Introduction to Optical/IR Interferometry: history and basic principles

Abstract: The present notes refer to a lecture delivered on 27 September 2017 in Roscoff during the 2017 Evry Schatzman School. It concerns a general introduction to optical/IR interferometry, including a brief history, a presentation of the basic principles, some important theorems and relevant applications.The layout of these lecture notes is as follows. After a short introduction, we proceed with some reminders concerning the representation of a field of electromagnetic radiation. We then present a short history of interferometry, from the first experiment of Fizeau and Stefan to modern optical interferometers. We then discuss the notions of light coherence, including the van Cittert - Zernicke theorem and describe the principle of interferometry using two telescopes. We present some examples of modern interferometers and typical results obtained with these. Finally, we address three important theorems: the fundamental theorem, the convolution theorem and the Wiener-Khinchin theorem which enable to get a better insight into the field of optical/IR interferometry.

• The paper details the evolution of optical/IR interferometry from early experimental methods to sophisticated high-resolution instruments.
• It introduces core theoretical frameworks such as the Huygens-Fresnel principle and key theorems that underpin interferometric analysis.
• The study examines practical implementations like VLTI and CHARA, showcasing strategies to overcome observational challenges.

Overview of Optical/IR Interferometry: History and Basic Principles

The paper "Introduction to Optical/IR Interferometry: History and Basic Principles," authored by Jean Surdej, provides a comprehensive look into the development and core concepts of optical and infrared interferometry. This exploration traces the evolution from early experiments to the intricacies of modern interferometers, encapsulating both theoretical frameworks and practical applications.

Historical Context and Evolution

The journey of optical/IR interferometry roots back to the pivotal realizations by Fizeau and Stephan in the mid-19th century, highlighting that angular resolution could be improved using multiple apertures instead of a single large one. Michelson and Pease's work in the early 20th century marked the first successful measurement of a stellar diameter through interferometric techniques, laying the groundwork for advancements in the field. This paper traces key milestones, including the transition from direct observation methods, like Galileo's early attempts, to sophisticated techniques involving complex arrangements like the Michelson Stellar Interferometer.

Theoretical Foundations

The paper elaborates on key theoretical principles underlying optical/IR interferometry. These include:

1. Complex Representation of Electromagnetic Waves: Utilizing complex notation simplifies understanding wave properties and behaviors, crucial for analyzing interferometric setups.
2. Huygens-Fresnel Principle: Serving as the conceptual framework for diffraction and interference, this principle forms the basis for understanding light interaction in interferometers.
3. Key Theorems: The document discusses several theorems pertinent to interferometry:
   • Convolution Theorem: Applied to comprehend the effects of finite aperture sizes on observed images.
   • Wiener-Khinchin Theorem: Used to derive the relationship between a system's point spread function and its frequency content.
   • Zernicke-Van Cittert Theorem: Connects the visibility of interference fringes directly with the Fourier transform of the source's brightness distribution.

Practical Implementations

The paper discusses several implementations of optical interferometry, detailing both historical and contemporary instruments. Examples include:

• I2T/NPOI, GI2T, VLTI, and CHARA: These instruments showcase varying configurations used to overcome challenges such as atmospheric disturbances and technological limits. They illustrate the evolution toward high precision and broader applicability of optical and IR interferometry.
• Beam Recombination Techniques: These include Fizeau-style homothetic recombination and Michelson's co-axial beam recombination, each with distinct operational advantages and limitations.

Implications and Future Directions

The paper posits that continuing advancements in interferometric technologies will likely expand observational capabilities, enhancing angular resolution and sensitivity significantly. This progression holds potential for increased discoveries not only in the characteristics of distant stars and exoplanets but also in studying stellar surfaces and circumstellar environments. Theoretical insights, particularly in the mathematical treatment of light coherence and imaging, are expected to push the boundaries of what's observable far beyond current limitations.

Conclusion

This work serves as an essential reference for researchers endeavoring to understand optical/IR interferometry's past and theories driving its future developments. It lays a solid foundation for appreciating how historical insights and theoretical breakthroughs continue to shape the field of high-resolution astronomical observations.