Aperture synthesis imaging in the radio was first realized by Ryle and Vonberg [1] in the 1940s, while optical/infrared interferometry has been feasible for just over 20 years (Baldwin et al. [2]), and yet it has only been in the past several years that serious efforts toward new beam combination schemes have been able to spur future generations of imaging instruments and techniques, and their associated data analysis and modeling software. Very high resolution radio (e.g., VLBI) and optical/infrared ground-based interferometry (e.g., VLTI, CHARA) will remain the only feasible methods to routinely attain milliarcsecond resolutions for the next few decades at least. This fact will not change with the launch of any currently planned space facilities, or with the deployment of extremely large monolithic telescopes, even when their associated multi-conjugate adaptive optics systems are implemented.

The scientific demand for this unparalleled angular resolution can be understood from a few examples: a one milliarcsecond resolution allows scientists to assess fundamental physics on spatial scales equivalent to 1 astronomical unit (AU) at one kiloparsec, or produce images with 18 cm resolution on objects in geosynchronous orbit. In astrophysics at optical wavelengths, these types of spatial scales finally allow us to answer questions related to details of stellar systems such as mass loss or mass transfer for binary systems, as well as interrogating the detailed physics of collapsing disks around protostars, or the star spots on giant stars a kiloparsec away. For the aerospace community, these types of resolutions would allow a satellite owner to determine if a solar panel or boom was misoriented, or if an uncommunicative satellite was possibly tumbling out of control. The promise of these results has sparked a new wave of research into pushing the boundaries of what is possible today with aperture synthesis techniques.

The first paper in this issue is a tutorial to orient readers new to the techniques on the principles of interferometric image reconstruction (Thiébaut and Young [3]). Along with a general introduction to the methodology, the paper walks readers through the reconstruction of a simple model with sparse Fourier data. Several existing reconstruction codes used in the optical/infrared astronomical community are discussed in terms of their optimization algorithms, their ability to treat multispectral data, and their regularization methods. It is shown that it is critical to understand regularizations when trying to interpret the goodness of any image produced in this way, and thus a method is presented to help place any data analysis into context.

Five articles presented in this issue cover a breadth of relevant topics in the field of aperture synthesis. Stafford et al. [4] experimentally demonstrate how an aperture synthesis technique can be used to improve ladar resolution in the azimuthal cross-range direction. This allows for resolution of objects in three dimensions, even when initially unresolved in two directions. Krug and Rabb [5] present a digital piston correction method for treating partially coherent data that occurs when subapertures of an aperture synthesis system have path differences which reduce system resolution. When applied correctly using the anamorphic pupil relay, the method enables lossless correction of the phase errors. A design for electronically tuned dynamic metasurface antennas which allows for the generation of custom electromagnetic waveforms which can be used in synthetic aperture radars is presented by Boyarsky et al. [6]. This type of system has the advantage of fewer articulated moving parts, producing cheaper and easily light-weighted systems. Several examples of other potential improvements for these types of synthetic aperture radars are also shown, leading to a multitude of new applications. In the fourth featured paper, Böhm et al. [7] present a method for reducing the effects of vibrations of the mechanical structure of a telescope on interferometric observations. They use a software feed-forward estimator to provide estimates of mirror motion based on accelerometer data. This allows for improvement of the sky coverage, performance, and robustness of the Large Binocular Telescope (LBT) Interferometer and its associated facility subsystems. The final paper discusses the development by the team of Mourard et al. [8] for a new beam-combining instrument, SPICA, for the Center for High Angular Resolution Astronomy (CHARA) array. This single-mode, fiber-fed, visible wavelength instrument will combine all six telescopes of the CHARA array at either low or moderate resolutions with higher precisions than previously available in order to measure diameters of fainter targets, or make images of brighter ones.