Justin Kiehne
CRESST Summer Internship
Next Generation X-ray Observatory
I.
Introduction
X-ray astrophysics and astronomy require increasingly high resolution optics for the study of the
most distant stellar objects. As resolution improves, light weight must be maintained for increasing
photon collection area. X-ray observations are made from space so limitations imposed by launch
must be considered when attempting to increase satellite size. NGXO investigates methods for
meeting both of these requirements.
Four parameters characterize any x-ray optics technology. The first is angular resolution. The second
is effective area per unit mass. The third is production cost per unit effective area, and the fourth is
production rate or schedule. NGXO efforts have focused on improving the first two parameters.
II.
Overview
NGXO is committed to building telescopes in the manner of the recently launched NuStar x-ray
telescope. This is a segmented method based on a collection of mirror modules, shown below.
There are three main components to the module construction process, demonstrated in the figure
to follow. The first is mandrel fabrication. Next is mirror segment production, followed by mirror
installation. All three are made up of a myriad of intermediate steps.
The goal is to improve and perfect each of these steps such that the products meet the minimum
requirements for angular resolution and weight while minimizing cost and production time. To
minimize cost and production time, the manufacturing processes must be perfected such that optics
can be manufactured reliably and efficiently.
III.
Forming Mandrel Fabrication
I worked primarily in the production of the forming mandrels. Utilizing a precision polisher (shown
in the figure below), mandrels are polished with a variety of tool shapes until they meet the
minimum optical requirement of 2.5” Half Power Diameter. These forming mandrels provide the
optical figure to the mirror segments that ultimately determine imaging performance of the
telescope.
Mandrel measurements are taken using a Zygo 24-inch-aperture Fizeau interferometer and a phase
shift laser at the Goddard Space Flight Center building 5.
Resolution is measured at the micro scale and plotted against the ideal parabola shape to determine
tool shape. The figure above demonstrates the gradual change that occurs with each polishing run.
Analysis of mandrel error provides dimensions of the new polishing tool.
The figure above shows the error profiles that correspond to the previously shown mandrel profiles.
Each of these error profiles is used to create a new polishing tool.
Polishing tools are poured and carved using pitch. Pitch is the preferred material for many optical
polishing purposes due to its viscoelastic qualities. Rather than scratch or break, pitch is a solid that
flows very slowly at room temperature.
IV.
Remarks
In addition to working in mandrel fabrication with Peter Blake, I performed a number of optical
fabrication tasks with Peter Hill, another optician working in Building 5. He is working on developing
new methods of diamond turning high precision optics for future projects. If he is successful, NASA
programs will be able to keep the production of high precision optics in-house and save significant
expenditures. These tasks included interferometer measurements, metrology, and some low-grade
optics lapping.