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

Citation

Belland, Brent Roy (2021) Modeling Focal Ratio Degradation, Its Implications for Upcoming Fiber Spectrographs, and the Dynamics of NGC 6822. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/rbce-f185. https://resolver.caltech.edu/CaltechTHESIS:05292021-004454950

Abstract

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.

Thesis Files