The Polstar Mission seeks to study the evolution of massive stars including their effect on the interstellar medium and their behavior in binary systems using a 60 cm telescope with a UV Spectropolarimeter within MIDEX mission constraints on cost cap, throughput, coating requirements, and system-level dimensional stability. The mission is in a high-earth orbit and must ensure precise and repeatable polarimetric observations. Design-to-cost paradigms are exercised throughout all design phases and heritage approaches to structure and mirrors are evoked. In terms of classical error budgets, designing for diffraction-limited performance at 1.2 μm is sufficient, however, there are special design concerns at these wavelengths which require maximizing throughput of photons. Special coatings and minimum reflections are mandatory with meticulous attention to cleanliness throughout the entire mission life cycle. Decontamination heaters must be employed shortly after launch, prior to opening the door, and periodically throughout the mission lifetime. Additionally, spectropolarimetry requirements impose constraints on symmetry and control of phase and amplitude. The secondary mirror must have adjustment capability in three degrees of freedom (tip, tilt, and focus) to address drifts from thermal perturbations, aging, and possibly even spacecraft jitter. We present in-process design approach and analyses to meet the challenges of ultraviolet wavelengths and polarization stability..
In the next decade, NASA envisions a large space-based telescope that will perform unprecedented astronomy focused on the detection and characterization of Earth-like exoplanets. Recent advances in optical coronography enable this mission, but the technology imposes challenging requirements on telescope dynamic stability and vibration isolation. An integrated non-contact pointing and vibration isolation system called the Disturbance Free Payload (DFP) provides a means to achieve this stability. This system provides an ideal non-contact state (with only residual coupling from power and data cables and actuator effects) while allowing for the necessary degree of rigid-body payload control to meet required telescope pointing and system line-of-sight (LOS) agility. A subscale demonstration of the DFP technology on a CubeSat operating in 6 degrees of freedom in the space environment is one of several developments needed to advance the DFP architecture to TRL 6. This paper describes the mission goals and the preliminary payload and experiment design.
An actuated hand-held impedance-controlled ultrasound probe has been developed. The controller maintains a
prescribed contact state (force and velocity) between the probe and a patient's body. The device will enhance the
diagnostic capability of free-hand elastography and swept-force compound imaging, and also make it easier for a
technician to acquire repeatable (i.e. directly comparable) images over time. The mechanical system consists of an
ultrasound probe, ball-screw-driven linear actuator, and a force/torque sensor. The feedback controller commands the
motor to rotate the ball-screw to translate the ultrasound probe in order to maintain a desired contact force. It was found
that users of the device, with the control system engaged, maintain a constant contact force with 15 times less variation
than without the controller engaged. The system was used to determine the elastic properties of soft tissue.
Tissue deformation in ultrasound imaging poses a challenge to the development of many image registration
techniques, including multimodal image fusion, multi-angle compound image and freehand three-dimensional
ultrasound. Although deformation correction methods are desired to provide images of uncompressed tissue structure,
they have not been well-studied. A novel trajectory-based method to correct a wide range of tissue deformation in
ultrasound imaging was developed. In order to characterize tissue deformation under different contact forces, a force
sensor provides contact force measurement. Template based image-flow techniques were applied to RF A-lines under
different contact forces. A two-dimensional displacement trajectory field was constructed, where pixel coordinates in
each scan were plotted against the corresponding contact force. Nonlinear extrapolation algorithms are applied to each
trajectory to relocate the corresponding pixel to where it would have been had there been no contact, thereby correcting
tissue deformation in the images. This method was validated by using a combination of FEM deformation and ultrasound simulation. It was shown that deformation of the simulated pathological tissue could be corrected. Furthermore, nonlinear polynomial regression was found to give better estimates, than linear regression, when large deformation was present. Estimation accuracy was not improved significantly for a polynomial regression larger than second order.
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