IVC: a simulation and model-fitting toolfor optical-IR interferometry

Abstract

I present a new software tool, called “Interferometry Visibility Computations” (IVC), for sparse uv-coverage optical/IR interferometry. It is aimed to compute interferometric observables from a large grid of geometrical toy models, or user supplied multi-wavelength radiative transfer simulations. It is designed to perform multi-baseline model fitting, taking into consideration various instrumental effects, and preventing high frequency aliasing in visibility computation from imaging models. The program can be called from a user’s code to allow simultaneous model fitting of different types of observations, e.g. interferometric and spectroscopic data at the same time. Hereafter I describe the software and its usage: some parts of it are not yet fully implemented, and all the updates will be notified in the distribution web page.

Li Causi G.

INAF – Osservatorio Astronomico di Roma, Via Frascati 33, Monteporzio Catone (RM), Italia

The software:

IVC is a tool designed for interferometric calculations of sparse uv-coverage interferometers, tailored for model fitting grids of geometric or radiative transfer models. It is freely available at www.mporzio.astro.it/~licausi/IVC/ and can be run via the free IDL Virtual Machine. It can work both in a stand-alone mode, with a Graphical User Interface (GUI), and in a Function mode, not interactive and callable from another program. The GUI mode is intended for preparing interferometric observations, when the need is to browse among the expected visibilities, closure phases and differential visibilities given a model and a set of baselines; all the outputs are presented in a tabbed window whose plots are updated upon the selections of the buttons panel (Fig.1). The Function mode is useful for browsing the observation data and for model fitting, and is mainly intended to allow simultaneous fitting of different kinds of data, like e.g. interferometry and spectroscopy at the same time.

Input description:

The user will feed IVC with source models described in three different ways: i) if the source has a circular symmetry, like spherical structures or pole-on views, IVC expects a 1D radial profile for any given wavelength, Oλ(r), allowing for either uniform or non uniform sampling of the radius; ii) if there is no circular symmetry the input must be a set of 2D FITS images, containing the brightness distribution at each wavelength, Oλ(x, y); iii) otherwise, one can use the IVC internal Toy Models Tool to produce analytic or imaging models formed by simple geometric entities, like a uniform disc, a gaussian envelope, a ring, a binary star and more, or any hierarchical combination of them (Fig.2).

Instrument description:

The parameters describing the instrument and the baselines are passed to IVC by an ASCII configuration file, as well as the target position in the sky, the hour angle, the wavelength range and the spectral resolution, or are directly read from the OI-FITS header if an observation file is loaded. All the effects that cannot be compensated by the data processing are taken into consideration to make the simulations matchable to the data, like the wavelength smearing, due to the finite width of the spectral channels, the temporal smearing, due to the baseline variation over the acquisition time, and the spatial filtering, which introduces a gaussian illumination affecting the visibility of the extended sources. More effects, like atmospheric turbulence in fiber injection, and some specific instrument behaviors are under study to improve the matching between models and real data.

Models sampling:

Sampling is critical for V2 computation: the true object Oλ(x, y) could have arbitrary high spatial frequencies, depending on its smallest feature scale, or edges, and any pixellization to a smaller maximum frequency causes an alteration of the whole visibility curve due to the aliasing, so that a sampling at the “interferometer resolution” of λ/Bmax does not guarantee a correct V2. Imaging models must be oversampled until having negligible power above the Bmax/λ frequency and analytical descriptions must be used when this is not possible, e.g. for a binary of two point sources. IVC takes that under control and allows to check the sampling error with the Toy Models Tool.

Observation fitting:

Fig.3 shows the IVC model fitting work-flow in its three fitting schemes: i) the GUI-mode loads a set of external models and find which of them best fits the data; ii) in Function-mode an external radiative code provides the parametric models, within its own optimization scheme, and calls IVC to get the 2 of the interferometric fit; this mode also allows simultaneous fitting with other kinds of data (like e.g. spectroscopic observations) so to obtain the best fit with respect to all the observation sets; iii) the internal Toy Models Tool can be used to generate a set of geometrical models, that IVC can use to fit the observations with a global optimization of the 2.

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Fig.1: The GUI with the tabbed graphical windows and the control buttons panel.

Fig.3: The work-flow of the model fitting in the three cases of GUI mode (left), Function mode (center) and Toy Models mode (right).

Fig.2: An example of an imaging model generated by the Toy Models Tool.