High angular resolution imaging with stellar intensity interferometry using air Cherenkov telescope arrays

Abstract

Optical stellar intensity interferometry with air Cherenkov telescope arrays, composed of nearly 100 telescopes, will provide means to measure fundamental stellar parameters and also open the possibility of model-independent imaging. In addition to sensitivity issues, a main limitation of image recovery in intensity interferometry is the loss of phase of the complex degree of coherence during the measurement process. Nevertheless, several modelindependent phase reconstruction techniques have been developed. Here we implement a Cauchy-Riemann-based algorithm to recover images from simulated data. For bright stars (m v ∼ 6) and exposure times of a few hours, we find that scale features such as diameters, oblateness and overall shapes are reconstructed with uncertainties of a few per cent. More complex images are also well reconstructed with high degrees of correlation with the pristine image. Results are further improved by using a forward algorithm.

Capabilities of future intensity interferometers for observing fast-rotating stars: imaging with two- and three-telescope correlations

“…A more detailed discussion of the sensitivity of CTA for SII is given by Nuñez et al (2012a), and according to their simulations, tens of hours of integration time are required for observing bright (mv ∼ 6) stars at blue wavelengths.…”

“…Since the phase of the complex visibility is not measured with two-point intensity correlations, image reconstruction from two-point correlations is non-trivial and has benefited from the use of several phase retrieval methods and image reconstruction techniques (Holmes & Belen'kii 2004;Nuñez et al 2012a;Strekalov, Kulikov & Yu 2014).…”

“…We first follow the strategy described in detail by (Nuñez et al 2012b): The first step consist in estimating the phase from Fourier magnitude data alone, using the "Cauchy-Riemann" phase reconstruction method, based on the theory of analytic functions (Holmes & Belen'kii 2004;Nuñez et al 2012a). The main idea behind the Cauchy-Riemann method is that the Fourier Transform of an object of finite extent is an analytic function.…”

“…In order to simulate SII data, we follow the procedure described in detail by Nuñez et al (2012a). First we set the spatial scale of the image: we concentrate on the case of a bright star of mv = 3, which can have an angular diameter of ∼ 0.5 mas as long as the temperature of the star is more than 1 ∼ 10, 000 K. Larger angular diameters than our choice of 0.5 mas are acceptable as long as they are not fully resolved by baselines smaller than ∼ 100 m. If needed, the pupil of the largest telescopes (∼ 24 m, presumably located towards the center of the array) could be segmented into subpupils to allow observations with angular diameters much larger than 0.5 mas.…”

“…Over half a century after the success of the Narrabri Stellar Intensity Interferometer (Hanbury Brown 1974), a future facility: the Cherenkov Telescope Array (CTA), has been proposed to be used in part as an Intensity Interferometer (Le Bohec & Holder 2006;Dravins et al 2013;Nuñez et al 2012a). When used as an optical Intensity Interferometer, CTA will provide thousands of simultaneous baselines as large as a kilometer, corresponding to sub-milliarcsecond angular resolution at visible wavelengths, and thus enable to resolve the surfaces of many nearby stars.…”