10.1 Introduction

An interferometer is an instrument that processes the electromagnetic wavefront prior to detection to reveal physical properties of the radiation. This predetection processing enables spectrometry and reveals information on the spatial structure of the source as well as the quality of the wavefront in the optical system. Interferometers have five important applications to the astronomical sciences:

1. spectral analysis (spectrometry),

2. fabrication, test, and measurement of optical system performance,

3. optical metrology and wavefront sensing,

4. astrometry, and

5. analysis of the spatial frequency structure in an image.

An interferometer coupled with a detector system measures the statistical correlation or coherence between wavefields. In the optical region of the spectrum, most astronomical sources emit incoherent radiation. Astronomical sources that emit single emission lines are incoherent because there is very little or no temporal coherence. These sources have neither spatial nor temporal coherence and are therefore incoherent. Reviews of some these topics appear in the literature. Here, we examine the broader topic of interferometry and how it relates to the design of telescopes and instruments and to their applications.

10.2 Historical Perspective

10.2.1 Young's double-slit experiment

Understanding the physics foundation of Young's double-slit experiment is critical to an understanding of interference and coherence. Thomas Young was one of the last great polymaths of the 19th century. He made pioneering contributions to physics (he demonstrated the wave theory of light), engineering (he derived the modulus of elasticity), Egyptology (he was fluent in Egyptian hieroglyphs and translated the Rosetta Stone), and physiology (he derived the mechanism of vision). Young supported himself as a practicing physician and not on grants or patronage. The apparatus Thomas Young used for demonstrating the wave nature of light is shown in Fig. 10.1.