Discussion

We have shown that the goniometer on the Philips CM200 can be characterized and modeled to significantly improve the accuracy for absolute positioning. At this stage, the model incorporates corrections for the slew rate for the x and y axis. Using the model a target may be located and repositioned to within approximately 100 nm when moving over a distance of approximately 10,000nm. This is an approximately tenfold improvement over the accuracy that is obtained without modeling. This accuracy is sufficient to achieve our objectives for the task of automated data collection from a vitreous ice specimen. Further improvements in the accuracy could be achieved by incorporating a model of the X and Y axis headings if the dependence of the Y axis heading on X axis position was modeled.

We have also carried out a partial characterization of a second CM200 TEM compustage to make certain that the observed behavior was not a peculiarity of our particular microscope. We thus used the instrument located in the laboratory of Dr. Ron Milligan at Scripps and determined that this second goniometer exhibited similar periodic behavior which can again be modeled and corrected with a Fourier series.

The measurements and analysis used to characterize the goniometer are automated and can be completed within a few hours with minimal operator intervention. The performance of the goniometer can be periodically checked over time. Over the four months that we have been making these measurements, the slew rate characteristics have not varied. The techniques that we have developed for characterizing the goniometer could also be used as a service tool to detect improperly functioning systems. When we began these experiments, the measurements of the X axis slew rate were extremely noisy although still characterized by a periodic function. Following a service call by the manufacturer, the noise was reduced and the performance of the system was significantly improved. These characterization techniques could also be used for acceptance tests and quality control.

We would expect that the Z and a and b axis of the five axis compustage goniometer could be similarly characterized and modeled. Ideally, the new generation of TEM’s could incorporate these models directly into the hardware to provide a goniometer of superior performance.

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