Surface Metrology Methods for X-ray Telescope Mirrors, Freeforms, and Heliostats

Abstract

Modern optical systems require or greatly benefit from freeform or non-rotationally symmetric optics. Increasingly stringent system performance requirements demand high accuracy surface shapes, which drives the need for surface metrology beyond state-of-the-art. This dissertation discusses three projects aimed at filling the need for more accurate or more flexible metrology methods to enable the construction of next generation systems. First is axial shift mapping, a self-referencing metrology technique to measure spaced based X-ray telescope mirrors. X-ray telescopes are composed of nested off-axis parabolic and hyperbolic surfaces, which are difficult to characterize due to their acylindrical shape. I present a shifting Fizeau interferometry technique that decouples contributions from the surface under test in the interferogram from the contributions due to the reference surface. I will present experimental results from using axial shift mapping to characterize a cylindrical mirror. This technique will allow better characterization of X-ray telescope mirrors on the path to a diffraction limited X-ray telescope. Second is the Virtual Ball Probe, an optical profiler being developed at Apre Instruments, Inc. Typically, optical profilers require the probe tip to be normal to the surface. This requires complicated stage geometry and can block certain areas of optics such as steep concave surfaces. The Virtual Ball Probe is designed to measure optical freeforms with surface slopes up to 50 degrees without the need for tilting of the probe tip to be normal with the surface. This allows for simple stage geometry and can accurately measure steep internal optical surfaces. I will discuss the system design and show current system performance. This system fills the need for an accurate yet flexible metrology system for modern freeform optics. Third is Grating Embedded Mirrors for single shot heliostat optical metrology. Commercial concentrated solar power plants are required to accurately monitor the surface slope error and canting error of thousands of heliostats to maintain plant efficiency. We have fabricated test Grating Embedded Mirrors (GEMs), which are float glass mirrors with phase gratings written into the bulk glass using an ultra-fast laser. We use these gratings to direct light to non-specular directions. I placed these grating embedded mirrors in front of a metrology system dubbed Diffractive Auto-Stigmatic Hartmann Camera (DASHCam) to measure the mirror surface slope error. I will compare the results gathered by DASHCam to the surface slope error as measured by a Fizeau Interferometer. GEM’s flexibility of design and ease of measurement is aimed at providing a compact, accurate, and high-speed heliostat slope error metrology system that is robust to harsh environmental conditions for the next generation of concentrated solar power plants. Together, these metrology systems advance the state-of-the-art by increasing flexibility while lowering uncertainty to meet the increasingly stringent requirements of next generation systems.