| DESCRIPTION |
Instrument Overview
===================
The Mercury Dual Imaging System (MDIS) on the MESSENGER spacecraft
provides critical measurements tracing Mercury's origin and evolution.
MDIS consists of a monochrome narrow-angle camera (NAC) and a
multispectral wide-angle camera (WAC). The NAC is a 1.5 degree field-of-
view (FOV) off-axis reflector, coaligned with the WAC, a four-element
refractor with a 10.5 degree FOV and 12-color filter wheel. The focal
plane electronics of each camera are identical and use a 1024x1024 Atmel
(Thomson) TH7888A charge-coupled device (CCD) detector. Only one camera
operates at a time, allowing them to share a common set of control
electronics. The NAC and the WAC are mounted on a pivoting platform that
provides a 90 degree field of regard, extending 40 degrees sunward and 50
degrees anti-sunward from the spacecraft +Z-axis, the boresight direction
of most of MESSENGER's instruments. Onboard data compression provides
capabilities for pixel-binning, remapping of 12-bit data into 8 bits, and
lossless or lossy compression.
Scientific Objectives
=====================
The MESSENGER spacecraft made three flybys of Mercury, in January
2008, October 2008, and September 2009, during which regions unexplored by
Mariner 10 were imaged by MDIS. In March 2011, the spacecraft began the
orbital phase of its mission, which including the Primary Mission and two
Extended Missions continues through spacecraft impact in March or April
2015.
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 the orbital 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 thermal
distortions.
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 surface
at a low altitude.
For monochrome global maps, data are acquired with the NAC for southern
latitudes when spacecraft altitude is high, and with the WAC at lower
altitudes over the northern hemisphere. This two-camera strategy results
in near-uniform global coverage with an average spatial resolution
of about 200 m/pixel. Off-nadir mosaics for stereo were acquired under
nearly identical lighting geometries to the nadir map to facilitate
automated stereo matching.
Subsystems
==========
MDIS consists of two cameras, a monochrome narrow-angle camera (NAC) and a
multispectral wide-angle camera (WAC), coaligned on a common pivot
platform. The passively cooled detectors in each camera are thermally
tied to the complex thermal system. This allows the detectors to be
maintained within their operating temperature of -10C, except during the
hottest portion of the orbit at Mercury. The pivot platform provides an
added degree of freedom to point the dual cameras with minimal impact on the
spacecraft. MDIS is mounted in MESSENGER's payload adaptor ring and faces
in the nadir direction in Mercury orbit along the spacecraft +z axis, with
the pivot aligned in the y-z plane where -y is toward the sun.
Specifications of the two cameras are given below:
Narrow Angle Wide Angle
Field of view 1.5 x 1.5 degrees 10.5 x 10.5 degrees
Pivot range Both -40 deg (toward Sun) to
+50 deg (away from Sun)
Exposure time 1 ms to 9989 ms 1 ms to 9989 ms
Frame transfer time 3.4 ms 3.4 ms
Image readout time 1 s 1 s
Spectral filters 1 filter 12 filters
Spectral range 725-783 nm 395-1040 nm
Focal length 550 mm 78 mm
Collecting area 462 mm**2 48 mm**2
Detector Both TH7888A CCD, 1024x1024
14-micron pixels
IFOV 25 microrad 179 microrad
Pixel FOV 5.1 m at 200-km altitude 35.8m at 200-km altitude
Quantization Both 12 bits/pixel
Compression 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 well
as two redundant external Data Processing Units (DPUs). The dual camera
assembly without the DPUs is simply referred to as 'MDIS'. Only one DPU
may be active at a time, and due to thermal constraints only one camera
will operate at a time; however, observations with the two cameras can be
interleaved at 5-second intervals. A separate electronics assembly
accommodates switching between the various modes of operating with the
redundant DPUs. The pivot platform has a large range of motion (240
degrees) to allow the cameras to be 'tucked away' to protect the optics
from contamination. The pivot motor drive-train provides precision
rotation over the 90-degree operational range of motion about the
spacecraft +Z axis.
A spectral calibration target is mounted on the inside of the payload
adapter ring. Early in the MESSENGER mission it was possible to tilt the
spacecraft in order to provide solar illumination on the calibration
target. The large range of motion of the pivot assembly enabled either
camera to point at the target permitting an absolute inflight radiometric
calibration and flat-field measurement.
Optical Design
==============
The WAC consists of a 4-element refractive telescope having a focal length
of 78 mm and a collecting area of 48 mm**2. The detector located at the
focal plane is an Atmel (Thomson) TH7888A frame-transfer CCD with a
1024x1024 format and 14-micron pitch detector elements that provide a 179-
microrad pixel (instantaneous) field-of-view (IFOV). A 12-position filter
wheel provides color imaging over the spectral range of the CCD detector.
Eleven spectral filters spanning the range from 395 to 1040 nm are defined
to cover wavelengths diagnostic of different potential surface materials.
The twelfth position is a broadband filter for optical navigation. The
filters are arranged on the filter wheel in such a way as to provide
complementary 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 and
a collecting area of 462 mm**2. The NAC focal plane is identical to the
WAC's, providing a 25-microrad IFOV. The NAC has a single medium-band
filter (50 nm wide), centered at 750 nm to match to the corresponding WAC
filter for monochrome imaging.
One of several impacts of the thermal environment on calibration accuracy
is the relative responsivity of the CCDs at wavelengths longer than about
700 nm. Response at longer wavelengths increases strongly with
temperature. If data are acquired over a large temperature range,
inaccuracies in correction for temperature-dependent response
introduce systematic errors in spectral properties at 850-1000 nm that
ultimately could lead to false mineralogic interpretations. To protect the
MDIS CCDs from wide temperature swings, incoming thermal infrared
radiation (IR) is rejected in the optics by heat-rejection filters on the
first optic of each camera. In the WAC, this rejection is accomplished
using a short-pass filter as the outer optic; for the NAC the bandpass
filter has a specially designed heat-rejection coating on its first
surface.
WAC Design
==========
Telescope Design. The WAC consists of a refractive telescope, dictated by
the required wide FOV and short focal length. The design approach was to
select the simplest lens design that gives acceptable image quality over
the field; however, an important constraint on the design is the limited
selection of glasses because of the radiation environment. The design
requirements were achieved by starting from a simple Cooke triplet and
splitting the central negative element. The resulting design, a Dogmar,
gives good image quality although not over the full 395-1040 nm spectral
range. The uncorrected axial chromatic aberration was reduced by varying
the thickness of each filter.
The WAC is focused at infinity with a 78-mm focal length to spread the
10.5 degree FOV across the 14.3-mm detector. The 14-micron square pixels
of the CCD provide an instantaneous FOV (IFOV) of 179 microrad. The field
curvature 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 of
the field, less than a pixel. The design is diffraction-limited over
the entire field, and MTF degradation is dominated by the pixel size.
Spectral Filter Design. A 12-position multispectral filter wheel provides
color imaging over the spectral range of the CCD detector (395-1040 nm).
Eleven spectral filters are defined to cover wavelengths diagnostic of
common silicate minerals and glasses and have full-width half maximum
(FWHM) bandwidths from 5-40 nm. A broadband clear filter is included
for 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 desired
passband. The variation in filter thickness used to remove residual
chromatic aberration results in a small variation in the focal length of
the camera between filters. The extreme filters give a focal length of
about 78 mm at 480 nm and about 78.5 mm at 1020 nm respectively. This
difference results in a variation in the image scale of 0.7 percent.
By positioning the filter in front of the detector, the size of the
filters is minimized. However, the incident angle of the beam on the
filter varies with field angle, causing a shift in the spectral bandpass
of the interference filters across the field. The effect of the incident
angle of the converging rays on the filter is much more serious than
the spatial variations across the surface of the filter. With a maximum
angle of 7.6 deg. at the corner of the field, spectral shifts of about 3 nm
are 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 large
collecting area is not required for sensitivity. The reflective design has
a long focal length that required folding the optical path in order to fit
in the available volume. The monochromatic design has a single medium-
band filter centered at 750 nm with a FWHM of 50 nm. The center
wavelength was chosen to match a corresponding WAC filter to
facilitate 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 was
selected over a Cassegrain in order to avoid the central
obscuration of the secondary mirror. The ellipsoidal primary mirror and
hyperboloidal secondary mirrors are gold-coated aluminum. In this design,
the image plane is tilted at an angle of approximately 9 degrees to
locate the optimal image quality.
A bandpass filter is the first optical component of the assembly and
defines the spectral range of the instrument. A specially designed
interference coating serves as a heat-rejection filter to minimize
admittance of IR radiation from Mercury that would heat the CCD during
imaging and increase background levels.
Due to mass limitations the aperture is less than
required for diffraction limited performance, so the Airy disk is
approximately 2 pixels across. Low distortion of the image was an
important design specification for the NAC. At the edge of the FOV this
amounts to 1.28 pixels. The all-aluminum assembly of the NAC makes it
insensitive to thermal distortions over the operational temperature range
of the instrument.
Filters Bandpasses
==================
The WAC camera utilizes a twelve position filter wheel with band passes
between 415 and 1020 nm. The NAC is a broadband imager with a bandpass
centered near 750 nm.
Filter Filter Center Bandpass
Number letter wavelength width
in file (nm) (nm)
name
1 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 are
controlled directly by the DPU software to move the platform. The phase
pattern can be adjusted by software to move the platform forwards or
backwards. The pivot platform's range of motion is mechanically
constrained by hard stops. The range of motion is further constrained by
soft stops applied by the software. The total range of motion of MDIS is
about 240 degrees, limited by hard mechanical stops in the pivot motor.
The hard stops are fixed at -185 degrees and 55 degrees from the
spacecraft +z axis. The pivot motor drive-train provides precision
rotation over the 90 degrees operational range of motion 40 degrees
sunward and 50 degrees antisunward of the spacecraft +z axis.
The MDIS pivot actuator is capable of accurately stepping in intervals of
0.01 degrees (about 150 microrad) per step. Pointing knowledge is
determined by first 'homing' the instrument, which is accomplished by
driving the actuator into one of the mechanical hard stops for a period of
time sufficient to ensure the orientation of the instrument if it had been
previously stopped at the opposite extreme of travel. Once the location
of the pivot actuator is known, the flight software retains this knowledge
and subsequent pointing commands are achieved by counting pulses (steps)
to the motor.
Thermo-mechanical Design
========================
The MESSENGER spacecraft is protected from direct solar illumination by
the spacecraft sunshade. The thermal environment of the instruments is
largely benign with the exception of short intervals during the 12-hour
orbital period when Mercury's illuminated surface subtends a large solid
angle. The passive thermal design of MDIS buffers temperature changes to
prevent the instrument from exceeding the CCD's high operational
temperature limit (-10C to limit the noise contribution from dark
current) except for occasional short periods. Survival heaters ensure
that cold operational limits are not violated.
The DPU is directly coupled to the spacecraft deck and blanketed with
multi-layer insulation (MLI). The -40 C to +50 C operating temperature of
the DPU follows the deck temperature. The temperature of the imager
interface board and the low-voltage power supply board are measured
internal to the DPU and reported in the instrument housekeeping data. The
MDIS temperature sensors are mounted at key locations on the instrument
and are reported in image headers. The CCD temperature is the most
important temperature for calibration.
Thermal Design of the MDIS Platform Assembly. Each camera has its own
thermal control system, and each CCD is mounted to a heat sink that bolts
onto a bracket that is thermally isolated from the focal plane unit (FPU).
Titanium spacers minimize heat conduction from the FPU to the CCD while
maintaining focus over the operating temperatures of the instrument.
A thermal link made of made of multiple sheets of thin aluminum foil
ties 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 a
conductive epoxy, thermally ties the wax pack to the evaporator
portion 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 common
radiator assembly made of three beryllium plates mounted to the instrument
with 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. During
normal operation, heat flowing from the CCD and through the wax pack
is absorbed at the evaporator. Liquid butane is evaporated and the
gaseous butane flows down the center of the tube toward the radiator to be
condensed on the cold walls of a condenser. An internal stainless steel
wire mesh acts as a wick to transport the liquid from the condenser back
to the evaporator, continuing the fluid loop.
A separate pipe, the liquid trap, is attached to the evaporator section
that allows the heat pipe to act as a diode, i.e. allowing heat to flow
only in one direction. A small tube connects the liquid trap to the
evaporator. The diode action occurs when the condenser/radiators becomes
warmer than the evaporator and the butane liquid in the condenser
vaporizes. The now gaseous butane flows down the pipe and condenses onto
the walls of the relatively cool (-10C) evaporator and liquid trap.
Since there is no wick in the small tube between the liquid trap and the
evaporator, the liquid cannot flow out of the trap, back to the
evaporator, and up to the condenser for re-evaporation. As the radiator
plates continue to heat up, more liquid is vaporized and flows down to the
liquid trap area. This process continues until most of the liquid is
located in the liquid trap and no more liquid can flow back up the pipe to
the condenser. In this state, the heat pipe is effectively shut off and
there is no longer any heat transfer between the evaporator and condenser.
Once the radiators cool back down and the condenser again drops below the
evaporator temperature, the heat pipe will start back up and conduct heat
like a regular heat pipe.
On orbit at Mercury, when the spacecraft is away from the hot planet, the
MDIS radiator plates do not see the planet and are only exposed to deep
space. In this condition, the radiator plates become very cold (-77C)
and the diode heat pipes operate as normal heat pipes. The gas internal to
the pipe moves freely along the length of the pipe, transporting heat from
the wax pack out to space. The heat from the CCD flows from the CCD
through the thermal link, through the wax pack and thermal strap, to the
evaporator of the heat-pipe. As the liquid in the pipe evaporates, heat
flows along the heat pipe to the condensers and finally to the radiator
panels. When the paraffin in the wax pack is all frozen, the wax pack,
thermal link, and CCD continue to cool below the freezing point of
the paraffin.
As the spacecraft starts to approach the planet, the radiator plates begin
to absorb heat from the hot planet. When the temperature of the condenser
rises above the temperature of the evaporator, most of the heat-conducting
gas in the pipe condenses in the liquid trap, effectively shutting off the
heat pipe. Heat that is still flowing from the CCD will now be absorbed
into the wax. The paraffin warms up to its melting point and remains
at that temperature until all 240 g of paraffin has melted. The amount of
wax was selected so that all the wax would not completely melt during the
hours surrounding periapsis. The melting wax clamps the temperature of
the CCD at warm operating temperature limit (-10C). As the spacecraft
moves farther away from the planet, the MDIS radiator and condenser
temperatures fall below the evaporator temperature and allow the heat
pipe to operate normally again. With the condenser temperature below the
evaporator temperature, heat flows from wax pack to the radiators and
begins refreezing the paraffin. The partially melted wax pack remains
at its melting (freezing) point until all the paraffin solidifies. At
that point, the wax pack temperature continues to drop down to -45C
where heaters and thermostats control the minimum temperature.
Other than the CCD-to-heat pipe heat path, the majority of the camera
components are thermally connected to the main platform. Heaters on the
platform prevent the camera optics and FPU electronics from dropping below
-35C. There are no radiators associated with the platform. The cameras
and platform are isolated from the environment by MLI blankets and do
not overheat due to the relatively small amount of heat dissipated in the
electronics.
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 uses
frame transfer to obtain electronic exposure control. The active image
forms an array 1024x1024 pixels (14.3x14.3 mm) in size in which
optical energy is accumulated. At the end of the exposure period, the
accumulated charge in each pixel is transferred in about 3.4 ms to
the masked-off 1024x1024 memory zone. A tantalum radiation shield not
only protects the CCD from ionizing radiation but also blocks
illumination to the memory zone. The combined image and memory zones'
1024x2048 pixel array has two on-chip output amplifiers, though
only one amplifier is used in the MDIS design. Manual and autoexposure
control from 1 ms to 9989 ms permit imaging over a broad range of
intensities.
The CCD provides 4 specially shielded columns to provide an accurate
measure of the dark level in a full resolution image (1024x1024). In the
case of 2x2 binning, the corresponding columns are two wide but shifted
out of the shielded area of the CCD.
The antiblooming feature of the CCD causes a scene-dependent nonlinearity
at high DN levels. Small sources do not typically exceed 3500 DN, but on
larger sources where antiblooming is overwhelmed, higher DN values can
occur.
Electronics
===========
The main electronics systems of MDIS include the DPUs, the DPU Interface
Switching Electronics (DISE box), and the FPU camera electronics.
Focal Plane Electronics. The detector electronics for both the WAC and NAC
are identical. Each CCD is bonded to a camera-specific mounting bracket
(heat-sink) prior to assembly into the FPU electronics. The NAC heat-sink
is tilted 9 degrees to match the optimal orientation of the NAC focal
plane, whereas the WAC heat-sink has no tilt. Due to thermal, power, and
operational constraints, only one camera operates at a time. The frame
rate of each FPU is fixed at 1 Hz; however, the frame rate to the
spacecraft is not fixed but cannot exceed 1 Hz.
The focal plane electronics are reduced in size to save mass. A type of
construction of the printed circuit boards known as rigid-flex enabled the
electronics to be folded into a small space. The clock drivers use an
integrated circuit (IC) designed to drive the gates of power metal oxide
semiconductor field effect transistors (MOSFETs) and are almost ideal to
drive the capacitive gates of a CCD. The TH7888A has a relatively high
internal gain of 6 microV/e. This value is large enough to drive the
correlated double sampler analog-to-digital converter (ADC) integrated
circuit directly while meeting the system noise requirements. The output
is a 12-bit digital word for each CCD pixel value. A Field-Programmable
Gate Array (FPGA) is used to provide clocks for the CCD and ADC derived
from an input clock from the DPU. The DPU also provides a coded exposure
time signal for the camera, as the autoexposure algorithm is in the DPU.
The FPGA also formats the data for transfer back to the DPU. The
interface between cameras and DPU uses low-voltage differential signaling
for low noise and to tolerate differences in grounding between the units.
DISE. The MDIS instrument contains six major electrical subsystems: two
FPUs, a filter wheel motor, a filter wheel motor resolver, a platform
pivot motor, and a platform pivot motor resolver. The pivot motor
contains redundant windings with one set going to each DPU. The remaining
systems 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 while
minimizing the connections through the rotary twist-capsule feedthrough.
There are two switching functions: one selects which DPU is master and the
other selects which camera will be active. After the DPU asserts itself
as master, it then applies power to the camera of interest. Each DPU
provides separate power lines for each camera. The DISE board uses these
power connections to power its internal circuitry that selects the correct
camera as well as powering the camera of interest. This selection is done
in a master/slave configuration with the NAC having priority. If power is
supplied to both cameras at the same time, signals from the NAC will be
transmitted. This arrangement ensures that the master DPU communicates
with the intended camera.
In addition to delivering each camera with the 15 V provided by the master
DPU, the DISE box linearly regulates a 6-V power supply. This regulation
is done in the DISE box to limit power dissipation and reduce mass in the
cameras. To ensure good thermal conduction of these components so that
they can safely dissipate their heat, they are mounted directly to the
lower corners of the DISE box frame. The DISE box switches control lines
using the low-voltage differential signaling (LVDS) protocol to and from
each camera. Each camera has a single LVDS interface, and each DPU has a
single LVDS interface. Switching between them is done with cold sparing
LVDS 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 the
instruments and the spacecraft, and it provides complete support for the
MDIS camera electronics. Two separate sets of electronics boards are
physically stacked and packaged together.
The Event Processing Unit (EPU) board provides data processing capability
and creates the telemetry data packets passed (through the DPU) to the
spacecraft. Its design is based on the Intersil (formerly Harris) RTX2010
16-bit processor. In addition to the RTX2010, the EPU board contains 64
kB of fuse-link programmable read-only memory (PROM) for boot code, 256 kB
of electrically erasable PROM (EEPROM), and 256 kB of SRAM. An Actel
RT54SX32S FPGA provides all the logic necessary to interconnect the
processor with the stacking connector data bus. The EPU also receives
(via the DPU) a once per second timing pulse from the spacecraft. This
synchronization pulse is a key part of the overall timekeeping design to
maintain <1-ms end-to-end timing error between the mission operations
center and the payload.
The Low Voltage Power Supply (LVPS) board converts spacecraft primary
power, ranging from 20 V to 36 V, to isolated secondary power. The LVPS
board utilizes an in-rush limiter, a VPT electromagnetic interference
(EMI) 461 filter, and two VPT hybrid dual output converters (one for +/-5
V and the other for +/-12 V). These converters provide better than
1 percent load and line regulation for nominal operation. Two radiation-
hardened opto-field effect transistor (FET) relays were used to switch up
to 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 to
monitor four voltages and six currents on the board, and five analog
voltages from off the board; the remaining channel measured board
temperature.
Communication between each DPU and the instruments (other than MDIS) takes
place through a Universal Asynchronous Receiver/Transmitter (UART) hub
board with eight 38.4-kbaud, RS-422, full duplex UART channels. Each
channel has a separate first circuit interface for maximum fault
tolerance. Independent SRAM-based receive buffers indexed by FPGA-based
registers are used to offload byte-by-byte operations from the DPU
software.
Communications between each DPU and the spacecraft is accomplished through
MIL-STD-1553 buses. A 1553 Interface Board has a design is based on the
UTMC SUMMIT UT69151DXE protocol/transceiver chip and also includes an
Actel RT14100A FPGA and 64 KB of SRAM. Each DPU is a remote terminal on
the spacecraft 1553 bus. All payload commands and telemetry, except MDIS
image data, are communicated via the 1553 bus.
All image acquisition control and hardware image processing occurs in the
Imager Interface Board. It utilizes two Actel RT54SX72S FPGAs, 2 MB of
SRAM, and a number of LVDS receiver and transmitter circuits to
communicate both with the FPUs and the spacecraft high-speed
Integrated 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 received
from the active FPU and buffered in SRAM. If the DPU receives a command
to record an image, the image data are transferred to either of the two
redundant 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 LVDS
interface at 16 MHz. A full 1024x1024 image requires approximately 850 ms
to 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 sent
to the IEM at 1 Hz; however, full-frame images can be sent only at 0.25
Hz. This interface limitation to the IEM requires the imager-interface
FPGA to buffer the images in SRAM. The main functions of the RTX-bus FPGA
are to store a 1024-bit image header and the FPU command words generated
by software running on the RTX processor in the DPU. This FPGA is also
capable of generating a 32-bin histogram created from the pixel values
produced by the FPU. This histogram is made available to the RTX
processor and is integral to the MDIS autoexposure algorithm.
The MDIS Imager Power Board provides switched +15 V to the NAC and WAC
electronics. A separate MOSFET-switched output is used for each NAC and
WAC FPU electronics.
Each of the pivot and filter wheel actuators has a dedicated Motor
Controller Board. Although the characteristics of the two actuators are
different, the board designs are effectively identical. Each board
utilizes an EMI filter, an Actel A1020 FPGA, and a bipolar UDS2998 motor-
driver to generate the appropriately phased motor drive waveforms. The
angular position of each actuator is measured via an AD2S80A motor
resolver readout hybrid.
Software
========
The APL-developed MESSENGER instruments share common software, an
instrument domain library. The library was created in 1994 to serve the
NEAR instruments. It is currently used in MESSENGER as well as the
Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument
on 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-specific
software. The first layer provides standard services, for example,
command handling and telemetry packet queuing. Another layer below the
service layer is a host abstraction layer. This layer abstracts the
input/output (I/O) interfaces used by the instrument to communicate with
the spacecraft. The common software includes a boot program. Its most
frequent use is to boot application code by copying it from EEPROM to RAM
and then start the application. However, the boot program also has
commands for loading memory from the ground, dumping memory back to the
ground, and copying memory to and from EEPROM. These features can be used
to upload new instrument programs. The boot program is stored in non-
volatile ROM. In the event that an instrument's program in EEPROM gets
corrupted, the boot program will allow the instrument software to be
reloaded.
The common command software can store and run sequences of commands,
called macros. Macros can loop and call other macros. Up to 64 macros
can be executing at any time. The common software's telemetry service
provides formatting and queuing of telemetry packets. The software
gathers the packet's data and constructs a Consultative Committee for
Space Data Systems (CCSDS) header with the appropriate flags, sequence
number, and other information. The packet is queued for transmission to
the spacecraft. The common software also includes status/housekeeping
packet generation, time management, voltage and current monitoring, and
I2C bus management services.
MDIS-specific software controls the imager hardware, exposure time, and
mechanisms. Commands are accepted from the spacecraft and forwarded to
the appropriate instrument. Telemetry is collected from the instrument
and forwarded to the spacecraft.
Imager Control. Only one of the MDIS cameras, either the NAC or WAC, can
be used at any given time. The MDIS software selects the camera to use
based on user command. Images are collected in three stages: expose the
image in the FPU, read the image from the FPU to the DPU, and finally
transmit it from the DPU to the spacecraft. Different processing steps
are applied to the image during each of these stages. The steps include
exposure, 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 by
command or automatically by the software. In manual mode, a full 1 ms to
9989 ms range of exposure times is available. In automatic mode, the
exposure time of the next image is computed by the software but limited to
between 1 ms and 989 ms. This computation has two distinct steps.
The first step computes a new exposure time based on the brightness of a
previous image. The second step anticipates filter wheel motions and
adjusts the computed exposure time accordingly.
During the read stage of the image pipeline, the hardware generates a
histogram of the image. The histogram is analyzed by the software to
determine if the image is overexposed or underexposed. First, the
histogram is scaled by a factor of four if it comes from a 2x2 binned
image. If the brightest histogram value exceeds a saturation threshold
(typically 3500 DN), the image is considered overexposed and the exposure
time is scaled back. Otherwise the image is considered underexposed.
Histogram values are accumulated starting from the brightest bin down
towards the dimmest bin, until a commandable, allowed number of pixels
exceeding a target brightness is reached. The brightness value that
causes the sum to exceed the allowed value is the actual image brightness.
The exposure time is scaled by the ratio of the commanded target
brightness minus a background brightness (dark current) to the actual
brightness minus the background brightness. The algorithm uses
uploadable parameters for the saturation threshold, overexposure fallback,
background brightness, target brightness, and the number of pixels allowed
to 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 what
was used in the test exposure. This is done using an uploadable 'filter
transmissivity table' containing relatively sensitivities of the 12 WAC
filters and the NAC in binned and unbinned modes (26 values). The exposure
time is scaled by the ratio of the transmissivity of the old setup to the
transmissivity of the new setup. Finally, the computed exposure time is
forced 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 controlled
by stepper motors. The pivot can move about 1.1 deg/s. Similarly, the
filter wheel can move 75 deg/s. Given twelve filters, an adjacent filter
can be reached in 0.4 s and any filter within 2.4 s.
Image Compression
=================
The MESSENGER mission requires compression to meet its science objectives
within the available downlink. Several successive options are available at
the instrument level using the DPU, and using the spacecraft main
processor (MP).
At the instrument level,
(1) 2x2 binning is available on-chip to reduce the 1024x1024 images to
512x512 format.
(2) 12-bit data number (DN) levels can be converted to 8 bits using any of
eight look-up tables.
(3) Data can be compressed losslessly using the Fast algorithm.
The strategy for image compression at the instrument level is to acquire
all monochrome data in 8-bit mode, all color data in 12-bit mode, and to
compress all data losslessly to conserve recorder space.
After data are written to the recorder, they can be uncompressed and
recompressed by the MP more aggressively using any of several options:
(4) Additional pixel-binning can be performed, including options for 2x2
or 4x4 compression of the input image. In practice, no more than 4x4 total
compression (including on-chip and by the MP) is applied to preserved the
dark columns.
(5) Up to 5 subframes of the image can be saved, or the whole image can be
retained.
(6) For the special case of optical navigation images, there is a
'jailbar' option that saves selected lines of an image at a fixed interval
for optical navigation images of Mercury during flyby approaches.
(7) Compression using an integer wavelet transform can be applied, except
when jailbars are used. The strategy for MP compression is that key
inflight calibrations will be lossless compressed by specifying a
compression ratio of 1:1, whereas all science data will be lossy
compressed. Compression ratios are typically 8:1 for monochrome data and
4:1 for orbital color data. Color imaging but not monochrome imaging may
be 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 a
host of support equipment. The largest of these chambers, the instrument
chamber, has an internal diameter of 1.3 m and a length of 2 m and permits
mounting the instrument on a three-axis motion stage. The motion stage can
rotate the instrument in azimuth and elevation through a range limited
only by the instrument harness and mounting hardware. Translation is
possible over a limited range, but it is sufficient to center the
instrument in the collimated beam of the chamber. The interior and ends of
the instrument vacuum chamber are surrounded by cold walls, within which
an external refrigerator circulates a cooling fluid to reduce the internal
temperature to approximately -40C. Most of the calibrations were performed
at approximately -30C. However, additional thermal configurations over
the range -34C to +25C were also performed, to bracket the behavior of the
instrument over the range of expected flight conditions.
Inside the instrument chamber, there are two sources of calibrated
illumination. In one configuration the instrument views a 20-inch diameter
integrating sphere through a quartz window. Dark cloth was draped around
the window and sphere opening in order to block unwanted external light
from entering the instrument optical path. Sphere spectral radiance was
calibrated by the manufacturer just prior to the start of instrument
calibration, for each configuration of the sphere's four bulbs: two 45-W
lamps and two 150-W lamps. One photometer was coupled through a fiber
optic to monitor the internal sphere brightness during all calibration
runs. Two additional photometers were mounted on the camera and allowed
additional monitoring of incident irradiance.
Rotating the azimuthal stage of the OCF 180 degrees enabled the instrument
to 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 a
grating monochromator. The monochromator may be illuminated by an
incandescent lamp with quartz optics. At the input slit, a set of neutral
density filters may be used to attenuate the light by known amounts. At
the exit slit, a set of long-pass filters is used to remove higher orders
(i.e., shorter wavelengths) from the grating. The wavelength can be
changed manually or can be set to scan under computer control. The grating
may be positioned to zeroth order, which allows the full incandescent
spectrum to be passed.
A MgF2 window partitions the monochromator from the unit under test,
allowing the monochromator to be removed without breaking vacuum so that
other sources may be placed at the focus of the collimator. These sources
include a point source (pinhole), pinhole array, and test samples
illuminated by an incandescent source and viewed off a silver fold mirror.
Four networked computers were used to acquire and monitor the calibration
data.
Calibration Equation
====================
Laboratory and flight measurements were used to derive values for the
terms of the calibration equation for both the WAC and NAC. Both
instruments measure relative light intensity in engineering units referred
to as DNs. The raw engineering units are converted to the physical units
of 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 CCD
temperature 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, row
y, through filter f, at CCD temperature T and exposure time t, for binning
mode b, and Mission Elapsed Time (MET),
Dk(x,y,T,t,b,MET) is the dark level in a given pixel, derived either from
the 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 this
pixel location,
Resp(f,b,T) is the responsivity, relating dark-, flat-, and
smear-corrected DN per unit exposure time to radiance,
Correct(f,MET) is a time-variable correction to responsivity
describing a sudden decrease in transmission of the WAC optics
on 24 May 2011 and subsequent recovery to normal values,
interpreted as due to contamination associated with MESSENGER's
first periapse season over Mercury's hot pole and the subsequent
bake-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 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.
Inversion of 12 to 8 bit Compression
====================================
The equation above assumes that DN levels are in their original
12-bit form. For any image that has compressed to 8 bits, the
inversion is performed using the inverse look-up tables provided
in the CALIB directory.
Dark Level
==========
The measured signal from the CCD, in the absence of incident photons, is
the sum of three major components: (1) dark current from thermal
electrons, (2) an electronic offset, or bias, of about 240 DN
intentionally added to the readout to prevent occurrence of negative
values that would appear as zero DN words, and (3) noise picked up by
instrument electronics. There has been no evidence of picked-up noise. In
the following discussion, the sum of dark current and bias is treated
together as 'dark level.'
The response of pixel-dependent dark level to temperature and exposure
time was characterized separately for the NAC and WAC without on-chip
binning ('full-frame') and with on-chip binning, over the CCD temperature
range 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 mean
decreases with increasing temperature. This relationship originates in the
amplifier in the read-out electronics and has a temperature-dependent
offset. After an image has been exposed, large current pulses are sent
from the power supply to the clocking signals controlling the movement of
charge in the CCD. These high-current clocking pulses increase the overall
signal levels for a very short time, as observed in the measured DNs. At
longer exposure times (>200 ms) and higher CCD temperatures (> -30C) the
effects of dark current, which increases exponentially with temperature,
dominate. In binned images, because on-chip binning adds the outputs of
four pixels prior to quantization, the impact of the dark current is
larger and the onset of increasing dark level with temperature and
exposure time occurs at lower temperature and shorter exposure time.
WAC dark levels are dominated by the increase in dark current with higher
temperature and longer exposure time.
Detector Temperature Calibration:
CCD temperature is measured by a temperature sensor bonded to the CCD
heat-sink mounting plate on each camera. This temperature sensor, whose
value is reported in instrument housekeeping, was calibrated during dark
level testing, where it was possible to install multiple thermocouples
throughout the MDIS instrument. In particular, a thermocouple located in
close proximity to the thermal link on the wax pack of each FPU was used
as the best measurement of the actual CCD temperature. A linear regression
was used to fit the measured response of the CCD temperature sensor with
the calibrated thermocouple response. The other instrument temperature
sensors, located on each FPU housing and the WAC filter wheel and NAC
telescope, were calibrated in a similar fashion. The final expressions to
convert the digital raw output to temperature are of the form
T(deg c) = A * raw_counts + B
Camera Temperature Name A B
WAC CCD temperature MESS:CCD_TEMP 0.2718 -318.455
WAC FPU temperature MESS:CAM_T1 0.5022 -262.258
WAC filter wheel temp. MESS:CAM_T2 0.553 -292.760
NAC CCD temperature MESS:CCD_TEMP 0.2737 -323.367
NAC FPU temperature MESS:CAM_T1 0.5130 -268.844
NAC 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 as
four different sensors: NAC full-frame, NAC binned, WAC full-frame,
and WAC binned. Two dark models were derived from the OCF calibrations for
each of the four modeled sensors: (1) a forward model of the dark current
as a function of column number, row number, exposure time, and raw CCD
temperature (DN), and (2) a backward model that uses the row-dependent
dark levels in the dark strips and extrapolates them across the FOV using
the same variables as the forward model. For both the forward and backward
models, dark level is treated as a linear function of column number, row
number, and exposure time. The exponential temperature dependence was
approximated with a third-order polynomial for ease of calculation. While
it is known that the CCD temperature response will change with time due to
exposure to radiation in space, this specific time dependence will be
determined 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 level
is modeled linearly where an offset alpha(y, t, T) is the value at column
x=0, and a slope beta(y, t, T) is the increase per column (per increment
of x). Then, to find the dark level at a given column, the model for this
row, 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 second
level of the model, the offset alpha and slope beta are themselves a
function of row position y. Thus, linear expansions are:
alpha(y,t,T) = A(t,T) + B(t,T) * y
beta(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 exposure
time and temperature. A third level of linear regression can be added to
the model, such that A(t,T), B(t,T), M(t,T), and N(t,T) can each be
expressed as its own linear function of exposure time t, using a
temperature-dependent offset C(T), O(T), E(T), or Q(T) respectively, and a
temperature-dependent slope D(T), P(T), F(T), or S(T) respectively. The
expanded 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, 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 or
0-511 for a binned image, t is in units of milliseconds, and T is in
UNCALIBRATED raw counts of CCD temperature.
The forward dark current model was derived by first finding a linear fit
of dark level as a function of column number for each row in the dark
calibration data. Second, the bias and dark DN accumulation rate per
column increment were fitted linearly as functions of row. A, B, M, and N
were fitted as functions of exposure time, for data binned into fifteen
difference CCD temperature bins (each bin 10 DN wide) covering the range
1000-1150 DN (-40.8C to -6.8C). Finally, the linear fit coefficients as
functions of exposure time at different temperatures were fitted as a
function 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 transferred
to the memory zone of the CCD, from which the analog signal is digitized
line by line. Accumulation of signal continues during the finite duration
of frame transfer, inducing 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. 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 t
in binning mode b,
Dk(x,y,t,b) is the dark level 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 ms, 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, i.e., 1024 for
full-frame images or 512 for binned images.
Response Linearity Correction
=============================
Calibration of the signal accumulation rate per unit time per unit
radiance at the sensor was conducted in the OCF for the WAC and the NAC
over a broad range of exposure times and source light intensities using a
white integrating sphere. The exposure times to saturation were determined
empirically in initial set-up. Using a variety of temperatures (-34C to
25C) and source intensities, images were acquired after varying the
exposure times until saturation occurred. Source intensities were stepped
in a fixed decreasing-light pattern through the eight levels that could be
achieved with the integrating sphere's two 150-W and two 45-W bulbs. The
bulbs were allowed about 20 minutes of warm-up to asymptotically
approaching constant light output. The WAC calibration procedure began
with all lamps on then subsequently turned lamps off in sequence. The
exposure sequence was repeated for each filter for the WAC, but only a
single exposure sequence was required for the single filter of the NAC.
The relationship between the raw DN output from the camera per unit
exposure time and radiance, as a function of variation in exposure time or
radiance is referred to as response linearity. Using dark-corrected,
desmeared DN values taken from the center quarter of the images, response
linearity was measured with both the NAC and WAC binned and full-frame.
Linearity was first examined separately for (a) linearity with respect to
exposure time at individual light levels, and (b) linearity with respect
to radiance at individual exposure times. The results are not
significantly different, so all data were merged to examine linearity with
respect to measured photons (the product of radiance and exposure time).
At corrected DN levels >1000, the CCDs are linear to within the error
bars. However, there is a decrease in responsivity at lower DNs, by up to
2% at the lowest DN levels.
To restore or 'linearize' WAC image data, the following corrections should
be applied after correction of dark current, bias, and smear.
For DN_dark > 1
DN_lin = DN_dark_smear/[0.008760 * Ln(DN_dark_smear) + 0.936321]
For DN_dark <= 1
DN_lin = DN_dark_smear/0.936321
To restore or 'linearize' NAC image data, the following procedure should
be applied after correction of dark current, bias, and smear.
For DN_dark > 1
DN_lin = DN_dark_smear/[0.011844 * Ln(DN_dark_smear) + 0.912031]
For DN_dark <= 1
DN_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 detector
responsivity, so that the responsivity coefficients can be expressed as
scalars for each filter. The flat field is derived using a linearized,
dark-corrected, desmeared image of a field- and aperture-filling, uniform
source and normalizing the mean value to unity over the center quarter
of the images.
Two factors make the ground-derived flat-field less than ideal. First,
MDIS's thermal blanketing generates reflections so that in the calibration
chamber, the illuminated source created glint off the blanketing that
reflected off the chamber window, adding spatially non-uniform stray light
to the measurements. Second, 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.
The flat-fields have been rederived inflight using images of a part of
Venus near the sub-solar point, with a photometric correction applied to
remove effects of variation of solar incidence angle across the scene.
Flat-fields for different filters and binning states are given in the
CALIB directory.
Responsivity
============
Responsivity is linearized, dark-corrected, desmeared DN per unit time per
unit radiance. Responsivity through each filter in each binning state
(except WAC filter 2, which is too sensitive to be characterized with this
experimental setup) was determined onground using the integrating-sphere,
whose radiances was calibrated by Labsphere using each of the four lamps
separately, over a wavelength range of 350-1100 nm in 5-nm steps. To
determine radiances through each of the WAC and NAC filters, the band
passes described below were convolved with the sphere's spectral radiance.
For data taken at room temperatures, the chamber door was open, but for
lower-temperature measurements the sphere was viewed through the quartz
window in the OCF chamber door. Therefore for the low-temperature data, an
additional correction was applied to the sphere radiance for each filter
to account for the window. This correction was derived from room-
temperature data, as the ratio of corrected DNs per unit time with the
door closed to that with the door open, for a single configuration of
sphere bulbs.
It is expected the temperature-dependence of responsivity over the
temperature range used will be approximately quadratic. The best
correction available from the combination of flight and ground data is
quadratic 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 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) * 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 and
binning state b, and
Correct(f, MET) is a temporal correction to responsivity for
filter f at time MET.
Values for Resp(f,T,b) and Correct(f, MET) for different filters and binning
states are given in the CALIB directory.
Filter Bandpasses:
The NAC uses a single filter, while the WAC views the scene through a
filter wheel outfitted with 12 filters of varying widths. The WAC filters
are labeled by their position counter-clockwise around the wheel as viewed
from the CCD. Filter 2 is a clear fused silica filter spanning the entire
passband of the CCD (395-1040 nm), and filters 1 and 3-12 are narrowband
filters.
Using the OCF monochromator, NAC and WAC filter transmissions were
measured as functions of wavelength for CCD temperatures ranging from -35C
to about 26C. The source appeared in the data as a bright rectangle in the
center of each image. After subtracting the dark model from the image
values and performing smear corrections, a mask was derived from each
image based on bright and dark pixel distributions. The values in the mask
were set to zero for the dark regions and unity for the source rectangle.
After multiplication, the pixel values for the resulting image were summed
to achieve maximum response with varying source wavelength. For summed
signal at each monochromator wavelength, the data were normalized to the
highest sum, at the most sensitive wavelength.
The center bandpass was solved assuming a square function, by summing the
normalized signals times the delta wavelength range covered by each. The
50% point of the cumulative distribution was defined as the center
wavelength.
The derived center wavelengths and bandpasses are given above, and the
relative filter sensitivities as a function of wavelength (used to model
the radiance of the white integrating sphere as sampled by MDIS) are
given in the CALIB directory.
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) = L(f) * pi * (SOLAR_DISTANCE/149597870.691)**2 / F(f)
where
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 CALIB directory.
Operational Considerations
==========================
(1) WAC clear filter: Filter 2 on the wide-angle camera is broad-band and
designed for star imaging. It is expected that typically even the minimum
exposure time will saturate on Mercury. Therefore flat-field and
responsivity corrections for WAC filter 2 were not derived.
(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. Wavelet compression applied to science images is
typically lossy. Pre-launch, a study was done in which the appearance of
artifacts at progressively higher wavelet compression ratios was
investigated using raw images from the NEAR multispectral imager.
Artifacts were identified by differencing raw images and the same images
after compression and decompression. Spatially coherent artifacts were
evident in monochrome images at compression ratios >8, and in color ratio
images >4. Hence wavelet compression ratios of 8 and 4 were selected for
nominal application to monochrome mapping and multispectral images
respectively.
(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.
(5) Data Quality Index. A data quality index in the PDS label for each
image is used to encode figures-of-merit into one parameter. The 16-byte
data quality index includes 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 a well-calibrated
range, and completeness of data within the commanded selection of
subframes or full frame.
(6) Temperature effects on 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. Mercury and Venus flybys are thermally benign. However in
Mercury orbit the noon-midnight orbit experiences large thermal
perturbations and errors in reported MDIS attitude of up to 350
microradians can be expected.
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