CONDITIONS FOR ELECTRON TOMOGRAPHIC DATA-ACQUISITION

The conventional algorithms employed in electron microscope tomography require that the series of images obtained from different orientation s of the specimen each represent a parallel orthographic projection un der uniform magnification. Electron microscope optics can produce dist ortions in images that may affect the accuracy of a tomographic recons truction, These distortions result in images with differential rotatio n and magnification of regions of a thick or highly tilted specimen lo cated at different distances with respect to the plane of focus, The d istortions increase in magnitude and may significantly affect the accu racy of the tomographic data when images are acquired under current ce nter misalignment or non-eucentric positioning of the specimen, An add itional source of error can be introduced if the condenser lenses are adjusted to compensate for the intensity attenuation due to the increa sed beam path length as the specimen is tilted, The change in beam cro ssover position due to adjustment of the condensers alters the beam di vergence and may introduce systematic changes in magnification over th e tilt series, The process of alignment of the tilt series will propag ate these errors to more central regions of the image, In most cases, with proper consideration, it is possible to minimize these errors to levels where they will have negligible effects on the resolution of th e tomographic reconstruction.