| INSTRUMENT_DESC |
The camera state description was provided by the NAVCAM
instrument Science Lead, Dr. Raymond L. Newburn, Jr., while the
rest of the description was copied from ``STARDUST Navigation
Camera Instrument Description Document'' with permission from the
Stardust project.
Navigation Camera State History
===============================
A year after launch the NavCam suffered its one known failure
when the filter wheel refused to move when commanded. Fortunately
for the overall success of the STARDUST mission, it stuck on one
of the two wide bandpass filters, the OpNav (Optical Navigation)
filter, a filter which transmits light from about 400 to 900 nm
and has the greatest total throughput of any of the eight
filters. This filter serves most engineering needs perfectly
well. The camera, however, has a Petzval lens system, and over
such a wide wavelength range suffers from some chromatic
aberration. As a result, the intrinsic point spread function is
about 2.3 pixels. (The high resolution filter, by comparison, had
a point spread of a quarter pixel.) Further, this camera lens was
manufactured in the early 1970s for the Voyager program, and its
antireflection coatings 30 years later leave something to be
desired. As a result, all images exhibit a broad shallow skirt of
scattered light.
When first used after launch, the camera was observed to be
heavily contaminated by a coating of unknown source and
composition. Total sensitivity was down by a factor of almost
100. A mild heating of the detector to 9 C for 143 hours,
utilizing an internal heater, resulted in a slight improvement in
performance, reducing the sensitivity loss to about a factor of
ten. Every star image still showed a huge halo of scattered
light. Turning the spacecraft to place direct sunlight on the
radiator, normally used to cool the detector, raised the detector
temperature to 24 C for 30 minutes and resulted in great
improvement. The camera now showed sensitivity approaching that
originally expected and significantly reduced scattered light.
Following passage through perihelion and Earth gravity assist,
images were acquired of the Moon and of star fields for geometric
calibration. It was obvious that some re-contamination had
occurred during the previous three months when the spacecraft
(but not the cooled detector) was warmest. A third heating cycle
resulted in the best images since the camera left the calibration
laboratory. A five second exposure reached magnitude 11.7 with a
signal to noise of three. The point spread was essentially the
2.3 pixels expected for the filter, though there still was a
broad very shallow skirt of scattered light caused by internal
reflections in the lens and by residual contamination. Camera
performance remained essentially constant for the next six
months.
After a year in deep space where power was low, communication
bandpass limited, and no imaging was attempted, a calibration
lamp image once again showed small re-contamination.
Interestingly, the periscope, which is not used for most imaging,
showed great improvement compared to two years earlier. Before
beginning the Annefrank approach, 60 hours of heating to a
temperature just above freezing was carried out with the internal
heater. No check of the results of this heating cycle has yet
been possible. Calibration checks will be performed before and
after the Wild 2 encounter, but these were not carried out for
the Annefrank encounter, which was conducted as an engineering
test and not to gather scientific data. Great caution must
therefore be used when attempting to interpret the Annefrank
images, since they do contain considerable scattered light.
Images acquired May 21, 2003 showed that the camera resolution
was still quite good, although a faint halo of scattered light
appeared around each image. A calibration lamp image, taken on
this date, showed loss in filament resolution, apparently caused
principally by the scattered light. An image through the
periscope was considerably improved over earlier images, but
showed a great deal of scattered light on one side, presumably
from the launch adapter ring that actually occults a bit of the
periscope on one side. A new feature was a line that was some 10
dn above the background in column 221. This line appeared some
time between January 28 and May 21 and has remained to this day.
We were not able to take new images until October 8 and October
11, 2003. These indicated that we had acquired some 2.5
magnitudes (a factor of ten) of obscuration over the previous 4.5
months. Another heating cycle reduced this to about 0.5 magnitude.
This state would have been adequate for the encounter but not
what was desirable.
On October 29, 2003 a new problem appeared. An image of the
calibration lamp showed nothing. We did not know whether the bulb
had burned out (something that had never happened before on any
spacecraft) or the shutter didn't open (again something that
had never happened before) or, after some thought, the
possibility the solar flare that hit us at this time flipped a
bit somewhere in the logic circuitry. The next images, taken on
November 8, 2003 showed that the shutter was working just fine.
There was concern that the failure of the lamp could indicate a
short circuit somewhere, so the lamp was not tried again until
after the Wild 2 encounter. At that time, January 13, 2004, the
lamp was working just fine, leaving us with the somewhat unlikely
conclusion that a solar particle had flipped a bit!
Three images, each with three second exposures, taken on October
14, 2003 were a first attempt to locate the comet. It hinted at
being present on single images, but adding the three together
convinced us that we had found 81 P/Wild 2 on the first try.
Images three days later with five second exposures absolutely
confirmed this. This began the optical navigation effort. Over
the next month the images showed some degradation, and there was
concern that this might hinder the final days of navigation, to
say nothing of the quality of the comet images. So, just five
days before the encounter, the Sun was once again turned on the
CCD radiator for about 35 minutes. This cleared things up
beautifully.
As the encounter approached, concern was expressed over the large
number of on-off cycles the camera was undergoing for the optical
navigation. It was decided to just leave the camera turned on, as
a safety measure. Unfortunately this raised the CCD temperature
by twelve degrees centigrade, and the images became loaded with
hot pixels. (The optics and detector sit on top of the
electronics box.) The hot pixels ruined the calibration run that
was attempted on December 18, 2003. So, the camera was again
turned on and off for every imaging sequence, which caused no
problem. The camera went into its encounter activities in a very
clean state and completed its imaging with great results. Another
calibration sequence was attempted on January 13, 2004 with no
problems, except that they were exposed on an anything but a rich
field. Post encounter images through the periscope showed it to
have been heavily damaged by the particle bombardment during
encounter, as was expected. And as mentioned previously, the
calibration lamp was working just fine. Unfortunately this cal
lamp frame was given a one second exposure rather than the
appropriate 20 ms. Every pixel was saturated.
Navigation Camera Overview
==========================
The NAVCAM, an engineering subsystem, is used to optically
navigate the spacecraft upon approach to the comet. This allows
the spacecraft to achieve the proper flyby distance, near enough
to the nucleus, to assure adequate dust collection. The camera
also serves as an imaging camera to collect science data. The
data includes high-resolution color images of the comet nucleus,
on approach and on departure, and broadband images at various
phase angles while nearby. These images can be used to construct
a 3-D map of the nucleus in order to better understand its
origin, morphology, and mechanisms, to search for mineralogical
inhomogeneities on the nucleus, and potentially to supply
information on the nucleus rotation state. The camera can provide
images, taken through different filters, that gives information
on the gas and dust coma during approach and departure phases of
the mission. These images provides information on gas
composition, gas and dust dynamics, and jet phenomena, if they
exist.
In order to meet these science and optical navigation objectives
the NAVCAM design was developed utilizing a Voyager Wide Angle
Optical Assembly. Additionally; the NAVCAM 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 NAVCAM is a framing charge
coupled device (CCD) imager with a focal length of 200 mm. The
NAVCAM has a focal plane shutter and filter changing mechanism of
the Voyager/Galileo type. The detector is a charge coupled device
(CCD), 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 (C&DH). NAVCAM command and telemetry functions are also
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 NAVCAM uses a data rate of 300 kpixels for transferring
data to the C&DH. There are 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 NAVCAM consists of the following major functional elements
(Newburn, 2003):
- Optics
- Filter Wheel and Shutter Mechanisms
- Detector
- Scan Mirror Mechanism
- Periscope
- Electronics and NAVCAM Control
Optics
------
The optics subassembly is inherited hardware 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 materials which 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 over the operating temperature range. The optical
barrel assembly mounts to the filter wheel and 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 modulation transfer
function (MTF) performance (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 is optimized to improve the
MTF over the filters passband.
Optics characteristics are:
Focal length 200 mm
Relative aperture f/3.5
Spectral Range 380 - 1100 nm
Resolution 60 microradian/pixel
Field of view 3.5 x 3.5 deg
Filter Wheel Subassembly
------------------------
The NAVCAM filter wheel assembly is inherited Flight Spare
hardware from the Voyager Project. The assembly contains an
eight position filter wheel and a driving mechanism. To actuate
the mechanism a pulse is sent that energizes the linear
solenoid, thereby rotating the rocker arm by means of the
connector rod. The pawl, pivoted on the rocker arm, is driven
toward the next wheel cog. At this point the pawl releases
latch A from the cog wheel, extends the drive spring and then
engages the next cog on the wheel. This puts the mechanism in
the cocked position. When the solenoid is de-energized, the
rocker arm and pawl are returned to their original positions by
the drive spring, which advances the filter wheel one position.
During this travel the A latch follows the pawl inward and is
in position to stop the filter wheel at the end of the stroke.
The back latch B ratchets over the cogs, preventing the wheel
from back lashing. A series of photo-diodes are uncovered by a
pattern of small apertures in the filter wheel, which are unique
for each filter position. Thus the filter that is in the
optical path is known for each image taken and is included as
part of the engineering telemetry.
The spectral response of the camera is controlled by bandpass
filters. The bandpass filters for Stardust are new and
installed into the filter wheel to replace the Voyager filters.
In this table, the filters are identified along with some of
their characteristics and their position location in the filter
wheel:
Filter Name OPNAV -- Optical Navigation
Central or Passband (nm) 698.8
FWHM(nm) 400
Transmission 92%
Blocking N/A
Wheel Position 0
Filter Name NH2 -- NH2 Emission
Central or Passband (nm) 665.1
FWHM(nm) 15
Transmission 70%
Blocking 10^-2@6500,6800 10^-3@6450,6850
Wheel Position 1
Filter Name OXYGEN -- Oxygen (0[1D]) Emission
Central or Passband (nm) 633.6
FWHM(nm) 12
Transmission 60%
Blocking 10^-3@6200,6500
Wheel Position 2
Filter Name C2 -- C2 (C2 delta v=0 band)
Central or Passband (nm) 513.2
FWHM(nm) 12
Transmission 65% and 52%:5099-5174
Blocking 10^-1@3000,5051 and 10^-1@5230,11000
Wheel Position 3
Filter Name YELLOW -- Yellow Continuum
Central or Passband (nm) 580.2
FWHM(nm) 4
Transmission 50%
Blocking 10^-2@5750 and 10^-2@5850
Wheel Position 4
Filter Name RED -- Red Continuum
Central or Passband (nm) 712.9
FWHM(nm) 6
Transmission 70%
Blocking 10^-1@3000-7082 and 10^-1@7175-11000
Wheel Position 5
Filter Name NIR -- NIR Continuum
Central or Passband (nm) 874.6
FWHM(nm) 30
Transmission 70%
Blocking 10^-2@8400 and 10^-3@9100
Wheel Position 6
Filter Name HIRES -- High Resolution
Central or Passband (nm) 596.4
FWHM(nm) 200
Transmission 85%
Blocking 10^-3: 3000-480 and 10^-3: 7000-11000
Wheel Position 7
All wavelengths are in nanometers.
Shutter Subassembly
-------------------
The NAVCAM shutter assembly is also inherited Flight Spare
hardware from the Voyager Project. The device is a two-blade
focal plane mechanism. Each blade is 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' blade. 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.
This double-bladed shutter has the property that in one
direction the exposures are 1.65 ms shorter than in the other.
Therefore a setting of 5 ms, which is the shortest possible,
results in alternate 5 and 3.35 ms exposures, those at 25 ms,
alternate 25 and 23.35 ms exposures, and so on. Occasionally
bias frames, which do not require shutter action, are
transmitted to Earth. This changes the ``parity.''
Detector
--------
The NAVCAM uses a charge coupled device (CCD) detector packaged
for the Cassini Imaging Science Subsystem (ISS). The operating
temperature range is -55 C to -25 C. The CCD is mounted in a
hermetically sealed package which is back-filled with argon. An
operating temperature of around -35 C is needed for suppression
of dark current and to minimize proton gamma and neutron
radiation effects. The NAVCAM employs passive radiative cooling
to maintain the detector operating temperature.
NAVCAM detector characteristics are:
Format 1024 x 1024 pixels
Pixel size 12 x 12 micrometers
Full well >= 100,000 e-
Dark current < 0.1 e-/pixel/sec at operating temperature
Charge transfer efficiency 0.99996 at operating temperature
Read Noise <= 15 e- rms
Scan Mirror Mechanism
---------------------
This mechanism enables the stationary wide angle optics (flying
sidewise during encounter) to keep the comet in view during
flyby. The scanning mirror, located some distance forward of
the camera lens faces 45 degrees away from the camera viewing
axis. Rotating the mirror about the camera axis at the proper
rate enables comet tracking during flyby. The mechanism is a
single degree of freedom device. It requires proper spacecraft
orientation so that the comet can be viewed in a viewing plane
originating at the scan mirror and oriented perpendicular to
the camera axis. The initial forward looking view (0 degree
position) is through a periscope which protects the scan
mirror. The mirror's home position is at -20 degrees, at which
the camera sees a black object on the spacecraft. Total mirror
rotation is 220 degrees, allowing views up to 20 degrees beyond
looking straight back. The maximum rotational rate is
approximately 3.1 degrees/sec.
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 which houses 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 100 degrees C temperature range. A smooth rotational
motion is further assured by a duplex bearing pair, by
precision gears and an anti backlash mechanism utilizing a
negator spring to produce a constant torque against the
rotational motion. This should suppress pixel smear to
approximately 2 pixels. The mirror, made of zerodur, is bonded
to flexures that attach to the rotational housing. Baffling
rings along the optical path assure that stray light is being
reflected away from the lens.
The drive unit next to the housing consists of the following
components: A brushless DC motor from American Electronics
Inc.: Vmax=36V, T=10oz-in, n~1200rpm. This motor was previously
space qualified for the MISR project. The motor is flanged onto
the four stage planetary gearbox made by American Technology
Consortium: e=252.6:1. The gearbox was previously space
qualified for the Mars Pathfinder project. A slip clutch at the
gearbox output shaft utilizes a set of Belleville springs to
keep the pinion's transmitted torque within a predetermined
limit. It prevents mechanical damage in the event of control
failures which might cause the mechanism to over-rotate and hit
the stops that limit travel. The pinion is engaged with the
main gear on the rotational part, providing a fifth
transmission stage. The overall gear ratio is 2518.6:1.
Periscope
---------
The periscope is an optical assembly that allows the scan
mirror to look over the protective Whipple shield while it is
pointed forward, in a direction parallel to the space craft +X
axis. This is to protect the scan mirror from particle
impingement, that would significantly degrade it's performance,
during cruise, upon approach and while flying through the comet
coma. The periscope contains two rectangular mirrors mounted at
45 degrees with respect to the space craft +X axis.
The mirrors are made out of aluminum to reduce the rate and
amount of degradation from particle impacting. For light
weighting, the mirrors are fabricated using an aluminum foam
core composite material with solid face sheets braised onto the
front and back surfaces. Single point diamond turning is used
to figure the reflective surface of the mirrors. Since the
forward looking mirror is exposed to the impacting particles it
is post polished and receives only a very thin protected
aluminum coating. While the mirror facing away from the
particle stream is 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 impact.
The periscope structure is a graphite/epoxy composite
construction. This material was chosen to make the structure
light and to reduce thermally induced distortions from the
spacecraft to the periscope assembly. Each mirror is
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 degrees down toward the spacecraft
-Z axis it is no longer imaging through the periscope. The
periscope was designed so that the images taken while the
mirror was partly looking through the periscope could still be
used for optical navigation.
Electronics and NAVCAM Control
------------------------------
The electronics for the NAVCAM consist of two major parts: the
camera electronics and the scan mirror electronics. The sensor
head electronics (part of the camera electronics) are mounted
on a chassis that is located behind the focal plane of the
optics while the rest of the camera electronics and the scan
mirror electronics are housed in the baseplate support. The
NAVCAM electronics control NAVCAM functions and process NAVCAM
commands and telemetry. The NAVCAM electronics are powered from
the spacecraft 28 volt regulated and 34 volt unregulated power
supplies.
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 quantized 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 NAVCAM ground support equipment. This port is
used for ground testing only.
The remainder of the camera electronics is called the main
electronics. The main electronics provide the power and perform
all NAVCAM control functions. This 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 Data Handling (C&DH) unit. A
high speed bus is used for transmission of image data and a low
speed bus is used for sending and receiving commands and
telemetry.
The NAVCAM scan mirror mechanism has its 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
motor rotation pulses, and 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 NAVCAM baseplate.
NAVCAM Commanding
=================
All commands are transmitted and received by the NAVCAM over the
low rate RS-422 bus. Commands received by NAVCAM are echoed back
to the spacecraft, including parity errors, so that commands with
errors can be ignored. This table contains a list of NAVCAM
commands:
Discrete Commands:
Command States/Contents
--------------------------- ---------------------------------
Camera power off Turn camera power off
Camera power on/reset state Turn camera power on/reset camera
Camera Function Commands:
Sample Analog telemetry 1 of 8 possible channels
Sample Digital telemetry 8 registers
Move filter wheel 1- 8 positions
Take picture (exposure time) shutter exposure and return image
data/digital telemetry
Select analog telem channel 1 of 8 possible channels
Calibration lamp On or Off
Data compression On or Off
Shutter bulb mode Open or close
Scan Mirror Commands:
Command States/Contents
--------------------------- ---------------------------------
Sample telemetry 4 registers
Move mirror Mirror is rotated at specified
velocity
Scan Motor power On or Off
Scan Mirror Heater On or Off
Telemetry Collection
====================
The NAVCAM collects pixel data, engineering data and status
data. This data is divided into three categories:
Camera Digital:
Image data
Shutter exposure time
Lamp status (on/off)
Compression status (on/off)
FIFO status
Filter Wheel move steps
Filter Wheel position
Camera Analog:
Filter Wheel voltage
CCD Temperature
+ 5 Volt supply voltage
- 5 Volt supply voltage
+ 12 Volt supply voltage
- 12 Volt supply voltage
Scan Mirror
Mirror velocity
Scan motor status (on/off)
Heater Status (on/off)
Motor direction
Motor rotation pulses (tick marks)
Housekeeping telemetry consists of engineering and status data
only, packetized with appropriate header information into packets
called housekeeping packets. This telemetry is used when the
NAVCAM is in an ON power state. This telemetry is only used when
the NAVCAM is actively taking data.
Effective Data Rates
====================
NAVCAM electronics provide a single data rate of 300 kilo-pixels
per second.
Encoding and Compression
========================
The pixel data from the NAVCAM can be processed within the NAVCAM
in several ways. The default processing is to transmit the
converted 12 bit data. When 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.
Power Management
================
The camera electronics are required to draw less than 8 watts and
the scan mirror less than 10 watts steady state. Operational
constraints are placed on the NAVCAM to limit the power drawn by
NAVCAM from the spacecraft. This table contains a list of the
power operating states.
State Definition
------------ ------------------------------------------------
Camera Off 28 volt power to the NAVCAM is off. Heaters
powered directly by the spacecraft can still be
on.
Camera On 28 volt power is applied to the NAVCAM to
receive commands, send telemetry and take data.
Scan motor Off Power supply to scan mirror is off. Heater can
still be on.
Scan motor On Power is applied to scan motor to receive
commands, send telemetry and scan.
At power turn on, the NAVCAM 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 remains in this
state until the first command is received. The states of all
mechanisms are what they were when the camera was last turned
off.
NAVCAM Safe State
=================
In response to a concern that the NAVCAM boresight may, in a
spacecraft fault condition, be exposed to the sun (accidentally
incident sunlight), a method to protect the shutter and focal
plane of the camera was developed. The NAVCAM safe state is
defined as placing a narrow band filter in the optical path and
opening the shutter. To reset the NAVCAM to a normal operating
state a power on reset clears the FPGA lockup.
|
