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An Innovative Instrument Design
- Instrument overview
- System and performance highlights
- Design challenges
- How CRISM works
Instrument Overview
CRISM is a visible-infrared imaging spectrometer with a scannable field
of view. CRISM can cover wavelengths from 0.362 to 3.92 microns (362 to 3920 nanometers)
at 6.55 nanometers/channel, enabling the CRISM team to identify a
broad range of minerals on the Martian surface.
CRISM consists of three boxes:
- the Optical Sensor Unit (OSU), which
includes the optics, gimbal, focal planes, cryocoolers, radiators, and
focal plane electronics
- the Gimbal Motor Electronics (GME), which
commands and powers the gimbal and analyzes data from its angular position encoder in a feedback loop
- the Data Processing Unit (DPU),
which accepts and processes commands from the spacecraft and accepts
and processes data from the OSU and communicates it to the spacecraft
Instrument System Properties
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Mass
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32.92 kg (72.5 lbs)
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Power (during normal operations)
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44.4-47.3 W
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Power (during standby with subsystems off)
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16.1 W
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Key Performance Characteristics
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| Aperture |
100 mm
(3.94 inches)
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Focal length |
441 mm
(17.36 inches)
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| Field of view |
2.12 degrees
(37 milliradians)
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Instantaneous FOV (pixel angular size) |
0.0035 degree
(61.5 microradians) |
| Spectral range |
VNIR: 362-1053 nm
IR: 1002-3920 nm |
Spectral sampling |
6.55 nm/channel |
Swath width
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9.4 to 11.9 km at 300 km altitude
(5.8 to 7.4 miles at 186 miles altitude)
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Spatial sampling
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15.7 to 19.7 m/pixel; resolves 38-m spot at all wavelengths
(51 to 64 feet/pixel; resolves 125-foot spot at all wavelengths)
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Pointing
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+/-60 degrees along the spacecraft ground track
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Scan jitter
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25 microradians from the aimpoint
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Design Challenges
CRISM's design came about as the combination of several innovative
technical solutions to some very demanding and sometimes conflicting needs by an instrument like CRISM.
Challenge #1: Conflicting resolution and signal-to-noise ratio requirements. CRISM simultaneously needs high spectral resolution, high spatial
resolution, and a high signal-to-noise ratio. (Signal-to-noise ratio is
a measure of stability of the measurements.) High resolution means
dividing up the collected photons of light into small spatial and spectral bins, whereas a high
signal-to-noise ratio requires measurement of as many photons of light as
possible.
Solution: A gimbaled sensor system. Reconciling
the conflicting demands requires some type of active pointing
so that a target can be tracked and long exposure times can be used to
collect as many photons as possible without smearing the images.
This could be done using a scan mirror or by gimbaling the whole
sensor system. CRISM uses the gimbaling approach, because it
accommodates an ample telescope baffle to keep stray light to a minimum.
CRISM's gimbal uses sophisticated software control that tracks a target
on the surface. Simultaneously,
the field of view is scanned across the target to build up a hyperspectral image. An angular position encoder
measures the gimbal's deviation from its commanded curve and adjusts the gimbal rate
accordingly. This approach keeps the gimbal on-target to within a half
pixel.
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CRISM's
gimbal motor and position encoder keep the field - of - view on a
pre-programmed angular profile to track a target on the Martian surface.
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Challenge #2: Cool the IR detector without adversely impacting spacecraft operations. The detector that measures the 1-3.92 micron part of CRISM's
wavelength range needs to operate at a very low temperature, colder
than -163°C (-260°F). There are two ways to get the detector this cold,
passively or
actively. In passive cooling, the detector would be connected to a
radiator
that points to cold, deep space and radiates away the detector's heat.
However, MRO needs to point to a variety of locations on Mars, and it
would be nearly impossible to keep Mars out of the radiator's view. If
the radiator looked at even a part of Mars' day side with any
regularity, it couldn't get the detector
cold enough. The other approach is to use a mechanical cooler,
basically a cryogenic refrigerator. As CRISM was designed and
built, two kinds of
mechanical coolers were available: small ones that may not have enough
lifetime to last through
MRO's mission, or a long-life cooler that would be too massive to fit
in CRISM. There was, at first, no obvious solution.
Solution: Three small coolers, multiplexed to the detector so that any one at a time can operate. No
existing cryogenic system met CRISM's requirements, so the team
worked with Swales Aerospace to invent one. Three small coolers were ganged
together so that if one failed, it had two backups. The three coolers
not only meet, they exceed MRO's operational lifetime requirements. Any
one of the three
provides enough cooling capacity to get the IR detector below its
temperature goal. One cooler at a time is selected, using a passive
thermal switch called a cryogenic diode heat pipe. Each cooler is
linked to the detector with one of these heat pipes. When a cooler is
turned on
and begins to cool, its heat pipe provides thermal conductivity to the
detector so that the cooler removes the detector's heat and gets it cold. The heat pipes on the "off" coolers isolate
them, so that their warmth doesn't leak into the cold IR
detector. CRISM's small mechanical coolers and its heat pipes
were both existing technology, but they were used together in a new and
innovative way.
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Ganged
coolers (left) and an assembly of cryogenic diode heat pipes inside a protective shroud
(right) provide a technical solution to CRISM's otherwise conflicting
requirements for cooling its IR detector.
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Challenge #3: Keep the IR detector and spectrometer cold, while keeping other parts of the instrument warm. The
OSU (the sensor part of CRISM) is only the size of a small microwave
oven, yet within that small space different components need to be kept
at temperatures that differ by as much as 150°C (255°F). For example, the
spectrometer optics need to kept at -85°C (-121°F). Any warmer, and
at CRISM's longer infrared wavlengths (>3 microns), the optics would glow
a dull red. The IR detector is mounted right on the spectrometer, but needs to
be 88°C (140°F) colder. The VNIR detector, also mounted on the spectrometer, needs to be 25°C (45°F)
warmer. And just inches away, the coolers' motors - their warm parts - need to be 65°C (117°F) warmer. All this has to be accomplished using no more than a few watts
of power. This is like saying, "keep a pot of water boiling and
an ice cube frozen on a dinner plate, while keeping the plate at room temperature. And use no more electricity than a
Christmas tree bulb."
Solution: Different zones of the instrument naturally radiate away heat at different rates. Most spacecraft instruments are covered in special blanketing to keep
them warm. Instead, CRISM is hanging out exposed and cold. In fact, it's covered
with special
white paint to make it even colder. Two passive radiators face Mars
and
naturally keep the coolers and telescope at their separate, optimal
temperatures. The coolers' operation provides just enough heat to keep
them warmer than the telescope. An "antisunward radiator," made from a
special, high thermal conductivity
cloth-epoxy composite, faces toward Mars' evening limb and passively
cools the spectrometer to its own optimal temperature. Just a few watts
of power are needed to cool the IR detector and warm the VNIR detector.
Special mounting assemblies designed at APL keep the detectors
thermally isolated from the spectrometer, even while holding them
firmly in place for optical alignment.
This cut-away view of the OSU shows the different thermal zones inside it.
Challenge #4: Maintain accurate calibration of the instrument across a wide range of operating temperatures and conditions. Spacecraft
optical instruments and their detectors are notoriously difficult to
maintain accurately
calibrated, as they get exposed to different temperature conditions and
accumulate radiation exposure in space. Properties of
optics change, and the detectors' senstivity changes. CRISM has an
additional challenge: even its very cold temperatures don't completely
eliminate the instrument's dull red glow at the
longer IR wavelengths (>3 microns), and that glow inside the
instrument creates a kind of glare through which Mars has to be
observed. What's more, the IR glow changes measurably
if the instrument temperature changes by only about 1/100th degree. In
the course of a typical orbit, some parts of the instrument change
temperature by several degrees.
Solution: Onboard calibration sources allow the effects of radiation and temperature to be measured and subtracted out of the data. A special 3-position shutter is key to this approach. In the "open"
shutter position, CRISM sees Mars. In its "closed" position the shutter
blocks the
field of view, so that the IR background glow and other similar effects
can be measured. In a third position, the shutter provides a view
into a small
"integrating sphere." This is a sphere with an opening to see the
inside, which contains a small incandescent light bulb. A photodiode inside the sphere measures the bulb's brightness and controls the bulb's
current so that the brightness repeatedly comes back to the same, reproducible value.
The light bouncing around inside the sphere provides a smooth scene of
known brightness to which CRISM data can be "tuned," much as a flag can
be used to color-balance a television image.
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This
computer rendering shows CRISM's optical bench, a rigid plate
to which the telescope and spectrometer are mounted. Critical parts of
the calibration subsystem are embedded in the optical bench, including
the shutter mechanism (center) and internal integrating sphere (left).
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Challenge #5: Use CRISM as a color camera to fill the gaps between targeted observations. Targeted observations exercise the full capabilities of CRISM, yielding 544-color, 18 meter
(60 feet) per pixel images. But targeted observations take so much data
volume (about
200 megabytes apiece) that only about 1% of Mars can be imaged in this
way. OMEGA data cover most of the planet, but on average they are a
factor of 50 lower in resolution. Some intermediate product is
needed to provide context for CRISM targeted observations and to find
targets for CRISM that might be too small for OMEGA to see.
Solution: Electronic switching allows CRISM to select a lower resolution operating mode and any subset of wavelengths. CRISM has two reduced resolution modes, and any combination of
wavelengths whatsoever can be selected by command. These two
capabilities are used to switch CRISM's mode of operation to that of a multispectral imager - a reduced resolution mode in which only 72 carefully selected
wavelengths are saved. These 72 were chosen based on analysis of OMEGA
data, to capture known mineralogic absorptions, but they can be updated
as needed to accommodate CRISM's discovery of new minerals.
How CRISM Works
- Optical design
- Key optical components
- Detectors
- Thermal control
- Pointing control
- Image acquisition
Optical Design
CRISM uses a 100-mm aperture, 441-mm focal length Ritchey-Chretien
telescope that focuses light onto a long, narrow slit. The
telescope is protected by a baffle that reduces scattered light from
outside the field of view. The end of the baffle is covered by
a hinged, one-time deployable cover to protect the telescope during
MRO's launch and cruise to Mars.
Following the
slit are the spectrometer optics. A beamsplitter reflects the
visible/near-infrared (VNIR, 0.36-1.05 microns) while transmitting the
infrared (IR, 1-3.92 microns), each
to its own spectrometer and detector. Each of the spectrometers uses
mirrors and a diffraction grating to spread the white light from each
spatial pixel into a spectrum that is brought to focus on its
detector.
The slit is a narrow rectangular opening in a piece
of foil held in a mounting assembly. Its shape traces out one line of a
spatial image. Multiple frames of data are taken to fill out a full two-dimensional spatial image.
The spectrometer optics are all enclosed in an aluminum box, painted black inside to minimize scattered light.
The spectrometer mounts to the back of the optical bench,
and the telescope mounts to the front of it. Below, the fully assembled
optics are shown held in the jig that was used to align the detectors
to the optics.
The detectors mount to the side of the optics
assembly. Each of the detectors is held in a special assembly that
keeps it accurately aligned in a fixed location but minimizes the
conduction of heat to or from the spectrometer housing. The IR assembly
has a baffle that blocks the >3-micron "red glow" of the inside of
the instrument from all angles except the path of the light coming in
to focus on the detector.
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The VNIR focal plane assembly showing the mounted detector, its thermal isolation mount, and connecting cables.
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The IR
focal plane assembly including its scattered light baffle and
connecting cables. The thermal isolation mount is hidden by the
scattered light baffle.
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The detectors themselves have filters mounted right on them, to block
stray light from wavelengths not being measured on that part of the
detector. The filters are striped, because there are 2 filters for
different VNIR wavelength ranges and 3 filters for different IR
wavelength ranges.
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With the detectors fully mounted on the optics, the thermal
control subsystem is hooked up. The illustration below shows its 20
major components including coolers, heat pipes, and radiators. Key
components come from Maryland, California, Pennsylvania,
Utah, Massachusetts, New York, and Israel.
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(1) Planet
Facing Radiator, built by APL. (2) Cryocooler, built by Ricor, Israel.
(3) Cooler-radiator flex link, built by Space Dynamics Laboratory
(SDL), Utah. (4) Cooler cold tip-diode heat pipe condenser, SDL. (5)
Diode heat pipe assembly, Swales Aerospace, MD. (6) Heat pipe
evaporator-IR FPA flex link, SDL. (7) Heat pipe shroud bracket, APL.
(8) Diode heat pipe-optical bench flex link, SDL. (9) Optical bench,
SSG Precision Optics, MA. (10) IR FPA isolation assembly, APL. (11) IR
FPA mount and packaging, Judson, PA. (12) IR detector and ROIC,
Rockwell Science Company, CA. (13) Order sorting filter, Rockwell
Science Company, CA. (14) Circuit board assembly, APL. (15) Manganin
wire cable assembly, Tayco, CA. (16) IR FPA alignment shim, APL. (17)
Spectrometer housing, SSG. (18) Spectrometer-Anti-sunward radiator flex
link, SDL. (19) Radiator isolator, APL. (20) Anti-sunward radiator, XC
Associates, NY.
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