Mission Critical Space Optics                Large Optical Measurements with a Remote Fizeau Cavity

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In the past atmospheric thermal effects have been a major hindrance to measuring good optical wavefront for larger optics. This is because changes in air temperature and pressure in the interferometer’s beam path cause distortions in the fringe pattern. If these distortions are random over time they can be averaged out, if the acquisition time to capture a single wavefront is fast enough. With simultaneous phase-shifting this is now possible. Figure 3A shows the Intellium™ H2000 measuring a 1.5 meter mirror six meters away. The thermal currents can easily be seen by subtracting two consecutive wavefront measurements. If these currents vary slowly then it becomes difficult to average them out as seen in Figure 3B (lower left). A fan can be used to mix up the air in order to randomize these currents, allowing them to be averaged out as seen in Figure 3B (lower right). The mirror’s final wavefront can then be measured accurately as shown in Figure 3B (upper right). These types of measurements have so difficult to make in the past, some customers have responded with their new found capabilities as follows: “We are using the Intellium™ H2000 every day and are ecstatic. We would like to acquire at least one more.” Optical Surface Technologies LLC.

                        

Figure 3A: The H2000 interferometer (left) measuring a 1.5 meter mirror (in the far back to the left ). A fan (in the far back to the right) is used to mix up the air, removing static thermal currents. The monitor on the right shows the interferogram. Courtesy of EOST, Tucson AZ.		Figure 3B: Wavefront data (upper right) of a 1.5 meter mirror acquired in heavy vibrations and a turbulent atmospheric environment. One of three interferograms is shown in the (upper left). The static thermal currents seen in the lower left are randomized by a fan resulting in random thermal pockets (lower right).

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Mission Critical Space Optics

Many times scientists do not have the luxury of measuring mission critical space optics in well behaved laboratory setups. The H1000 is being used to measure the beryllium mirrors of the James Web Space Telescope (JWST) at Ball Aerospace Corp. For measurement, the mirrors are mounted in configurations similar to how they will be mounted in the actual telescope. This allows the effects of stress and other factors to be incorporated into the wavefront measurement. In the past, these types of measurements were impossible due to instability and vibrations. With the ability to acquire wavefront data in a matter of milliseconds in these simulated environments, the performance of these telescopes have a much better chance performing as expected in space. Because of this Ball Aerospace as quoted “ We currently have an Intellium™ H1000 and a MiniScatR™. Over the next 2 years we may buy several more systems.”

	Figure 4: James Webb Space Telescope Primary Mirror (1/6 actual size). Courtesy Ball Aerospace & Technology Corp.

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Large Optical Measurements with a Remote Fizeau Cavity

Large optics are often difficult to measure due to the long optical paths lengths. Long optical path lengths allow many potential errors to be introduced into the measurements. The errors can be due to vibration, atmospheric turbulence, and mechanical drift of heavy parts. Although the Intellium™ H2000 can handle these errors as mentioned previously, there is an additional elegant solution. A remote Fizeau cavity can be used, where the transmission optic (TR) and test optic are closely spaced from each other a remote distance away from the interferometer. Since the test and reference beams are orthogonally polarized, all phase-shifting is performed inside the interferometer even though the TR is positioned far away. A real-world example is shown in Figures 5A and 5B where both the TR and test flat are approximately 8.0 meters away from the interferometer. The 1.6 “test” meter mirror (lower mirror in Figure 5A) is being measured with a smaller 1.0 meter transmission flat (the remote fizeau cavity). Since the transmission flat is smaller than the test flat, the test flat must be measured many times while being rotated. The overlapped measurements are then stitched together using ESDI’s stitching software. This requires data to be acquired extremely fast to minimize errors as mentioned previously.

          

	Figure 5A: Test Setup for measuring multiple apertures of a larger optic. On the left, the diagram shows the beam propagating from the interferometer (far left) to an off axis parabola (top left), then down to the transmission flat and test flat. In the right image, the top optic is the calibrated 1.0 meter transmission flat and bottom optic is the 1.6 meter test flat. The remote Fizeau cavity is the gap between these two flats. Courtesy of College of Optical Sciences ( University of Arizona).

           

	Figure 5B: Surface map of test optic (Figure 1) created by stitching 12 sub apertures.(left) and corresponding average radial profile (right). Surface irregularity from stitching – power and astigmatism removed. Final Result: 7.3 nm rms, 116 nm PV.

Conclusion

The H2000 offers true vibration insensitivity, high interferogram resolution, and the advantages of a true common path Fizeau interferometer. Since the H2000 uses common Fizeau type accessories, it can easily be integrated into existing research and production systems.