Snowflakes are a natural subject for microscopy, but the conditions under which they form, and the minutes-long working time imposed by sublimation, are substantial technical barriers to field microscopy of snow crystals. Building on the work of earlier pioneers, we developed a portable, actively cooled, semi-automated microscope system capable of making z-stacked images of individual snowflakes and small groups of crystal specimens at ultrahigh resolution. Field trials of the system in Alaska and Canada produced encouraging results and also suggested directions for further improvement.

Ultrahigh-resolution imaging may be achieved using modifications of the off-axis holography scheme in a scanning transmission electron microscopy (STEM) instrument equipped with one or more electrostatic biprisms in the illuminating system. The resolution is governed by the diameter of a reference beam, reduced by channeling through a line of atoms in an atomic-focuser crystal. Alternatively, the off-axis holography may be combined with the Rodenburg method in which a four-dimensional data set is obtained by recording a nanodiffraction pattern from each point of the specimen as the incident beams are scanned. An ultrahigh-resolution image is derived by computer processing to give a particular two-dimensional section of this data set. The large amount of data recording and data processing involved with this method may be avoided if the two-dimensional section is derived by recording the hologram while the four beams produced by two perpendicular biprisms are scanned in opposing directions across the specimen by varying the voltages on the biprisms. An equivalent scheme for conventional TEM is also possible. In each case, the complex transmission function of the specimen may be derived and resolutions of about 0.05 nm may be expected.