Thomas Kester

Large-mirror figure measurement by optical profilometry techniques

In most cases, conventional interferometric methods can be used to test the figure of large spherical and flat optical components. There are, however, certain types of unconventional surfaces, such as those used in grazing incidence x-ray applications, that are nearly impossible to test by conventional means. These cylindrical aspheres are usually tested by some type of scanning optical profiler. We discuss the use of a versatile slope measuring scanning interferometer system, the Long Trace Profiler (LTP), in measuring the figure error of large surfaces, particularly those that have extremely long radii of curvature in the tangential direction. Use of this instrument in different configurations has permitted measurements to be made on cylindrical asphere segments that are over one meter long, on water-cooled high heat load mirrors in ultra high vacuum synchrotron beam lines under actual operating conditions, and on complete x-ray telescope mirror shells and mandrels that are mounted in a vertical configuration to minimize gravity sag errors. Each of these different configurations has its own particular advantages and shortcomings. The ultimate performance of the LTP depends upon the thermal stability of the local environment. We illustrate the effects that temperature variations on the order of plus or minus 0.1 degrees Celsius have on the errors in the measurement of a long radius sphere.

Calibration of a Vertical-Scan Long Trace Profiler at MSFC

The long trace profiler (LTP) is the instrument of choice for the surface figure measurement of grazing incidence mirrors. The modification of conventional LTP, the vertical- scan LTP, capable of measuring the surface figure of replicated shell mirrors is now in operation at Marshall Space Flight Center. A few sources of systematic error for vertical-scan LTP are discussed. Status of systematic error reduction is reported.

Incoming Metrology of Segmented X-Ray Mandrels at MSFC

The baseline design of the Constellation-X Spectroscopy X-ray telescope (SXT) employs segmented x-ray mirrors, to be replicated from precision mandrels. Thus far, the Constellation-X Project has procured and received three (3) flight-scale mandrels, for use in development of mirror technologies. Complementary to 30° sections of 10-m-focal-length Wolter-1 optics of diameters 1.6, 1.2, and 1.0 m, the mandrels’ primary (parabolic) and secondary (hyperbolic) optical surfaces are each 0.5-m long. In order to generate surface maps for x-ray performance predictions, NASA’s Marshall Space Flight Center (MSFC) is conducting incoming metrology. Using a combination of instruments, this metrology measures axial-slope deviations and axial profiles, slope differences, roundness, absolute radius, and micro-roughness. This paper describes the mandrels, the metrology requirements, and MSFC’s metrology instrumentation and procedures.

Progress of multi-beam long trace-profiler development

The multi-beam long trace profiler (LTP) under development at NASA’s Marshall Space Flight Center[1] is designed to increase the efficiency of metrology of replicated X-ray optics. The traditional LTP operates on a single laser beam that scans along the test surface to detect the slope errors. While capable of exceptional surface slope accuracy, the LTP single beam scanning has slow measuring speed. As metrology constitutes a significant fraction of the time spent in optics production, an increase in the efficiency of metrology helps in decreasing the cost of fabrication of the x-ray optics and in improving their quality. Metrology efficiency can be increased by replacing the single laser beam with multiple beams that can scan a section of the test surface at a single instance. The increase in speed with such a system would be almost proportional to the number of laser beams. A collaborative feasibility study has been made and specifications were fixed for a multi-beam long trace profiler. The progress made in the development of this metrology system is presented.

Development of multi-beam long trace profiler

In order to fulfill the angular resolution requirements and make the performance goals for future NASA missions feasible, it is crucial to develop instruments capable of fast and precise figure metrology of x-ray optical elements for further correction of the surface errors. The Long Trace Profilometer (LTP) is an instrument widely used for measuring the surface figure of grazing incidence X-ray mirrors. In the case of replicated optics designed for x-ray astronomy applications, such as mirrors and the corresponding mandrels have a cylindrical shape and their tangential profile is parabolic or hyperbolic. Modern LTPs have sub-micro radian accuracy, but the measuring speed is very low, because the profilometer measures surface figure point by point using a single laser beam. The measurement rate can be significantly improved by replacing the single optical beam with multiple beams. The goal of this study is to demonstrate the viability of multi-beam metrology as a way of significantly improving the quality and affordability of replicated x-ray optics. The multi-beam LTP would allow one- and two-dimensional scanning with sub-micro radian resolution and a measurement rate of about ten times faster compared to the current LTP. The design details of the instruments optical layout and the status of optical tests will be presented.

Forming mandrels for making lightweight x-ray mirrors

Future x-ray astronomical missions, similar to the proposed International X-ray Observatory (IXO), will utilize replicated mirrors to reduce both mass and production costs. Accurately figured and measured molds (called mandrels) - on which the mirror substrates are thermally formed, replicating the surface of the mandrels - are essential to enable these missions. The Optics Branches of the Goddard Space Flight Center (GSFC) and Marshall Space Flight Center (MSFC) have developed fabrication processes along with metrologies that yield high-precision mandrels; and through the SBIR program, they encourage small businesses to attack parts of the remaining problems. The Goddard full-aperture mandrel polisher (the MPM-500) has been developed to a level where mandrel surfaces match the 1.5 arcsec HPD level allocation in a 5 arcsec telescope program. This paper reviews this current technology and describes a pilot program to design a suite of machine tools and process parameters capable of producing many hundreds of these precision objects. A major challenge is to keep mid-spatial frequency errors below 2 nm rms - a severe specification; but we must also note the factors which work to our advantage: e.g., how the figure departs from a pure cone by only one micron, and how the demanding figure specifications which apply in the axial direction are relaxed by an order of magnitude in the azimuthal. Careful study of other large optical fabrication programs in the light of these challenges and advantages has yielded a realistic plan for the economical production of mandrels that meet program requirements in both surface and quantity.

Mounting and Alignment of Full-Shell Replicated X-Ray Optics

We are developing grazing-incidence x-ray optics for astronomy. The optics are full-cylinder mirror shells fabricated using electroformed-nickel replication off super-polished mandrels. For space-based applications where weight is at a premium, very-thin-walled, light-weight mirrors are required. Such shells have been fabricated at MSFC with < 15 arcsec resolution. The challenge, however, is to preserve this resolution during mounting and assembly. We present here a status report on a mounting and alignment system currently under development at Marshall Space Flight Center to meet this challenge.

Development of a Direct Fabrication Technique for Full-Shell X-Ray Optics

Future astrophysical missions will require fabrication technology capable of producing high angular resolution x-ray optics. A full-shell direct fabrication approach using modern robotic polishing machines has the potential for producing high resolution, light-weight and affordable x-ray mirrors that can be nested to produce large collecting area. This approach to mirror fabrication, based on the use of the metal substrates coated with nickel phosphorous alloy, is being pursued at MSFC. A model of the wear pattern as a function of numerous physical parameters is developed and verified using a mandrel sample. The results of the polishing experiments are presented.

Status of multi-beam long trace-profiler development

The multi-beam long trace profiler (MB-LTP) is under development at NASA’s Marshall Space Flight Center. The traditional LTPs scans the surface under the test by a single laser beam directly measuring the surface figure slope errors. While capable of exceptional surface slope accuracy, the LTP single beam scanning has slow measuring speed. Metrology efficiency can be increased by replacing the single laser beam with multiple beams that can scan a section of the test surface at a single instance. The increase in speed with such a system would be almost proportional to the number of laser beams. The progress for a multi-beam long trace profiler development is presented.

A stainless-steel mandrel for slumping glass x-ray mirrors

We have fabricated a precision full-cylinder stainless-steel mandrel at NASA Marshall Space Flight Center. The mandrel is figured for a 30-cm-diameter primary (paraboloid) mirror of an 840-cm focal-length Wolter-1 telescope. We have developed this mandrel for experiments in slumping-thermal forming at about 600°C-of glass mirror segments at NASA Goddard Space Flight Center, in support of NASAs participation in the International X-ray Observatory (IXO). Precision turning of stainless-steel mandrels may offer a low-cost alternative to conventional figuring of fusedsilica or other glassy forming mandrels. We report on the fabrication, metrology, and performance of this first mandrel; then we discuss plans and goals for stainless-steel mandrel technology.