A focusing aid for the X-ray projection microscope

Summary Part of the electrons reflected by the target of an X-ray microscope pass the demagnifying lens in opposite direction and form a magnified image of the focus in the electron source. This secondary image can be caught on a fluorescent screen and observed. The position, shape and size of the image give information on alignment, focusing conditions and image errors. The brightness is a factor 10³ to 10⁴ higher than that of the X-ray fluorescent image under normal focusing conditions. If a magnetic objective is used, centering can be done very accurately (angle of tilt < 1/600 radians). In practice this method proves to be very satisfactory, even for voltages as low as 6 kV and less. At these voltages the depth of penetration of the electrons in gold is less than 500 Å, so that for a 0.1 resolution there is no need for a thin target.