The 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 nearby.

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 jet phenomena.

In 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 type.

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 (C&DH).

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:

Optics

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.

Optics Characteristics

Shutter Subassembly

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" 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.

Detector

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 temperature.

Scan Mirror Mechanism

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.

Periscope

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 strikes.

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.

Electronics 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 unregulated supplies.

Camera Electronics

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.

Scan Mirror Electronics

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.

NC Commanding

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 be ignored.

Effective Data Rates

NC Electronics provides one data rate of 300 kbpxls per second.

Encoding and Compression

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.

Power Management

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.

Power 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.

NC Safe State

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.