Radio and Optical Interferometry: Basic Observing Techniques and Data Analysis

Abstract

Astronomers usually need the highest angular resolution possible when observing celestial objects, but the blurring effect of diffraction imposes a fundamental limit on the image quality from any single telescope. Interferometry allows light collected at widely-separated telescopes to be combined in order to synthesize an aperture much larger than an individual telescope thereby improving angular resolution by orders of magnitude. Because diffraction has the largest effect for long wavelengths, radio and millimeter wave astronomers depend on interferometry to achieve image quality on par with conventional large-aperture visible and infrared telescopes. Interferometers at visible and infrared wavelengths extend angular resolution below the milli-arcsecond level to open up unique research areas in imaging stellar surfaces and circumstellar environments.In this chapter the basic principles of interferometry are reviewed with an emphasis on the common features for radio and optical observing. While many techniques are common to interferometers of all wavelengths, crucial differences are identified that will help new practitioners to avoid unnecessary confusion and common pitfalls. The concepts essential for writing observing proposals and for planning observations are described, depending on the science wavelength, the angular resolution, and the field of view required. Atmospheric and ionospheric turbulence degrades the longest-baseline observations by significantly reducing the stability of interference fringes. Such instabilities represent a persistent challenge, and the basic techniques of phase-referencing and phase closure have been developed to deal with them. Synthesis imaging with large observing datasets has become a routine and straightforward process at radio observatories, but remains challenging for optical facilities. In this context the commonly-used image reconstruction algorithms CLEAN and MEM are presented. Lastly, a concise overview of current facilities is included as an appendix.

Long-baseline optical intensity interferometry

“…The data streams from multiple telescopes can then be cross-correlated in software, possibly applying noise filters and also computing other spatio-temporal parameters such as higher-order correlations between three or more telescopes, which could contain additional information. Such a capability was built into the AquEYE and IquEYE instruments developed at the University of Padova for very high time-resolution astrophysics, also with a view toward intensity interferometry, using similar SPAD detectors to those here (Capraro et al, 2010;Naletto et al, 2009Naletto et al, , 2013.…”

“…Limiting parameters include the requirement of optical and atmospheric stability to a fraction of an optical wavelength, and also the need to cover many interferometric baselines, given that optical light cannot be copied with retained phase, but has to be split up (and diluted) by beamsplitters to achieve interference among multiple telescope pairs. Overviews of those techniques are given by Millour (2008) and Quirrenbach (2009) and in more detail by Monnier (2003) and Monnier & Allen (2013), while recent results are highlighted in Berger et al (2012) and van Belle (2012).…”

ACTIVE GALACTIC NUCLEUS AND QUASAR SCIENCE WITH APERTURE MASKING INTERFEROMETRY ON THEJAMES WEBB SPACE TELESCOPE

“…Because of read noise, this limits ground-based phase measurements to the very brightest objects. As Monnier & Allen (2013) state: "Without valid phase information accompanying the visibility amplitude measurements, one cannot carry out the inverse Fourier Transform that lies at the core of synthesis imaging and the clean algorithm specifically. "…”

“…Mosaic fields are limited to the size of the isoplanatic patch using ground-based optical interferometers (due to atmospheric instability and the guide star field-of-view limitations of AO systems; Fried 1966). At L band, this is slightly larger than half an arcminute (Monnier & Allen 2013). Very wide-field mosaicking is more likely to be used for non-AGN science (though mapping of large scale jet interactions might call for such a technique), but we note the issue here for its importance to other subfields (e.g., interstellar medium, ISM).…”

“…More complete descriptions of the technique can be found in Monnier (2003), Monnier &Allen (2013), andIreland (2013). We assume the Fraunhofer approximation of Fourier optics, viz., the image plane complex amplitude a(k) is the Fourier transform of the pupil plane complex amplitude A(x).…”