Tuning the S.E.M. for Quality Photography
I could see that this new ETEC scanning electron microscope had great potential imaging capabilities when I first became acquainted with it in 1973. It had a brand new high resolution imaging CRT (cathode ray tube) in the copy camera system, that was capable of resolving 2500 lines ore more, according to the specs.
These new SEMs were fitted with a 4”x5” camera back but the only film used was Polaroid, via an insert holder for the 4x5 film back. The microscopes came from the manufacturer very poorly set up and you could only get a barely decent polaroid from it--not really very sharp. What was the problem? I had to find out.
I undertook to tune the entire imaging system of the microscope starting with the electronics. I improvised a hand held optical microscope to visualize the spot on the imaging CRT so that I could electronically tune it to focus at a very sharp point both in the center and at the edges. Once that was accomplished, I had to use a micrometer to focus the lens at a wide-open aperture (ƒ1.4)--in increments of .01”. I took a series of images using Kodak Plus-X 4x5 sheet film, and after developing the film and looking at the negatives under a microscope, determined the best critical focus distance for the camera. The lens was then stopped down to ƒ11 for subsequent photography.