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.
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.
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.