13.1 Introduction

Astronomers recognized in the mid-1960s that an expensive single large-aperture optical telescope could be divided into a series of less-expensive smaller apertures that were coherently interconnected to provide nearly the same performance as that given by the large single aperture. The penalties were an increase in the integration time and more-extensive ground data processing required to reconstruct an image. Also, extensive real-time control software and hardware were needed to maintain the alignment of the telescope subsystems.

The Michelson stellar interferometer (MSI) is an example of a two-aperture sparse-aperture telescope. The MSI samples the MCF at only one spatial frequency (determined by the separation of the telescopes) and at one azimuth of the two-dimensional MCF. For some astronomical applications, such as astrometry or stellar atmospheres studies, the MSI is a very productive system. The MSI and its astronomical applications are discussed in detail in Chapter 14.

Radio astronomers and electrical engineers developed innovative image-reconstruction techniques using sparse apertures with multiple radio telescopes. These innovators then digitally processed the data to create or synthesize an image. In the radio region of the spectrum, it is well known that sparse apertures are a cost-effective means of obtaining data to reconstruct very high-spatial-resolution images. At radio frequencies, the noise temperature of the receiver, sometimes called the antenna temperature, dominates the SNR. In the UV, optical, and near-IR region of the spectrum, the SNR is dominated by the signal-photon arrival rates, which are established by the nature (color and brightness) of the source and not the receiver(s). In the radio region of the spectrum, both the amplitude and the phase of the received signal are recorded. At optical frequencies, in white light (where a heterodyne process cannot be used), the phase cannot be directly recorded.

In optical/IR astronomy, a telescope fulfills two functions. The large continuous aperture collects energy to observe faint sources at the angular resolution that is characteristic of its extent. The outside area of the aperture collects the radiation needed to give high angular resolution.