Early this year the Section on Structural Biology acquired a Zeiss EM902, the first commercially available microscope to incorporate an imaging energy-loss spectrometer. The main thrust of the project--and the reason for procuring one of these instruments--has been to exploit its potentialities for imaging relatively thick specimens in vitreous ice. Preservation of the native structures of biological macromolecules by rapid freezing, and maintaining them in an aqueous environment (ice) during imaging has potentially been one of the most important breakthroughs of the last twenty years, and the Zeiss EM902 has considerable potential to advance this area of research. A Dage-MTI SIT-66 camera is used for image intensification because the microscope is often operated under minimal dose conditions. The video output is fed to a Data Translation DT 2651 frame grabber and DT 2658 processor board inside a MicroVAX II. A FORTRAN acquisition program using DT-IRIS subroutines has been developed that allows up to 128 512 x 512 pixel frames to be averaged and acquired in 1/4 real time. The averaged image is stored on disk in the same format used by the CSL image processing facility. Extensive computer image processing can be performed with the highly developed CSL software package (PIC). One present need is to add a shading correction routine to the acquisition program. This work should be completed in the near future.
