The Mars Orbiter Camera (MOC) narrow angle system is a 3.5 m focal length camera that has operated for the last five years as part of the Mars Global Surveyor (MGS) Mission. Folded into a total package (including electronics) of less than 1 m length and weighing just over 20 kg, MOC's Ritchey-Chretien optical design is extremely sensitive to primary-to-secondary despace. Because of this, providing proper focus over the range of operational conditions was the primary challenge of the MOC development effort. As initially proposed, the instrument used a graphite-epoxy metering structure to provide a completely athermal system. Given of the sensitivity of the design and large operational temperature range, this turned out not to be realizable. The first fallback from a completely athermal design was to model the response of the system over temperature, and set the detector so the system would be focus over the range of operational conditions. Prototype testing revealed this was also not a workable solution. Late in the development flow, the system was retrofitted with a set of heaters to control focus in flight by application of radial thermal gradients across the primary mirror. Despite the loss of the first copy of the MOC on Mars Observer in 1993, the MOC on MGS has been an outstanding success, returning over 140,000 images of Mars to date and making a number of new discoveries about the planet.
OCA Applied Optics has devised and optimized methods for designing, building and testing precision mirror systems whose performance is not compromised by large changes in environmental temperature. A key to our approach is the use of a single material for the construction of all telesope components to minimize the differences in the contraction and expansion of the components with changes in the operating temperature. Specifically, we designed and built a simple, on-axis, monometallic telescope suitable for cryogenic testing in order to investigate and optimize methods for accurately testing the optical performance at cryogenic temperatures. We then designed and built a sophisticated, off-axis monometallic telescope representative of the current technology in advanced spectrometer instruments for deep-space applications. The novel design of this telescope facilitated assembly, alignment, and testing. We characterized the performance of the instrument in both laboratory and cryogenic environmental conditions. The results of these tests show that the instrument focus position and image quality showed negligible change at cryogenic temperatures, compared to a room temperature environment. This research has already contributed to improved performance and reduced cost for advanced reflective optical instruments for several space applications.
The Mars Observer Camera (MOC) is one of the instruments aboard the Mars Observer Spacecraft to be launched not later than September 1992, whose mission is to geologically and climatologically map the Martian surface and atmosphere over a period of one Martian year. This paper discusses the events in the development of MOC that took place in the past two years, with special attention given to the implementation of thermal blankets, shields, and thermal control paints to limit solar absorption while controlling stray light; vibration testing of Flight Unit No.1; and thermal expansion testing. Results are presented of thermal-vac testing Flight Unit No. 1. It was found that, although the temperature profiles were as predicted, the thermally-induced focus displacements were not.
The paper discusses the effect of thermal gradients on the optical performance of the primary mirror of Mars Observer Camera (MOC), which will be launched on the Mars Observer spacecraft in September 1992. It was found that mild temperature gradients can have a large effect on the mirror surface figure, even for relatively low coefficient-of-thermal-expansion materials. However, in the case of the MOC primary mirror, it was found that the radius of curvature (ROC) of the reflective surface of the mirror changed in a nearly linear fashion with the radial temperature gradient, with little additional aberration. A solid-state ROC controller using the thermal gradient effect was implemented and verified.
The Mars Observer Camera (MOC) is one of several instruments aboard the Mars Observer Spacecraft, which is scheduled to launch in September 1992, and begin monitoring the Martian surface (from Martian orbit) in December 1993. The MOC is the only instrument, however, that will record visual images of the surfe of Mars. The MOC comprises three separate optical systems: the Narrow-Angle system, for relaying high-resolution images of the Martian surface to Earth; the Red Wide-Angle system, for relaying limb-to-limb images in the 575 to 625 nanometer spectral range; and the Blue Wide-Angle system, for relaying limb-to-limb images in the 400 to 450 nanometer spectral range. The Mars Observer Project is conducted by the Jet Propulsion Laboratory (JPL), while the MOC experiment is conducted by the Arizona State University (ASU). Responsibility for the MOC instrument design, fabrication, integration, and test lies with the California Institute of Technology (Caltech). Perkin-Elmerts Applied Optics Operations (now OCA Applied Optics) and Composite Optics' involvement in the MOC program began in Iate-1986 when Caltech awarded this team a contact to fabricate the MOC Engineering Model and, later, two Right Models (one being a spare) based on the novel approach of utilizing graphite/epoxy composites for a majority of the MOC structure to achieve minimum weight. Since a majority of the structure is made of graphite/epoxy, including the sensitive metering structure between the pnmary and secondary mirrors of the Narrow-Angle system, charterizing the MOC structure became mandatory. Major concerns during the design of the MOC were not only structural integrity (designing the MOC such that its lightweight strucwre would withstand the shock and vibration of launch) and thermal stability (maintaining focus during the extreme thermal environment in Martian orbit), but also hygroscopic issues (paphite/epoxy absorbs atmospheric moisture and expands, causing defocus). This paper also briefly addresses the methods employed to reduce stray light from the walls of the graphite/epoxy structure.