Automated Very Low Magnification Imaging for TEM

Abstract

A typical TEM specimen grid provides approximately a 2×2 mm area that is available for imaging. in order to identify and locate suitable targets on the grid the microscopist must usually inspect the grids at magnifications that allow for only a small area of the grid in the field of view at a time. Systematically searching the grid and mentally keeping track of relative locations presents a challenge for a microscopist especially as the image normally rotates as the magnification is changed. We present an automated technique that creates a very low magnification (VLM) image of the entire available imaging area on the grid. The VLM image can then be used as a reference map for searching the grid at high magnification.

VLM images were obtained at a nominal magnification of 57x using a Philips CM200 TEM equipped with a Gatan CCD camera. The VLM image of the entire 2 mm specimen grid can be created using a mosaic of 49 images, each 512×512 pixels in size. The sampling distance between the images in the 7×7 array is 300μm and the pixel size is 680nm. The images are then automatically tiled and re-sampled to form a final VLM matrix of 4K×4K with a pixel size of 730nm. The resampling factors for the tiling operation include the scale and the relative angle between the camera and goniometer axes. These factors are determined automatically from a calibration process that characterizes the goniometer.