We compare the quality of interferometric image reconstructions for two different sets of data: square of the
visibility plus closure phase (e.g. AMBER like case) and square of the visibility plus visibility phase (e.g.
PRIMA+AMBER or GRAVITY like cases). We used the Multi-aperture image Reconstruction Algorithm for
reconstructions of test cases under different signal-to-noise ratios and noisy data (squared visibilities and phases).
Our study takes into account noise models based on the statistics of visibility, phase and closure phase. We
incorporate the works developed by Tatulli and Chelly (2005) on the noise of the power-spectrum and closure
phase in the read-out and photon noise regimes,1 and by Colavita (1999) on the signal-to-noise ratio of the
visibility phase.2 The final images were then compared to the original one by means of positions and fluxes,
computing the astrometry and the photometry. For the astrometry, the precision was typically of tens of
microarcseconds, while for the photometry, it was typically of a few percent. Although both cases are suitable
for image restorations of real interferometric observations, the results indicate a better performance of phase
referencing (V<sup>2</sup> + visibility phase) in a low signal-to-noise ratio scenario.
Classically, optical and near-infrared interferometry have relied on closure phase techniques to produce images.
Such techniques allow us to achieve modest dynamic ranges.
In order to test the feasibility of next generation optical interferometers in the context of the VLTI-spectro-imager
(VSI), we have embarked on a study of image reconstruction and analysis. Our main aim was to test the
influence of the number of telescopes, observing nights and distribution of the visibility points on the quality of
the reconstructed images. Our results show that observations using six Auxiliary Telescopes (ATs) during one
complete night yield the best results in general and is critical in most science cases; the number of telescopes is
the determining factor in the image reconstruction outcome.
In terms of imaging capabilities, an optical, six telescope VLTI-type configuration and ~200 meter baseline
will achieve 4 mas spatial resolution, which is comparable to ALMA and almost 50 times better than JWST will
achieve at 2.2 microns. Our results show that such an instrument will be capable of imaging, with unprecedented
detail, a plethora of sources, ranging from complex stellar surfaces to microlensing events.
One of the aims of next generation optical interferometric instrumentation is to be able to make use of information
contained in the visibility phase to construct high dynamic range images.
Radio and optical interferometry are at the two extremes of phase corruption by the atmosphere. While in
radio it is possible to obtain calibrated phases for the science objects, in the optical this is currently not possible.
Instead, optical interferometry has relied on closure phase techniques to produce images. Such techniques allow
only to achieve modest dynamic ranges. However, with high contrast objects, for faint targets or when structure
detail is needed, phase referencing techniques as used in radio interferometry, should theoretically achieve higher
dynamic ranges for the same number of telescopes.
Our approach is not to provide evidence either for or against the hypothesis that phase referenced imaging
gives better dynamic range than closure phase imaging. Instead we wish to explore the potential of this technique
for future optical interferometry and also because image reconstruction in the optical using phase referencing
techniques has only been performed with limited success.
We have generated simulated, noisy, complex visibility data, analogous to the signal produced in radio interferometers,
using the VLTI as a template. We proceeded with image reconstruction using the radio image
reconstruction algorithms contained in aips imagr (clean algorithm). Our results show that image reconstruction
is successful in most of our science cases, yielding images with a 4 milliarcsecond resolution in K band.
We have also investigated the number of target candidates for optical phase referencing. Using the 2MASS
point source catalog, we show that there are several hundred objects with phase reference sources less than 30
arcseconds away, allowing to apply this technique.
The VLTI Spectro Imager (VSI) was proposed as a second-generation instrument of the Very Large Telescope Interferometer
providing the ESO community with spectrally-resolved, near-infrared images at angular resolutions
down to 1.1 milliarcsecond and spectral resolutions up to R = 12000. Targets as faint as K = 13 will be imaged
without requiring a brighter nearby reference object; fainter targets can be accessed if a suitable reference is
available. The unique combination of high-dynamic-range imaging at high angular resolution and high spectral
resolution enables a scientific program which serves a broad user community and at the same time provides the
opportunity for breakthroughs in many areas of astrophysics. The high level specifications of the instrument are
derived from a detailed science case based on the capability to obtain, for the first time, milliarcsecond-resolution
images of a wide range of targets including: probing the initial conditions for planet formation in the AU-scale
environments of young stars; imaging convective cells and other phenomena on the surfaces of stars; mapping
the chemical and physical environments of evolved stars, stellar remnants, and stellar winds; and disentangling the central regions of active galactic nuclei and supermassive black holes. VSI will provide these new capabilities
using technologies which have been extensively tested in the past and VSI requires little in terms of new
infrastructure on the VLTI. At the same time, VSI will be able to make maximum use of new infrastructure as it
becomes available; for example, by combining 4, 6 and eventually 8 telescopes, enabling rapid imaging through
the measurement of up to 28 visibilities in every wavelength channel within a few minutes. The current studies
are focused on a 4-telescope version with an upgrade to a 6-telescope one. The instrument contains its own
fringe tracker and tip-tilt control in order to reduce the constraints on the VLTI infrastructure and maximize
the scientific return.
We present the work developed within the science team of the Very Large Telescope Interferometer Spectro-Imager (VSI) during the Phase A studies. VSI aims at delivering ~ 1 milliarcsecond resolution data cubes
in the near-infrared, with several spectral resolutions up to 12 000, by combining up to 8 VLTI telescopes. In
the design of an instrument, the science case plays a central role by supporting the instrument construction
decision, defining the top-level requirements and balancing design options. The overall science philosophy of
VSI was that of a general user instrument serving a broad community. The science team addressed themes
which included several areas of astrophysics and illustrated specific modes of operation of the instrument: a)
YSO disks and winds; b) Multiplicity of young stars; c) Exoplanets; d) Debris disks; e) Stellar surface imaging;
f) The environments of evolved stars; g) AGN tori; h) AGN's Broad Line Region; i) Supermassive black-holes;
and j) Microlensing. The main conclusions can be summarized as follows: a) The accessible targets and related
science are extremely sensitive to the instrument limiting magnitude; the instrument should be optimized for
sensitivity and have its own fringe tracker. b) Most of the science cases are readily achievable with on-axis fringe
tracking, off-axis fringe tracking enabling extra science. c) In most targets (YSOs, evolved stars and AGNs), the
interpretation and analysis of circumstellar/nuclear dust morphology requires direct access to the gas via spectral
resolved studies of emission lines, requiring at least a spectral resolution of 2 500. d) To routinely deliver images
at the required sensitivity, the number of telescopes in determinant, with 6 telescopes being favored. e) The
factorial increase in the number of closure phases and visibilities, gained in a single observation, makes massive
surveys of parameters and related science for the first time possible. f) High dynamic range imaging and very
high dynamic range differential closure phase are possible allowing the study of debris disks and characterization
of pegasides. g) Spectro-imaging in the near-infrared is highly complementary to ALMA, adaptive optics and
interferometric imaging in the thermal infrared.