<p> Just over two decades ago the first planet outside our solar system was found, and thousands more have been discovered since. Nearly all these exoplanets were indirectly detected by sensing changes in their host stars' light. However, exoplanets must be directly imaged to determine their atmospheric compositions and the orbital parameters unavailable from only indirect detections. The main challenge of direct imaging is to observe stellar companions much fainter than the star and at small angular separations. Coronagraphy is one method of suppressing stellar diffraction to provide high star-to-planet contrast, but coronagraphs are extremely sensitive to quasi-static aberrations in the optical system. Active correction of the stellar wavefront is performed with deformable mirrors to recover high-contrast regions in the image. Estimation and control of the stellar electric field is performed iteratively in the camera's focal plane to avoid non-common path aberrations arising from a separate pupil sensor. Estimation can thus be quite time consuming because it requires several high-contrast intensity images per correction iteration.</p><p> This thesis focuses on efficient focal plane wavefront correction (FPWC) for coronagraphy. Time is a precious commodity for a space telescope, so there is a strong incentive to reduce the total exposure time required for focal plane wavefront estimation. Much of our work emphasizes faster, more robust estimation via Kalman filtering, which optimally combines prior data with new measurements. The other main contribution of this thesis is a paradigm shift in the use of estimation images. Time for FPWC has generally been considered to be lost overhead, but we demonstrate that estimation images can be used for the detection and characterization of exoplanets and disks. These science targets are incoherent with their host stars, so we developed and implemented an iterated extended Kalman filter (IEKF) for simultaneous estimation of the stellar electric field and the incoherent signal. From simulations and testbed experiments, we report the increased FPWC speed enabled by Kalman filtering and the use of the IEKF for exoplanet detection during FPWC. We discuss the relevance and future directions of this work for planned or proposed coronagraph missions.</p>

<p> This thesis consists of the design, fabrication, and launch of a sounding rocket payload to observe the spectrum of the soft X-ray emission (0.1-1 keV) from the Cygnus Loop supernova remnant. This instrument, designated the Off-plane Grating Rocket for Extended Source Spectroscopy (OGRESS), was launched from White Sands Missile Range on May 2nd, 2015. The X-ray spectrograph incorporated a wire-grid focuser feeding an array of gratings in the extreme off-plane mount which dispersed the spectrum onto Gaseous Electron Multiplier (GEM) detectors. The gain characteristics of OGRESS's GEM detectors were fully characterized with respect to applied voltage and internal gas pressure, allowing operational settings to be optimized. The GEMs were optimized to operate below laboratory atmospheric pressure, allowing lower applied voltages, thus reducing the risk of both electrical arcing and tearing of the thin detector windows. The instrument recorded 388 seconds of data and found highly uniform count distributions over both detector faces, in sharp contrast to the expected thermal line spectrum. This signal is attributed to X-ray fluorescence lines generated inside the spectrograph. The radiation is produced when thermal ionospheric particles are accelerated into the interior walls of the spectrograph by the high voltages of the detector windows. A fluorescence model was found to fit the flight data better than modeled supernova spectra. Post-flight testing and analysis revealed that electrons produce distinct signal on the detectors which can also be successfully modeled as fluorescence emission.</p>

<p> On August 21, 2017 a total solar eclipse occurred over the United States commencing on the west coast moving across to the east coast providing an opportunity to observe how the rapid day-night-day transition changed the ionosphere&rsquo;s D-region electron density and how very low frequency (VLF) electromagnetic wave propagation was affected. To observe the solar obscurity effects, VLF receivers were deployed in two locations: one in the path of totality in Lakeside, Nebraska and another south of the totality path in Hugo, Colorado. The locations were chosen to achieve an orthogonal geometry between the eclipse path and propagation path of U. S. Navy VLF transmitter in North Dakota, which operates at 25.2 kHz and has call sign NML. VLF amplitude and phase changes were observed in both Lakeside and Hugo during the eclipse. A negative phase change was observed at both receivers as solar obscuration progressively increased. The observed phase changes became positive as solar obscuration reduced. The opposite trend was observed for the amplitude of the transmitted signal: growth as max totality approached and decay during the shadow&rsquo;s recession. The Long Wave Propagation Capability (LWPC) code developed by the US Navy was used to model the observations. LWPC is a modal solution finder for Earth-ionosphere waveguide propagation that takes into account the D-region density profile. In contrast to past efforts where a single ionosphere profile was assumed over the entire propagation path, a degree of spatial resolution along the path was sought here by solving for multiple segments of length 100-200 km along the path. LWPC modeling suggests that the effective reflection height changed from 71 km in the absence of the eclipse, to 78 km at the center of the path of totality during the total solar eclipse and is on agreement with past work.</p><p>

It is generally believed that molecular clouds are clumpy. In clumpy molecular clouds the large scale spatial distribution of the emission from the ionized, neutral, and molecular carbon species is significantly different than it would be in a cloud with a smooth density distribution. Thus it should be possible to derive some general properties of the clumps (i.e. size, volume filling factor, kinematics, etc.) by observing these species, even if individual clumps are not spatially resolved. We have built a receiver capable of observing both the J = 3 $\to$ 2 transition of $\sp{12}$CO and $\sp{13}$CO and the $\sp3$P$\sb1$ $\to$ $\sp3$P$\sb0$ transition of neutral atomic carbon, and a filterbank of novel design that acts as its backend. We have used this instrument to obtain a map of $\sp{12}$CO J = 3 $\to$ 2 in the Orion A region. The observed line shapes, and a comparison of the intensity of this line to that of the J = 1 $\to$ 0 transition, strongly suggests that this region is clumpy, that a kinetic temperature gradient exists within each clump, and that the source of the heating is situated on the surface of the cloud. Furthermore, an analysis of the smoothness of very high signal-to-noise $\sp{12}$CO and $\sp{13}$CO J = 1 $\to$ 0 spectra in the region indicates that the volume filling factor of the clumps increases from the surface to the core of the cloud. We develop a model of an edge-heated clumpy molecular cloud that takes into account the chemical and thermal effects of the UV photons on the density and temperature structure of the clumps. We show that this model can reproduce the observed $\sp{12}$CO intensities and line ratios in the region, and is in qualitative agreement with other characteristic features observed in this and other similar regions (i.e. widespread presence of warm CO, of neutral carbon, and of high dipole moment species). We discuss the implications of these results on the general properties of the clumps in the Orion region.

<p> We introduce an elastic constitutive model of gravity that enables the interpretation of cosmological observations in terms of established ideas from Solid Mechanics and multi-scale modeling. The behavior of physical space is identified with that of a material-like medium called "cosmic fabric," which exhibits constitutive behavior. This cosmic fabric is a solid hyperplate that is broad in the three ordinary spatial dimensions and thin in a fourth hyperspatial dimension. Matter in space is treated as fabric inclusions that prescribe in-plane (three-dimensional) strain causing the transverse bending of the fabric into the fourth hyperspatial dimension. The linearized Einstein-Hilbert action, which governs the dynamics of physical space, is derived from postulating Hooke's Law for the fabric, and the Schwarzschild metric is recovered from investigating matter-fabric interactions. At the continuum length scale, the Principle of Relativity is shown to apply for both moving and stationary observers alike, so that the fabric's rest reference frame remains observationally indistinguishable at such a length scale. Within the Cosmic Fabric paradigm, the structural properties of space at different hierarchical length scales can be investigated using theoretical notions and computational tools from solid mechanics to address outstanding problems in cosmology and fundamental physics. For example, we propose and offer theoretical support for the "Inherent Structure Hypothesis", which states that the gravitational anomalies currently attributed to dark matter may in fact be manifestations of the inherent (undeformed) curvature of space. In addition, we develop a numerical framework wherein one can perform numerical "experiments" to investigate the implications of said hypothesis. </p><p>