Navigation Camera (NC) is an engineering subsystem
used to optically navigate the Stardust spacecraft
upon approach to the comet. This will assist the
spacecraft in achieving a proper Comet Wild 2
flyby distance, near enough to the nucleus to
assure adequate dust collection, while keeping
it out of danger. The camera will also serve as
an imaging camera to collect science data. This
will include capturing high-resolution color images
of the comet nucleus, on approach and on departure,
and broadband images at various phase angles while
The Navigation Camera images will be used to
construct a 3-D map of the comet nucleus in order
to better understand its origin, morphology and
mechanisms, and to search for mineralogical irregularities
on the nucleus. It could potentially supply information
on the nucleus rotation state too. The camera
will provide images, taken through different filters,
to provide information on the gas and dust coma
during approach and departure phases of the mission.
These images may be able provide information on
gas composition, gas and dust dynamics and possible
order to meet these science and optical navigation
objectives the NC design was developed utilizing
a Voyager Wide Angle Optical Assembly. Additionally,
the NC has a newly developed scan mirror mechanism
to vary the camera viewing angle and a periscope
to protect the scanning mirror while the spacecraft
flies through the comet coma. The NC is a framing
charge coupled device (CCD) imager with a focal
length of 200 mm. The NC has a focal plane shutter
and filter changing mechanism of the Voyager/Galileo
At the heart of the camera is a charge coupled
device (CCD) used as a detector, cooled to suppress
dark current and shielded from protons and electrons.
The electronics contain the signal chain and CCD
drivers located in the sensor head, command and
control logic, power supplies, mechanism drivers,
a digital data compressor and two UARTs too interface
with the spacecraft Command and Data Handling
NC command and telemetry functions will also be
handled by the electronics, including storage
of science commands, collection of science imaging
data and telemetry, transmission of imaging data
and telemetry to C&DH and receipt of commands
from C&DH. The NC uses a data rate of 300
k pixels for transferring data to the C&DH.
There is also the option for data reduction with
12 bit to 8 bit square root compression, windowing
and error free compression within windows.
Major Functional Elements
The NC consists of the several major functional
elements. This is more technical and esoteric
information about these elements:
The optics subassembly is hardware originally
designed, built and tested for the Voyager Project.
It is a Petzval-type refractor lens with a 200
mm focal length, f/3.5 and a spectral range 380
nm - 1000 nm. The optical components, with the
exception of the filters, are manufactured from
LF5G15 and BK7G14. These materials are radiation
resistant. A new field flattener element, located
in front of the CCD window, was designed for Stardust
to reduce field curvature and to provide additional
CCD radiation shielding.
The optics are supported on three invar rods that
athermalize the system to keep the camera in focus
throughout the operating temperature range. An
optical barrel assembly mounts to the shutter
assembly, utilizing an aluminum truss structure.
The housing and truss are also inherited hardware
from Voyager. There is a small incandescent lamp,
spider mounted in front of the first lens element
that can be used for in-flight calibrations.
Because radiation resistant optical materials
were used to harden the optics, the lens has a
poor broad band MTF performance for axial color.
The theoretical MTF for the spectral range 380
nm to 1100 nm is 30% at 32 lp/mm. The thickness
of individual filters will be optimized to improve
the MTF over the filters passband.
- 1100 nm
x 3.5 degrees
The NC shutter assembly is also
inherited Flight Spare hardware from the Voyager
Project. The device is a two-blade focal plane
mechanism, with each blade actuated by its own
permanent rotary solenoid. The duration of the
exposure is controlled by the time interval between
two pulses - an open pulse and a close pulse.
The open pulse powers the "leading"
blade and the close pulse powers the "trailing"
The exposure sequence starts with the leading
blade covering the aperture. An open pulse moves
the leading blade, uncovering the aperture, and
the close pulse moves the trailing blade, in the
same direction, covering the aperture again. The
permanent magnets in the rotary solenoid of each
blade hold the blades in a detent position when
the shutter is not powered. Exposures can be taken
with the blades moving in either direction. A
total of 4096 exposure times are available that
range from 5 ms to 20 s, in 5 ms increments. There
is also a bulb command, for longer exposures,
that allows the shutter to be held open for any
desired length of time.
The NC uses a charge coupled device (CCD) detector
packaged for the Cassini Imaging Science Subsystem
(ISS). The operating temperature range is -55oC
to -25oC. The CCD is mounted in a hermetically
sealed package, back-filled with argon. An operating
temperature of around -35oC is needed for suppression
of dark current and to minimize proton gamma and
neutron radiation effects. The NC employs passive
radiative cooling to maintain the detector operating
This mechanism enables the stationary wide-angle
optics - flying sidewise during encounter - to
keep the comet in view. The scanning mirror, located
some distance forward of the camera lens, faces
45=83 away from the camera viewing axis. Rotating
the mirror around the camera axis at the proper
rate enables comet tracking during flyby.
The mechanism consists of a cylindrical section
with mirror and an anti-backlash mechanism, the
drive unit with motor, gearbox and slip clutch,
and a base housing the control electronics. The
cylindrical section is coaxial with the camera
lens. It consists of the rotational housing containing
the mirror and a stationary housing with an anti
backlash mechanism attached to it. The sections
of the housing, which hold the main bearings,
are made from titanium to enable accurate operations
over a 100C temperature range.
The periscope is an optical assembly allowing
the scan mirror to look over the protective Whipple
shield while it is pointed forward, in a direction
parallel to the spacecraft. This is to protect
the scan mirror from particle strikes, that would
significantly degrade its performance during cruise,
upon approach and while flying through the comet
coma. The periscope contains two rectangular mirrors
mounted at 45 with respect to the spacecraft.
The mirrors are made of aluminum to reduce the
rate and amount of degradation from particle collisions.
To keep the weight light, the mirrors were fabricated
using an aluminum foam core composite material
with solid face sheets braised onto the front
and back surfaces. Single point diamond turning
was used to figure the reflective surface of the
mirrors. Since the forward-looking mirror is exposed
to the particles it was post polished and received
only a very thin protected aluminum coating. The
mirror facing away from the particle stream was
nickel coated and post polished with a thin protected
aluminum coating. This process achieves a much
better mirror figure and smother surface finish
but tends to flake off when exposed to particle
The periscope structure is of graphite/epoxy composite
construction. This material was chosen to make
the structure light and reduce thermally induced
distortions from the spacecraft to the periscope
assembly. Each mirror was kinematically mounted
to the composite structure using three triangular
bipod flexures. The periscope is only utilized
when the scan mirror is looking forward. After
the scan mirror has rotated approximately 15-20
down toward the spacecraft -Z axis it no longer
imaging through the periscope. The periscope was
designed so that the images taken while the mirror
was partly looking through periscope could still
be used for optical navigation.
and NC Control
The electronics for the NC consists of two major
parts the camera and scan mirror electronics.
The sensor head electronics - part of the camera
electronics - is mounted on a chassis located
behind the focal plane of the optics, while the
rest of the camera electronics and the scan mirror
electronics are housed in the base plate support.
The NC electronics control NC functions and process
NC commands and telemetry. NC electronics is powered
from the spacecraft 28 volt regulated and 34 volt
The portion of the camera electronics mounted
behind the camera is called the sensor head electronics.
These electronics support the operation of the
CCD detector and the preprocessing of the detector
data. The pixel data is quanitized to 12 bits,
giving an intra-frame dynamic range of 4096. Detector
readout rate is fixed at 300 kpixels / second.
In addition, a direct access port is included
in the sensor head electronics to send telemetry
to the NC ground support equipment. This port
is used for ground testing only.
The remainder of the camera electronics is called
the main electronics. This area provides the power
and performs all NC control functions. It includes
a CCD clock generator, image compressor, image
buffer, mechanism and lamp drivers, telemetry
mux and converter, bus controller, UARTs and power
supplies. The spacecraft specified RS-422 Bus
is used for communication with the Command and
Handling (C&DH). A high-speed bus is used
for transmission of image data and a low speed
bus sends and receives commands and telemetry.
The NC scan mirror mechanism has it's own interface
with the spacecraft. This includes a separate
power interface, a bi-directional low speed RS-422
bus for telemetry and commanding transmission,
a low speed RS-422 bus for outputting for motor
rotation pulses, a discrete output for motor direction.
All interfaces with the scan mirror mechanism
are done through one 24 pin connector designated
J2 that is mounted in the NC base plate.
All commands are transmitted and received by the
NC over the low rate RS-422 bus. Commands received
by NC are echoed back to the spacecraft, including
parity errors, so that commands with errors can
NC Electronics provides one data rate of 300 kbpxls
The pixel data from the NC can be processed within
the NC in several ways. The default processing
is to transmit the converted 12 bit data. When
the data compression is turned on the 12 bit data
is compressed to 8 bits using a square-root compression
algorithm. This is accomplished via a look-up
table stored in ROM.
The camera electronics is required to draw less
than 8 watts and the scan mirror less than 10
watts steady state. Operational constraints are
placed on the NC to limit the power drawn by NC
from the spacecraft at any one time.
On/Initialization and Power Off
At power turn on, the NC registers are all set
to zero. At this point the camera is in an idle
mode with all clocks running, waiting to receive
commands. The camera will remain in this state
until the first command is received. The state
of the mechanism will remain what it was when
the camera was last turned off.
In response to a concern that the NC boresight
may, in a spacecraft fault condition, be exposed
to the sun, a method to protect the shutter and
focal plane of the camera was developed. The NC
safe state is defined as placing a narrow band
filter in the optical path and opening the shutter.
To reset the NC to a normal operating state a
power on reset will clear the FPGA lockup.
November 26, 2003