Steven R. Wassom

The coronal suprathermal particle explorer (C-SPEX)

The primary science objective of the Coronal Suprathermal Particle Explorer (C-SPEX) is to investigate the spatial and temporal variations of coronal suprathermal particle populations that are seeds for acceleration to solar energetic particles (SEPs). It is understood that such seed particle populations vary with coronal structures and can change responding to solar flare and coronal mass ejection (CME) events. Models have shown that higher densities of suprathermal protons can result in higher rates of acceleration to high energies. Understanding the variations in the suprathermal seed particle population is thus crucial for understanding the variations in SEPs. However, direct measurements are still lacking. C-SPEX will measure the variation in the suprathermal protons across various coronal magnetic structures, before/after the passage of CME shocks, in the post-CME current sheets, and before/after major solar flares. Understanding the causes for variation in the suprathermal seed particle population and its effect on the variation in SEPs will also help build the predictive capability of SEPs that reach Earth. The CSPEX measurements will be obtained from instrumentation on the International Space Station (ISS) employing well-established UV coronal spectroscopy techniques.

Toward end-to-end optical system modeling and optimization: a step forward in optical design

Optical system modeling is interdisciplinary by its very nature. Optics, thermal engineering, structural engineering, control systems, electrical engineering, and data analysis are among the disciplines required to perform such modeling. Each discipline tends to have various software tools at its disposal to perform the required design and analysis but the software tools have had only limited ability for interdisciplinary use. Optical design software can form the core for optical systems modeling in many instances but its capabilities must be extended, or it needs to be used in a non-traditional way, depending on the problem at hand. We have used optical design software to assist in or form the basis for solving a number of interdisciplinary optical systems modeling problems. As an example, we present our method of dynamic optical ray tracing here and show its application. We also mention an example of linking optical design software to external code to solve optical systems modeling problems. Although these modeling efforts were successful, they illustrate the associated difficulty and need for integrated software modeling tools.

Low-cost large-angle steering mirror development

The Space Dynamics Laboratory has combined internal funds with its background in space-rated mechanisms to develop a prototype low-cost large-angle 2-axis fine steering mirror (FSM). The FSM has a 75-mm clear aperture, 30-degree mechanical elevation angle, 120-degree mechanical azimuth angle, and a 70-Hertz small-amplitude bandwidth. Key components include a rotary voice coil, unique patent-pending feedback sensor, brushless DC motor and optical encoder. Average error is <1 arcsec and total mechanical mass is <1 kg. Additional accomplishments include a passive launch lock, launch vibration testing, portable demonstration electronics development, and thermal-vacuum testing to pressures down to 1e-7 torr and temperatures down to 164 K.

SOFIE jitter analysis

SOFIE (Solar Occultation for Ice Experiment) is a 16-channel radiometer that was launched into a polar orbit on NASAs Aeronomy of Ice in the Mesosphere (AIM) spacecraft on 25 April 2007. An in-depth jitter analysis was performed to verify that the spacecraft could meet the pointing requirements. The analysis was based on an integrated modeling capability which combines structural dynamics with dynamic ray tracing to determine the motion of the boresight on the focal plane array (FPA) in the presence of disturbances. Two approaches were used and compared: a frequency-based analysis and a time-based analysis. For the frequency approach, the spacecraft provider determined the peak amplitude of the disturbance motions within 10% of each SOFIE modal frequency. The transmissibility factor Q between disturbance motion input and boresight motion output was determined for each degree of freedom and modal frequency. The disturbance amplitudes were then multiplied by each Q and summed over all frequencies and degrees of freedom. For the time-based analysis, the disturbance time histories were applied directly to the integrated model to generate the motions of the boresight ray on the FPA. The resulting motions were input to the sun sensor simulation to determine if the sun tracking algorithm could stay in fine track mode, or lose lock and jump to coarse track mode. As expected, the jitter from the frequency-based analysis was worse than the time-based analysis due to the implied assumption that the disturbance frequencies lined up exactly with the modal frequencies. Even so, the worst-case result met the requirement of 35 arcsec peak-peak jitter. The sun sensor simulation showed that the algorithm would still remain in fine-track mode and not lose lock even under the worst-case condition. Actual on-orbit data is presented that verifies the validity of the analysis.