Modeling Focal Ratio Degradation, Its Implications for Upcoming Fiber Spectrographs, and the Dynamics of NGC 6822

Spectroscopy is a cornerstone of astronomical research, enabling the measurements of abundances and velocities of astronomical objects. Fiber spectroscopy, with its capability to acquire spectra from targets in densely packed fields and position fibers over targets over a large field of view, promises to greatly expand the number of astronomical objects with acquired spectra. Upcoming such projects include the Subaru Prime Focus Spectrograph, which plans to advance cosmology, galactic archaeology, and galactic evolution studies. However, with the introduction of fibers to the spectrographic optical path also comes the introduction of the issue of focal ratio degradation (FRD). FRD is the scattering of light traveling through a fiber due to effects such as imperfections in the core-clad interface of a fiber, end face preparation, and stresses imposed on the fibers; thus, FRD is inherent to any fiber-based instrument. Focal ratio degradation scatters light to larger angles, resulting in lost light at the spectrograph and redistribution of light in the light's point spread function at the spectrograph detector. These issues are compounded by the fact that each fiber has a unique FRD 'fingerprint' and stresses during telescope operation can vary and induce dynamically changing FRD. It is also difficult to measure precisely. This thesis focuses on characterization of FRD. It introduces a novel approach to measuring that permits extraction of the effect of angular misalignment and tests this model on fiber mounted to a Cobra fiber positioner, though this method could be utilized for any optical fiber. The effects of focal ratio degradation on the spectra are simulated and found to affect counts 3-4 pixels from the center of sky lines on the order of 1-2% for changes in FRD at the 3 milliradian level, highlighting the importance of a good understanding of FRD to model sky lines at the level desired for the Subaru prime focus spectrograph. Multiplexed spectroscopy in NGC 6822 from KECK/DEIMOS is also presented. The ancient red giant population in NGC 6822 is prolately rotating, an unusual configuration that has been used as evidence of merger histories in other dwarf galaxies. The distribution of metallicity as a function of radius shows the oldest, most metal-poor stars are the most dispersion dominated. They also reside at larger radii, possibly pointing to a disruptive star formation history that scattered stars from the center of the galaxy over time. Future results from upcoming spectrographic surveys such as the Subaru Prime Focus Spectrograph could illuminate what is driving these peculiar features in NGC 6822 among many other scientific discoveries.</p

Jovian Plasma Modeling for Mission Design

The purpose of this report is to address uncertainties in the plasma models at Jupiter responsible for surface charging and to update the jovian plasma models using the most recent data available. The updated plasma environment models were then used to evaluate two proposed Europa mission designs for spacecraft charging effects using the Nascap-2k code. The original Divine/Garrett jovian plasma model (or "DG1", T. N. Divine and H. B. Garrett, "Charged particle distributions in Jupiter's magnetosphere," J. Geophys. Res., vol. 88, pp. 6889-6903,1983) has not been updated in 30 years, and there are known errors in the model. As an example, the cold ion plasma temperatures between approx.5 and 10 Jupiter radii (Rj) were found by the experimenters who originally published the data to have been underestimated by approx.2 shortly after publication of the original DG1 model. As knowledge of the plasma environment is critical to any evaluation of the surface charging at Jupiter, the original DG1 model needed to be updated to correct for this and other changes in our interpretation of the data so that charging levels could beproperly estimated using the Nascap-2k charging code. As an additional task, the Nascap-2k spacecraft charging tool has been adapted to incorporate the so-called Kappa plasma distribution function--an important component of the plasma model necessary to compute the particle fluxes between approx.5 keV and 100 keV (at the outset of this study,Nascap-2k did not directly incorporate this common representation of the plasma thus limiting the accuracy of our charging estimates). The updating of the DG1 model and its integration into the Nascap-2k design tool means that charging concerns can now be more efficiently evaluated and mitigated. (We note that, given the subsequent decision by the Europa project to utilize solar arrays for its baseline design, surface charging effects have becomeeven more of an issue for its mission design). The modifications and results of those modifications to the DG1 model to produce the new DG2 model presented here and the steps taken to integrate the DG2 predictions into Nascap-2k are described in this repor

Focal Ratio Degradation for Fiber Positioner Operation in Astronomical Spectrographs

Focal ratio degradation (FRD), the decrease of light’s focal ratio between the input into an optical fiber and the output, is important to characterize for astronomical spectrographs due to its effects on throughput and the point spread function. However, while FRD is a function of many fiber properties such as stresses, microbending, and surface imperfections, angular misalignments between the incoming light and the face of the fiber also affect the light profile and complicate this measurement. A compact experimental setup and a model separating FRD from angular misalignment was applied to a fiber subjected to varying stresses or angular misalignments to determine the magnitude of these effects. The FRD was then determined for a fiber in a fiber positioner that will be used in the Subaru Prime Focus Spectrograph (PFS). The analysis we carried out for the PFS positioner suggests that effects of angular misalignment dominate and no significant FRD increase due to stress should occur

FRD characterization in large-scale for FOCCoS of Prime Focus Spectrograph for Subaru telescope

The focal ratio degradation effects on optical fibers, technically referred to as FRD, has been the subject of intense studies since the beginning of the use of optical fibers in the construction of instruments applied in astronomy. A number of studies attempt to relate FRD to light loss in the optical system and other studies attempt to qualify and quantify FRD as a function of the stress induced during assembly of the structures supporting the ends of the optical fibers. In this work, we present a large-scale study to characterize FRD in all the fibers that make up the cables of the FOCCoS, Fiber Optical Cable and Connectors System project. FOCCoS, has the main function of capturing the direct light from the focal plane of Subaru Telescope using 2400 optical fibers, each one with a microlens in its tip, and conducting this light through a route containing connectors to a set of four spectrographs. The optical fiber cable is divided in 3 different segments called Cable A, Cable B and Cable C. Multi-fibers connectors assure precise connection among all optical fibers of the segments, providing flexibility for instrument changes. Our study provides procedures and methods to analyze the effects of FRD on all cable segments for each type of termination involved. Special attention is devoted to the understanding of how angular deviations between the input surface of the fiber and the test beam can significantly influence the calculation of FRD in optical fibers

FRD characterization in large-scale for FOCCoS of Prime Focus Spectrograph for Subaru telescope

The focal ratio degradation effects on optical fibers, technically referred to as FRD, has been the subject of intense studies since the beginning of the use of optical fibers in the construction of instruments applied in astronomy. A number of studies attempt to relate FRD to light loss in the optical system and other studies attempt to qualify and quantify FRD as a function of the stress induced during assembly of the structures supporting the ends of the optical fibers. In this work, we present a large-scale study to characterize FRD in all the fibers that make up the cables of the FOCCoS, Fiber Optical Cable and Connectors System project. FOCCoS, has the main function of capturing the direct light from the focal plane of Subaru Telescope using 2400 optical fibers, each one with a microlens in its tip, and conducting this light through a route containing connectors to a set of four spectrographs. The optical fiber cable is divided in 3 different segments called Cable A, Cable B and Cable C. Multi-fibers connectors assure precise connection among all optical fibers of the segments, providing flexibility for instrument changes. Our study provides procedures and methods to analyze the effects of FRD on all cable segments for each type of termination involved. Special attention is devoted to the understanding of how angular deviations between the input surface of the fiber and the test beam can significantly influence the calculation of FRD in optical fibers

Orbital decay of hot Jupiters : coefficients of nonlinear tidal coupling as a function of the stellar host type

Thesis: S.B., Massachusetts Institute of Technology, Department of Physics, 2016.Cataloged from PDF version of thesis.Includes bibliographical references (page 43).Hot Jupiters raise strong tides in their host stars due to their large masses (around a Jupiter mass) and tight orbits (orbital periods P </~ 3 days). These tides remove orbital energy and angular momentum, causing the planet's orbit to decay. The rate of decay depends on the detailed structure of the host star through the coefficients of nonlinear tidal coupling. Here we calculate these coefficients over a range of stellar mass and age on the main sequence (0.8 </= M/M < 1.2). These coefficients provide crucial input for future studies of nonlinear tidal dissipation; our analysis enables these studies to be extended to systems with non-solar-type hosts.by Brent Belland.S.B

Orbital Decay of Hot Jupiters due to Weakly Nonlinear Tidal Dissipation

We study tidal dissipation in hot Jupiter host stars due to the nonlinear damping of tidally driven g -modes, extending the calculations of Essick & Weinberg to a wide variety of stellar host types. This process causes the planet’s orbit to decay and has potentially important consequences for the evolution and fate of hot Jupiters. Previous studies either only accounted for linear dissipation processes or assumed that the resonantly excited primary mode becomes strongly nonlinear and breaks as it approaches the stellar center. However, the great majority of hot Jupiter systems are in the weakly nonlinear regime in which the primary mode does not break but instead excites a sea of secondary modes via three-mode interactions. We simulate these nonlinear interactions and calculate the net mode dissipation for stars that range in mass from 0.5 M _⊙ ≤ M _⋆ ≤ 2.0 M _⊙ and in age from the early main sequence to the subgiant phase. We find that the nonlinearly excited secondary modes can enhance the tidal dissipation by orders of magnitude compared to linear dissipation processes. For the stars with M _⋆ ≲ 1.0 M _⊙ of nearly any age, we find that the orbital decay time is ≲100 Myr for orbital periods P _orb ≲ 1 day. For M _⋆ ≳ 1.2 M _⊙ , the orbital decay time only becomes short on the subgiant branch, where it can be ≲10 Myr for P _orb ≲ 2 days and result in significant transit time shifts. We discuss these results in the context of known hot Jupiter systems and examine the prospects for detecting their orbital decay with transit timing measurements