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.
Focal Length 200mm
Relative Aperture f/3.5
Spectral Range 380 - 1100 nm
Resolution 60 microradian/pixel
Field of View 3.5 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" 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.
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.