,background brightness, target brightness, and the number of pixels allowedto exceed the target brightness.

Next, the algorithm adjusts the exposure time further if the imager,binning mode or filter selected for the next exposure does not match whatwas used in the test exposure. This is done using an uploadable 'filtertransmissivity table' containing relatively sensitivities of the 12 WACfilters and the NAC in binned and unbinned modes (26 values). The exposuretime is scaled by the ratio of the transmissivity of the old setup to thetransmissivity of the new setup. Finally, the computed exposure time isforced to fall within an uploadable range but is always less than 989 ms;this latter step prevents too-long exposures to limit image motion smear.

Mechanism Control. The MDIS pivot platform and filter wheel are controlledby stepper motors. The pivot can move about 1.1 deg/s. Similarly, thefilter wheel can move 75 deg/s. Given twelve filters, an adjacent filtercan be reached in 0.4 s and any filter within 2.4 s.


Image Compression
=================


The MESSENGER mission requires compression to meet its science objectiveswithin the available downlink. Several successive options are available atthe instrument level using the DPU, and using the spacecraft mainprocessor (MP).

At the instrument level,
(1) 2x2 binning is available on-chip to reduce the 1024x1024 images to512x512 format.

(2) 12-bit data number (DN) levels can be converted to 8 bits using any ofeight look-up tables.

(3) Data can be compressed losslessly using the Fast algorithm.

The strategy for image compression at the instrument level is to acquireall monochrome data in 8-bit mode, all color data in 12-bit mode, and tocompress all data losslessly to conserve recorder space.

After data are written to the recorder, they can be uncompressed andrecompressed by the MP more aggressively using any of several options:
(4) Additional pixel-binning can be performed, including options for 2x2or 4x4 compression of the input image. In practice, no more than 4x4 totalcompression (including on-chip and by the MP) is applied to preserved thedark columns.

(5) Up to 5 subframes of the image can be saved, or the whole image can beretained.

(6) For the special case of optical navigation images, there is a'jailbar' option that saves selected lines of an image at a fixed intervalfor optical navigation images of Mercury during flyby approaches.

(7) Compression using an integer wavelet transform can be applied, exceptwhen jailbars are used. The strategy for MP compression is that keyinflight calibrations will be lossless compressed by specifying acompression ratio of 1:1, whereas all science data will be lossycompressed. Compression ratios are typically 8:1 for monochrome data and4:1 for orbital color data. Color imaging but not monochrome imaging maybe further pixel-binned.


Ground Calibration Facility
===========================


All calibration tests were performed in the Optical Calibration Facility(OCF) at APL. The OCF consists of large, linked vacuum chambers with ahost of support equipment. The largest of these chambers, the instrumentchamber, has an internal diameter of 1.3 m and a length of 2 m and permitsmounting the instrument on a three-axis motion stage. The motion stage canrotate the instrument in azimuth and elevation through a range limitedonly by the instrument harness and mounting hardware. Translation ispossible over a limited range, but it is sufficient to center theinstrument in the collimated beam of the chamber. The interior and ends ofthe instrument vacuum chamber are surrounded by cold walls, within whichan external refrigerator circulates a cooling fluid to reduce the internaltemperature to approximately -40C. Most of the calibrations were performedat approximately -30C. However, additional thermal configurations overthe range -34C to +25C were also performed, to bracket the behavior of theinstrument over the range of expected flight conditions.

Inside the instrument chamber, there are two sources of calibratedillumination. In one configuration the instrument views a 20-inch diameterintegrating sphere through a quartz window. Dark cloth was draped aroundthe window and sphere opening in order to block unwanted external lightfrom entering the instrument optical path. Sphere spectral radiance wascalibrated by the manufacturer just prior to the start of instrumentcalibration, for each configuration of the sphere's four bulbs: two 45-Wlamps and two 150-W lamps. One photometer was coupled through a fiberoptic to monitor the internal sphere brightness during all calibrationruns. Two additional photometers were mounted on the camera and allowedadditional monitoring of incident irradiance.

Rotating the azimuthal stage of the OCF 180 degrees enabled the instrumentto look along a beam tube into an off-axis parabolic collimating mirror,which has a focal length of 1.43 m, operates at F/7, and is focused on agrating monochromator. The monochromator may be illuminated by anincandescent lamp with quartz optics. At the input slit, a set of neutraldensity filters may be used to attenuate the light by known amounts. Atthe exit slit, a set of long-pass filters is used to remove higher orders(i.e., shorter wavelengths) from the grating. The wavelength can bechanged manually or can be set to scan under computer control. The gratingmay be positioned to zeroth order, which allows the full incandescentspectrum to be passed.

A MgF2 window partitions the monochromator from the unit under test,allowing the monochromator to be removed without breaking vacuum so thatother sources may be placed at the focus of the collimator. These sourcesinclude a point source (pinhole), pinhole array, and test samplesilluminated by an incandescent source and viewed off a silver fold mirror.

Four networked computers were used to acquire and monitor the calibrationdata.


Calibration Equation
====================


Laboratory and flight measurements were used to derive values for theterms of the calibration equation for both the WAC and NAC. Bothinstruments measure relative light intensity in engineering units referredto as DNs. The raw engineering units are converted to the physical unitsof radiance, following the calibration equation:
L(x,y,f,T,t,b) = Lin[DN(x,y,f,T,t,b,MET) - Dk(x,y,T,t,b,MET) -Sm(x,y,t,b)] / {Flat(x,y,f,b) * t * [Resp(f,b,T)/Correct(f,MET)]}
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 CCDtemperature T and exposure time t, for binning mode b,
DN(x,y,f,T,t,b,MET) is the raw DN measured by the pixel in column x, rowy, through filter f, at CCD temperature T and exposure time t, for binningmode b, and Mission Elapsed Time (MET),
Dk(x,y,T,t,b,MET) is the dark level in a given pixel, derived either fromthe dark strip or estimated from 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 at thispixel location,
Resp(f,b,T) is the responsivity, relating dark-, flat-, andsmear-corrected DN per unit exposure time to radiance,
Correct(f,MET) is a time-variable correction to responsivitydescribing a sudden decrease in transmission of the WAC opticson 24 May 2011 and subsequent recovery to normal values,interpreted as due to contamination associated with MESSENGER'sfirst periapse season over Mercury's hot pole and the subsequentbake-off of the contaminant,
and
t is the exposure time in milliseconds.

The above equation assumes that data are in the native 12-bit format inwhich they were read off the CCD, and that onboard application of 12-to-8bit lookup tables (LUTs) has been inverted.

This correction is done step-wise using the calibration tables andimages in this directory as follows.

Inversion of 12 to 8 bit Compression
====================================

The equation above assumes that DN levels are in their original12-bit form. For any image that has compressed to 8 bits, theinversion is performed using the inverse look-up tables providedin the CALIB directory.

Dark Level
==========

The measured signal from the CCD, in the absence of incident photons, isthe sum of three major components: (1) dark current from thermalelectrons, (2) an electronic offset, or bias, of about 240 DNintentionally added to the readout to prevent occurrence of negativevalues that would appear as zero DN words, and (3) noise picked up byinstrument electronics. There has been no evidence of picked-up noise. Inthe following discussion, the sum of dark current and bias is treatedtogether as 'dark level.'
The response of pixel-dependent dark level to temperature and exposuretime was characterized separately for the NAC and WAC without on-chipbinning ('full-frame') and with on-chip binning, over the CCD temperaturerange 25C to -40C.

In the NAC full-frame data, at short exposure times and low temperatures,the dark level is dominated by the electronic bias and the image meandecreases with increasing temperature. This relationship originates in theamplifier in the read-out electronics and has a temperature-dependentoffset. After an image has been exposed, large current pulses are sentfrom the power supply to the clocking signals controlling the movement ofcharge in the CCD. These high-current clocking pulses increase the overallsignal levels for a very short time, as observed in the measured DNs. Atlonger exposure times (>200 ms) and higher CCD temperatures (> -30C) theeffects of dark current, which increases exponentially with temperature,dominate. In binned images, because on-chip binning adds the outputs offour pixels prior to quantization, the impact of the dark current islarger and the onset of increasing dark level with temperature andexposure time occurs at lower temperature and shorter exposure time.

WAC dark levels are dominated by the increase in dark current with highertemperature and longer exposure time.

Detector Temperature Calibration:
CCD temperature is measured by a temperature sensor bonded to the CCDheat-sink mounting plate on each camera. This temperature sensor, whosevalue is reported in instrument housekeeping, was calibrated during darklevel testing, where it was possible to install multiple thermocouplesthroughout the MDIS instrument. In particular, a thermocouple located inclose proximity to the thermal link on the wax pack of each FPU was usedas the best measurement of the actual CCD temperature. A linear regressionwas used to fit the measured response of the CCD temperature sensor withthe calibrated thermocouple response. The other instrument temperaturesensors, located on each FPU housing and the WAC filter wheel and NACtelescope, were calibrated in a similar fashion. The final expressions toconvert the digital raw output to temperature are of the form
T(deg c) = A * raw_counts + B
Camera Temperature Name A BWAC CCD temperature MESS:CCD_TEMP 0.2718 -318.455WAC FPU temperature MESS:CAM_T1 0.5022 -262.258WAC filter wheel temp. MESS:CAM_T2 0.553 -292.760NAC CCD temperature MESS:CCD_TEMP 0.2737 -323.367NAC FPU temperature MESS:CAM_T1 0.5130 -268.844NAC Telescope temp. MESS:CAM_T2 0.4861 -269.718
Dark Current Model:
For the purpose of modeling dark current, the WAC and NAC were treated asfour different sensors: NAC full-frame, NAC binned, WAC full-frame,and WAC binned. Two dark models were derived from the OCF calibrations foreach of the four modeled sensors: (1) a forward model of the dark currentas a function of column number, row number, exposure time, and raw CCDtemperature (DN), and (2) a backward model that uses the row-dependentdark levels in the dark strips and extrapolates them across the FOV usingthe same variables as the forward model. For both the forward and backwardmodels, dark level is treated as a linear function of column number, rownumber, and exposure time. The exponential temperature dependence wasapproximated with a third-order polynomial for ease of calculation. Whileit is known that the CCD temperature response will change with time due toexposure to radiation in space, this specific time dependence will bedetermined in flight over the lifetime of the mission.

The forward is more accurate and is described below.

Consider the first of four variables to the dark model: column position x,row position y, exposure time t, and temperature T. For the column-dependence at a particular row, exposure time, and temperature, dark levelis modeled linearly where an offset alpha(y, t, T) is the value at columnx=0, and a slope beta(y, t, T) is the increase per column (per incrementof x). Then, to find the dark level at a given column, the model for thisrow, exposure time, and temperature is a linear function:
Dk(x,y,t,T) = alpha(y,t,T) + beta(y,t,T) * x
However, the column-dependence will vary with row, so that in the secondlevel of the model, the offset alpha and slope beta are themselves afunction of row position y. Thus, linear expansions are:
alpha(y,t,T) = A(t,T) + B(t,T) * ybeta(y,t,T) = M(t,T) + N(t,T) * y
Note that at this level of expansion of the dark model, the row-dependences of column-dependences are themselves dependent on exposuretime and temperature. A third level of linear regression can be added tothe model, such that A(t,T), B(t,T), M(t,T), and N(t,T) can each beexpressed as its own linear function of exposure time t, using atemperature-dependent offset C(T), O(T), E(T), or Q(T) respectively, and atemperature-dependent slope D(T), P(T), F(T), or S(T) respectively. Theexpanded dark model at this level becomes:
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
At the fourth and final level of expansion, C(T), D(T), E(T), F(T), O(T),P(T), Q(T), S(T) are all third-order functions of temperature, forexample:
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 or0-511 for a binned image, t is in units of milliseconds, and T is inUNCALIBRATED raw counts of CCD temperature.

The forward dark current model was derived by first finding a linear fitof dark level as a function of column number for each row in the darkcalibration data. Second, the bias and dark DN accumulation rate percolumn increment were fitted linearly as functions of row. A, B, M, and Nwere fitted as functions of exposure time, for data binned into fifteendifference CCD temperature bins (each bin 10 DN wide) covering the range1000-1150 DN (-40.8C to -6.8C). Finally, the linear fit coefficients asfunctions of exposure time at different temperatures were fitted as afunction of temperature with a third-order polynomial.

The coefficients of these expansions are provided in the CALIB directory.

Frame Transfer Smear Correction
===============================

An image is exposed for a nominal integration time and is then transferredto the memory zone of the CCD, from which the analog signal is digitizedline by line. Accumulation of signal continues during the finite durationof frame transfer, inducing a streak or frame-transfer smear in the wakeof an illuminated object in the field of view, parallel to the directionof frame transfer. Quantitatively, the smear correction is:
Sm(x,y,t,b) = SUMM(1,y-1) { t2/t * [DN(x,y,t,b) - Dk(x,y,T,t,b) - Sm(x,y,t,b)] / Flat(x,y,b,f)}
where
Sm(x,y,t,b) is the smear in column x and row y at exposure time tin binning mode b,
Dk(x,y,t,b) is the dark level in column x and row y at exposure time tand temperature T in binning mode b,
Flat(x,y,b,f) is the flat-field correction in column x and row y inbinning mode b and filter f,
t is exposure time in ms, and
t2 is the time for frame transfer (about 3.4 ms) divided by the number oflines in the image in the direction of frame transfer, i.e., 1024 forfull-frame images or 512 for binned images.

Response Linearity Correction
=============================

Calibration of the signal accumulation rate per unit time per unitradiance at the sensor was conducted in the OCF for the WAC and the NACover a broad range of exposure times and source light intensities using awhite integrating sphere. The exposure times to saturation were determinedempirically in initial set-up. Using a variety of temperatures (-34C to25C) and source intensities, images were acquired after varying theexposure times until saturation occurred. Source intensities were steppedin a fixed decreasing-light pattern through the eight levels that could beachieved with the integrating sphere's two 150-W and two 45-W bulbs. Thebulbs were allowed about 20 minutes of warm-up to asymptoticallyapproaching constant light output. The WAC calibration procedure beganwith all lamps on then subsequently turned lamps off in sequence. Theexposure sequence was repeated for each filter for the WAC, but only asingle exposure sequence was required for the single filter of the NAC.

The relationship between the raw DN output from the camera per unitexposure time and radiance, as a function of variation in exposure time orradiance is referred to as response linearity. Using dark-corrected,desmeared DN values taken from the center quarter of the images, responselinearity was measured with both the NAC and WAC binned and full-frame.
Linearity was first examined separately for (a) linearity with respect toexposure time at individual light levels, and (b) linearity with respectto radiance at individual exposure times. The results are notsignificantly different, so all data were merged to examine linearity withrespect to measured photons (the product of radiance and exposure time).
At corrected DN levels >1000, the CCDs are linear to within the errorbars. However, there is a decrease in responsivity at lower DNs, by up to2% at the lowest DN levels.

To restore or 'linearize' WAC image data, the following corrections shouldbe applied after correction of dark current, bias, and smear.

For DN_dark > 1DN_lin = DN_dark_smear/[0.008760 * Ln(DN_dark_smear) + 0.936321]
For DN_dark <= 1DN_lin = DN_dark_smear/0.936321
To restore or 'linearize' NAC image data, the following procedure shouldbe applied after correction of dark current, bias, and smear.

For DN_dark > 1DN_lin = DN_dark_smear/[0.011844 * Ln(DN_dark_smear) + 0.912031]
For DN_dark <= 1DN_lin = DN_dark_smear/0.912031
where
DN_dark is dark-corrected input DN
DN_dark_smear is the input dark- and smear-corrected DN.

DN_lin is linearized dark- and smear-corrected DN.


Flat-field Correction
==========================

The flat field correction removes pixel to pixel differences in detectorresponsivity, so that the responsivity coefficients can be expressed asscalars for each filter. The flat field is derived using a linearized,dark-corrected, desmeared image of a field- and aperture-filling, uniformsource and normalizing the mean value to unity over the center quarterof the images.

Two factors make the ground-derived flat-field less than ideal. First,MDIS's thermal blanketing generates reflections so that in the calibrationchamber, the illuminated source created glint off the blanketing thatreflected off the chamber window, adding spatially non-uniform stray lightto the measurements. Second, eliminating the backscattering off thechamber window required acquisition of flat-fields at ambient (room-temperature) conditions at which residuals from the dark currentcorrection introduce artifacts. Ultimately the latter approach was chosenfor ground derivations.

The flat-fields have been rederived inflight using images of a part ofVenus near the sub-solar point, with a photometric correction applied toremove effects of variation of solar incidence angle across the scene.

Flat-fields for different filters and binning states are given in theCALIB directory.


Responsivity
============

Responsivity is linearized, dark-corrected, desmeared DN per unit time perunit radiance. Responsivity through each filter in each binning state(except WAC filter 2, which is too sensitive to be characterized with thisexperimental setup) was determined onground using the integrating-sphere,whose radiances was calibrated by Labsphere using each of the four lampsseparately, over a wavelength range of 350-1100 nm in 5-nm steps. Todetermine radiances through each of the WAC and NAC filters, the bandpasses described below were convolved with the sphere's spectral radiance.
For data taken at room temperatures, the chamber door was open, but forlower-temperature measurements the sphere was viewed through the quartzwindow in the OCF chamber door. Therefore for the low-temperature data, anadditional correction was applied to the sphere radiance for each filterto account for the window. This correction was derived from room-temperature data, as the ratio of corrected DNs per unit time with thedoor closed to that with the door open, for a single configuration ofsphere bulbs.

It is expected the temperature-dependence of responsivity over thetemperature range used will be approximately quadratic. The bestcorrection available from the combination of flight and ground data isquadratic in form. The application of the linear correction is:

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 binningstate 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 CCDtemperature of 1060 DN (-30.3C),
correction_offset(f,b) is camera- and filter-dependent temperaturecorrection offset for filter f and binning state b,
correction_coef1(f,b) is the camera- and filter-dependent temperaturecorrection first-order coefficient for filter f and binning state b.

correction_coef2(f,b) is the camera- and filter-dependent temperaturecorrection 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) * Correct(f, MET))
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 milliseconds,
Resp(f,T,b) is the responsivity in filter f at CCD temperature T andbinning state b, and
Correct(f, MET) is a temporal correction to responsivity forfilter f at time MET.

Values for Resp(f,T,b) and Correct(f, MET) for different filters and binningstates are given in the CALIB directory.

Filter Bandpasses:
The NAC uses a single filter, while the WAC views the scene through afilter wheel outfitted with 12 filters of varying widths. The WAC filtersare labeled by their position counter-clockwise around the wheel as viewedfrom the CCD. Filter 2 is a clear fused silica filter spanning the entirepassband of the CCD (395-1040 nm), and filters 1 and 3-12 are narrowbandfilters.

Using the OCF monochromator, NAC and WAC filter transmissions weremeasured as functions of wavelength for CCD temperatures ranging from -35Cto about 26C. The source appeared in the data as a bright rectangle in thecenter of each image. After subtracting the dark model from the imagevalues and performing smear corrections, a mask was derived from eachimage based on bright and dark pixel distributions. The values in the maskwere set to zero for the dark regions and unity for the source rectangle.
After multiplication, the pixel values for the resulting image were summedto achieve maximum response with varying source wavelength. For summedsignal at each monochromator wavelength, the data were normalized to thehighest sum, at the most sensitive wavelength.

The center bandpass was solved assuming a square function, by summing thenormalized signals times the delta wavelength range covered by each. The50% point of the cumulative distribution was defined as the centerwavelength.

The derived center wavelengths and bandpasses are given above, and therelative filter sensitivities as a function of wavelength (used to modelthe radiance of the white integrating sphere as sampled by MDIS) aregiven in the CALIB directory.


Conversion from radiance to I/F
===============================

To convert from radiance to I/F (also known as radiance factor, the ratioof measured radiance to that which would be measured from a whiteperfectly Lambertian surface), the following expression should be applied:
I_over_F(f) = L(f) * pi * (SOLAR_DISTANCE/149597870.691)**2 / F(f)
where
L(f) is calibrated radiance calculated as described above for some filterf,
SOLAR_DISTANCE is that value for distance of the target object from thecenter of the sun in kilometers (as indicated by the keywordSOLAR_DISTANCE)

149597870.691 is the number of kilometers in 1 AU
F(f) is effective average solar irradiance sampled under the filterbandpass.

The effective average solar radiance for each camera and bandpass is givenin the CALIB directory.


Operational Considerations
==========================

(1) WAC clear filter: Filter 2 on the wide-angle camera is broad-band anddesigned for star imaging. It is expected that typically even the minimumexposure time will saturate on Mercury. Therefore flat-field andresponsivity corrections for WAC filter 2 were not derived.

(2) NAC PSF. Due to mass constraints, the NAC aperture is smaller thanwhat is required for diffraction-limited performance. The expected size ofthe Airy disk (approximately, the full-width at half-maximum of the point-spread function including only effects of diffraction) is > 2 pixels. Inpractice the PSF is further broadened by surface imperfections of opticalelements and scatter centers on optical surfaces.

(3) Compression. Wavelet compression applied to science images istypically lossy. Pre-launch, a study was done in which the appearance ofartifacts at progressively higher wavelet compression ratios wasinvestigated using raw images from the NEAR multispectral imager.
Artifacts were identified by differencing raw images and the same imagesafter compression and decompression. Spatially coherent artifacts wereevident in monochrome images at compression ratios >8, and in color ratioimages >4. Hence wavelet compression ratios of 8 and 4 were selected fornominal application to monochrome mapping and multispectral imagesrespectively.

(4) Frame transfer smear. At very short exposure times (<7 ms), the timefor frame transfer is close to the total exposure so that the correctionfor frame transfer smear may leave perceptible artifacts.

(5) Data Quality Index. A data quality index in the PDS label for eachimage is used to encode figures-of-merit into one parameter. The 16-bytedata quality index includes automated assessments of validity of theexposure time, presence of an excessive number of pixels at or approachingsaturation, validity of the reported pivot position, validity of thereported filter wheel position, quality of spacecraft attitude knowledgefrom the MESSENGER star cameras, CCD temperature within a well-calibratedrange, and completeness of data within the commanded selection ofsubframes or full frame.

(6) Temperature effects on attitude. The orientation of MDIS relative tothe spacecraft reference frame was determined inflight using starcalibrations to solve for WAC-NAC coalignment, the orientation of thepivot plane, and the origin of the reported pivot position within theplane. However these alignments can be affected by thermal state of thespacecraft. Mercury and Venus flybys are thermally benign. However inMercury orbit the noon-midnight orbit experiences large thermalperturbations and errors in reported MDIS attitude of up to 350microradians can be expected.