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.
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 (Solar Occultation for Ice Experiment) is a 16-channel radiometer that was launched into a polar orbit on
NASA's 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.
Space Dynamics Laboratory (SDL), in partnership with GATS, Inc., designed, built, and calibrated an instrument to conduct the Solar Occultation for Ice Experiment (SOFIE). SOFIE is the primary infrared sensor in the NASA Aeronomy of Ice in the Mesosphere (AIM) instrument suite. AIM's mission is to study polar mesospheric clouds (PMCs). SOFIE will make measurements in 16 separate spectral bands, arranged in 8 pairs between 0.29 and 5.3 μm. Each band pair will provide differential absorption limb-path transmission profiles for an atmospheric component of interest, by observing the sun through the limb of the atmosphere during solar occultation as AIM orbits Earth. A fast steering mirror and imaging sun sensor coaligned with the detectors will track the sun during occultation events and maintain stable alignment of the Sun on the detectors. This paper outlines the instrument specifications and resulting design. The success of the design process followed at SDL is illustrated by comparison of instrument model calculations to calibration results, and lessons learned during the SOFIE program are discussed. Relative spectral response predictions based on component measurements are compared to end-to-end spectral response measurements. Field-of-view measurements are compared to design expectations, and radiometric predictions are compared to results from blackbody and solar measurements. Measurements of SOFIE detector response non-linearity are presented, and compared to expectations based on simple detector models.
The SOFIE pointing control system (PCS) locates and tracks the top edge of the sun and periodically scans the solar disk for calibration. Primary hardware components are a steering mirror assembly (SMA), sun sensor, vibration isolation system (VIS), and associated electronics. The SMA has a 100-Hz control bandwidth and is capable of ±1.6 mechanical degree deflection in azimuth and elevation axes. The sun sensor uses a 1024x1024 pixel, radiation-hardened focal plane array and coarse and fine tracking algorithms to report the solar centroid and edge positions to the PCS. The PCS control law uses this information to command the SMA. A change in launch loads necessitated the development of the VIS, which features passive viscoelastic damping to protect the SMA. A rapid prototyping methodology was used to develop the control laws for the inner SMA feedback loop and outer PCS feedback loop. The methodology features integrated end-to-end modeling of structural dynamics, controls, and optics; automatic C-code synthesis from block diagrams; real-time hardware-in-the-loop (HIL) testing; and the ability to change control parameters "on the fly." Extensive testing of the PCS shows stable pointing performance of about 2 arcsec in the presence of 60-arcsec disturbances, compared to the requirement of 15 arcsec.
Space Dynamics Laboratory (SDL) recently designed, built, and delivered the Solar Occultation for Ice Experiment (SOFIE) instrument as the primary sensor in the NASA Aeronomy of Ice in the Mesosphere (AIM) instrument suite. AIM's mission is to study polar mesospheric clouds (PMCs). SOFIE will make measurements in 16 separate spectral bands, arranged in eight pairs between 0.29 and 5.3 μm. Each band pair will provide differential absorption limb-path transmission profiles for an atmospheric component of interest, by observing the sun through the limb of the atmsophere during solar occulation as AIM orbits Earth. A pointing mirror and imaging sun sensor coaligned with the detectors are used to track the sun during occulation events and maintain stable alignment of the sun on the detectors. This paper outlines the mission requirements and goals, gives an overview of the instrument design, fabrication, testing and calibration results, and discusses lessons learned in the process.
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.
Dynamic ray tracing is a new tool that combines optical ray tracing and dynamic simulation codes. The implementation presented in this paper is a customization of the commercial code ADAMS. The tool features a special subroutine that was written and linked to the code, enabling it to compute and display the paths and intersection points of reflected and refracted optical rays as the optical surfaces move dynamically. Its first intended use would be for analysis and control of high-frequency jitter and lower-frequency drift. In addition to "undesired" motions or deformations, the method may also be used to simulate intentionally moving optical components such as scanners or zoom systems. The main difference in this capability and that of the existing optical design codes is that this method yields visual dynamic results. In quasi-real time, the user can watch the ray trace move and the resultant image quality metric change due to unwanted or intentional motion of the optical elements. This approach will enable the user to more quickly understand and visualize the situation and will reduce the chances of error that arise when two codes have to be used (static ray tracing and dynamic simulation) to analyze the system.