Albert Slomba

Mirror Surface Metrology And Polishing For AXAF/TMA

The achievement of the derived goals for mirror surface quality on the Advanced X-ray Astrophyscis Facility (AXAF), Technology Mirror Assembly (TMA) required a combination of state-of-the-art metrology and polishing techniques. In this paper, we summarize the derived goals and cover the main facets of the various metrology instruments employed, as well as the philosophy and technique used in the polishing work. In addition, we show how progress was measured against the goals, using the detailed error budget for surface errors and a mathematical model for performance prediction. The metrology instruments represented a considerable advance on the state-of-the-art and fully satisfied the error budget goals for the various surface errors. They were capable of measuring the surface errors over a large range of spatial periods, from low-frequency figure errors to microroughness. The polishing was accomplished with a computer-controlled process, guided by the combined data from various metrology instruments. This process was also tailored to reduce the surface errors over the full range of spatial periods.

Performance prediction of the AXAF Technology Mirror Assembly using measured mirror surface errors

We have developed a math model relating the measured parameters of the Technology Mirror Assembly (TMA) to its final performance. This scalar scattering model is valid for large and small amplitude features. It allows the user to specify power spectral densities and/or autocovariance functions within any spatial bandwidth, including microroughness. We present new TMA data in the bandwidth of ~0.1-1000 mm-1, predicting performance and comparing them with x-ray test data. We also account for assembly, alignment, and particulate contamination. Finally, we comment on improved performance expected after repolishing.

Derivation Of Requirements For Surface Quality And Metrology Instrumentation For AXAF/TMA

The top level goal for residual image degradation due to all mirror surface errors for the Advanced X-ray Astrophysical Facility (AXAF) Technology Mirror Assembly (TMA) was a Root Mean Square (RMS) image diameter of 0.38 arc second, approximately one order of magnitude tighter than for the highly successful HEAO-B, which represented the previous state of the art. In this paper, we cover the subdivision of the top level goal into a detailed error budget for various surface errors, and from there the subdivision into requirements on the surface metrology instrumentation. The derivation of the detailed error budget for surface errors required the definition of a new set of mathematical functions to describe surface errors on cylindrical optics, and a new type of analysis of the effect of mid-frequency surface errors on high quality X-ray images. The derivation of requirements on the surface metrology instrumentation pointed the way for the conceptualizing and design of several new metrology instruments which were beyond the previous state of the art.

Technology Mirror Assembly Mirror Quality Requirements And Achievements

In order to demonstrate optical technology readiness for the Advanced X-ray Astrophysical Facility (AXAF), a Wolter Type I telescope was built. This telescope, called the Technology Mirror Assembly (TMA), was designed to have a system resolution of 0.6 arc second and a tight encircled energy performance specification. In order to meet these goals for encircled energy and resolution, an error budget was established, and specifications applied to each optical parameter. These tolerances are nearly an order of magnitude tighter than those required for HEAO-B, the highly successful forerunner of AXAF, which represents the present state of the art. Such stringent tolerances required a considerable advance in the metrology and polishing process. This paper describes the TMA error budget generation, details the metrology instrumentation and performance levels achieved, and discusses the computer controlled polishing process and equipment used to fabricate these X-ray optics. Finally, we illustrate how polishing progress was measured using a mathematical performance prediction model. Subsequent measurement of TMA focal plane performance shows a resolution of less than 0.5 arc second at X-ray wavelengths, which is in good agreement with these predictions and which represents the highest level of X-ray telescope performance ever achieved.

A Coaxial Interferometer With Low Mapping Distortion

A vacuum rated Coaxial Reference Interferometer (CORI) has been designed that satisfies the data input requirements of Computer Controlled Polishing (CCP) methods. The instrument obtains wavefront data at very high sampling intervals (50 to 100 fringes per aperture) and maintains precise mirror to focal plane mapping fidelity. Realistic as manufactured design goals are a 0.010% rms wavefront quality at f/2.3 and the ability to locate positions on a 2.4 meter diameter mirror to within 0.6mm from interferogram coordinates. Internal calibration means have been provided to independently verify interferometer performance as required. The instrument can be used to test both uncoated and coated mirrors. Interferograms are recorded on 70-mm film but may be monitored visually or by means of a vidicon.

In-Process Metrology For X-Ray Optics

Space-based astronomy at x-ray wavelengths has steadily progressed over the past 25 years. These advances have been made possible through the use of telescopes with increasingly higher resolution and collecting area, along with corresponding improvements in detectors, focal plane instrumentation, and pointing systems. In this paper, we discuss an in-process metrology system designed to provide sufficiently accurate data to the polishing process to allow fabrication of a half-arc second glancing angle of incidence telescope operating in the 1 to 7 KeV spectral region. This measurement system was developed in anticipation of, and applied to, the Advanced X-ray Astrophysics Facility (AXAF) Technology Mirror Assembly(TMA). The metrology instruments represent a considerable advance in the state of the art for measurements of large, thin, aspheric cylindrical optical elements. They allowed measurement of surface figure errors over a very wide range of spatial surface frequency with unprecedented accuracy during each phase of fabrication -- from coarse grinding through figuring and final polishing.

Interferometric Measurement Of Large Aperture Infrared Windows

A versatile interferometer has been designed and built to evaluate infrared windows as large as 51 cm in diameter. Transmitted wavefront measurements may be made with an accuracy of 0.05λ peak-to-valley at 10.6 µm. Variations in refractive index of transmitting materials as small as 5 x 10-6 may be measured and mapped. Through the use of an auxiliary mirror, systems or components requiring a spherical wavefront at f/6 or slower may also be tested.

Application Of Lasers To Measure The Optical, Thermal And Mechanical Properties Of Materials

The advent of the laser has had a profound effect in most areas of scientific and technological research. A particularly fertile area of application is in the measurement of the optical, thermal, and mechanical properties of materials. This paper discusses the application of laser measuring techniques in the fields of Raman spectroscopy, optical surface scatter, interferometry and optical testing, dimensional stability measurements, microyield and microcreep studies, thermal expansion coefficient measurements, and transient thermal phenomena in optical materials. In these areas of materials measurement, all of the lasers characteristics-temporal and spatial coherence, frequency tunability, collimation, polarization, and high brightness--have been usefully exploited at wavelengths from the ultraviolet to the far infrared.