A variety of devices have been proposed whose performance might be superior to that achieved when using highly underfocused images to generate contrast for cryo-EM specimens. Practical applications nevertheless continue to be limited because it has been difficult to prevent corruption of the images due to unwanted electrostatic charging that occurs during use. We have implemented two steps that - together - appear to overcome charging of one such device. This particular device consists of a microfabricated aperture that contains an opaque half circle at the center. The opaque half circle produces high-contrast “schlieren-optics” images of the low-resolution features encoded in the phase of the wave transmitted through the specimen. At the same time, however, the rest of the scattered wave passes through the open area of the aperture, resulting in conventional images at intermediate and high resolution. This hybrid double-sideband/single-sideband (Foucault knife edge) aperture is thus well suited for recording images at the optimal (Scherzer) value of defocus. Our first step in controlling unwanted charging is to coat all exposed surfaces with evaporated carbon, which ensures that the isopotential surfaces conform, as intended, to the physical surface of the aperture. The second step is to hold the device at a temperature of 300° C or more during use, in order to prevent the buildup of polymerized organic contamination during use. Although care must be taken to avoid hitting the opaque half circle directly with the unscattered electron beam, our preliminary experience is that this type of in-focus phase-contrast aperture can be used for at least a few days without showing apparent charging effects. Our proof-of-concept experiments confirm that this type of device is effective in producing high-contrast, in-focus images of cryo-EM specimens.