To obtain higher spatial resolution interferometric measurements, users of optical shop interferometers generally want to obtain the highest possible number of pixels in the field of view. When the optical surface being tested does not fill the interferometer’s field of view, zoom optics in the viewing system can provide a convenient means to fill the detector. Some users employ zoom to measure subapertures of a larger optical surface to observe mid-spatial frequency (MSF) features that may not be seen in a full aperture test. While the zoom obviously enables the detector to be filled, its capability to increase the MSF measurement performance of the instrument is more difficult to assess.<p> </p>To investigate how zoom affects the MSF measurement capability, we measured a spherical surface with significant MSF content over a range of lateral magnifications. Two methods were used to obtain equivalent lateral magnifications: zoom and changing the transmission sphere. Differences in the relative MSF content were observed between the two methods. For further comparison, the same surface was measured with the same transmission spheres on a different interferometer with a fixed magnification coherent viewing system. We report on differences observed in measured MSF content between the two interferometers.
Aspheric surfaces can provide significant benefits to optical systems, but manufacturing high-precision
aspheric surfaces is often limited by the availability of surface metrology. Traditionally, aspheric measurements have
required dedicated null correction optics, but the cost, lead time, inflexibility, and calibration difficulty of null optics
make aspheres less attractive. In the past three years, we have developed the Subaperture Stitching Interferometer for
Aspheres (SSI-A®) to help address this limitation, providing flexible aspheric measurement capability up to 200 waves
of aspheric departure from best-fit sphere.
Some aspheres, however, have hundreds or even thousands of waves of departure. We have recently
developed Variable Optical Null (VON<sup>TM</sup>) technology that can null much of the aspheric departure in a subaperture. The
VON is automatically reconfigurable and is adjusted to nearly null each specific subaperture of an asphere. The VON
provides a significant boost in aspheric measurement capability, enabling aspheres with up to 1000 waves of departure
to be measured, without the use of null optics that are dedicated to each asphere prescription. We outline the basic
principles of subaperture stitching and the Variable Optical Null, demonstrate the extended capability provided by the
VON, and present measurement results from our new Aspheric Stitching Interferometer (ASI<sup>TM</sup>).
Traditionally, the most accurate measurements of aspheric surfaces have relied on interferometric null tests. These
usually require "null correction" optics, which often take significant time and expense to design and fabricate, and are
specific to a particular asphere prescription. Alignment and calibration of the null correction optics can also be quite
difficult. Thus there is a significant benefit to a flexible, accurate, "operator-friendly" alternative to the null test.
Testing aspheres without null correction (using a spherical wavefront) has been very limited. A typical interferometer
can acquire only a few micrometers of fourth-order aspheric departure before the interference fringes become too dense
to resolve. Other "non-null" issues include accounting for the part's aspheric shape and optical aberrations of the
interferometer. QED's SSI-ATM addresses these limitations, allowing a standard Subaperture Stitching Interferometer
(SSI®) to automatically measure mild aspheric surfaces. The basic principles of how subaperture stitching enhances
asphere capability are reviewed. Furthermore, SSI-A measurements from real aspheres are presented, along with null test measurements where available.
Interferometric tests of aspheres have traditionally relied on so-called "null correctors". These usually require significant time and expense to design and fabricate, and are specific to a particular asphere prescription. What's more, they are tedious to align and calibrate. Aspheres can also be tested without null correction (using a spherical wavefront), but such capability is extremely limited. A typical interferometer can acquire only a few micrometers of fourth-order aspheric departure due to high-density interference fringes. Furthermore, standard software packages do not compensate for the impact upon a non-null measurement of (i) the part's aspheric shape or (ii) the interferometer's optical aberrations. While fringe density and asphere compensation severely limit the practical utility of a non-null asphere measurement, subaperture stitching can directly address these issues. In 2004, QED Technologies introduced the Subaperture Stitching Interferometer (SSI<sup>(R)</sup>) to automatically stitch spherical surfaces (including hemispheres). The system also boosts accuracy with in-line calibration of systematic errors. We have recently added aspheric capability, extending non-null aspheric test capability by an order of magnitude or more. As demonstrated in the past on annular zones of nearly nulled data, subaperture stitching can extend the testable aspheric departure. We present a more generally applicable and robust method of stitching non-null aspheric phase measurements. By exploiting novel compensation schemes and in-line system error calibration, our subaperture stitching system can provide significantly better accuracy and increased testable aspheric departure over an unstitched non-null test. Examples of stitched non-null tests are analyzed in this paper, and cross-tested against corresponding null tests.
Interferometers are often used to measure optical surfaces and systems. The accuracy of such measurements is often limited by the ability to calibrate systematic errors such as reference wave and image distortion. Standard techniques for calibrating reference wave include the two-sphere and random-ball test. QED Technologies® (QED) recently introduced a Subaperture Stitching Interferometer (SSI®) that has the integrated ability to perform reference wave calibration. By measuring an optical surface in multiple locations, the stitching algorithm has the ability to compensate for reference wave and imaging distortion. Each of the three reference wave calibration methods has its own limitations that ultimately affect the accuracy of the measurement. The merits of each technique for reference wave calibration are reviewed and analyzed. By using the SSI-computed estimate and the random-ball test in tandem, a composite method for calibrating reference wave error is shown to combine the benefits of both individual techniques. The stitching process also calibrates for distortion, and plots are shown for different transmission optics. Measurements with and without distortion compensation are shown, and the residual difference is compared to theoretical predictions.