LOW COST CCD CAMERAS FOR PHILIPS CM10 ELECTRON MICROSCOPE

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Our grant has funds, to be matched by departmental funds, to obtain a CCD camera system for our department's Philips TEM. The main purpose of this system is to allow direct digitization of images suitable for reconstructions from serial sections. Currently this work is done by taking pictures on film and digitizing with a video camera. The characteristics of images acquired in this way provide a benchmark for the performance of a CCD camera system. We arranged for tests to be performed on three cameras: the Gatan Bioscan 792, AMT's 1K by 1K camera, and a 1. by 1. camera from Princeton Instruments. Each manufacturer provided us with digital images of the edge of a slot grid, axonemes from Chlamydomonas in cross and longitudinal section (pixel size about 1 nm), and Golgi regions from NRK cells (pixel size about 2.5 nm). The image of an edge could be analyzed to determine the Modulation Transfer Function (MTF) of the camera system, using software developed in our laboratory for measuring MTF from such images. The other images could be compared with numerous similar images photographed on film and digitized with a video camera. Both Princeton Instruments and AMT cameras are lens-coupled and had a spatial resolution only half as good as when digitizing from film, which means that the effective number of pixels is only 500-600, not 1000-1200. The Gatan camera is fiber-optic coupled and has adequate resolution, but in these initial tests the phosphor was too sensitive and the camera saturated at relatively low electron doses. Thus it was impossible to obtain an image with an acceptable signal-to-noise ratio. Both AMT and Gatan have made improvements since these tests. AMT has developed a higher-resolution lens. We have evaluated images from the CCD camera with this lens and found that the overall resolution is better by about 35%, so that the effective number of pixels is now 730. Gatan has reduced the phosphor sensitivity in the Bioscan camera and the images acquired with it now appear to have a reasonable signal-to-noise ratio.