This paper compares two well-known types of interferometer arrays for optical aperture synthesis. An analytical model for both types describes the expected output, in terms of photon counts. The goal is to characterize the performance of both types of array for blind imaging of a wide-field or extended object that would be partially resolved by a single elementary aperture. The spectrum of the source is assumed to be constant over the source and in time, but broad-banded. The light levels are such that only a few photons per pixel or bin are received. The simulated interferometer responses are discussed. The process of reconstructing the source from the 'recorded' responses is presented, but not discussed in this paper. It turns out that both types of interferometer are capable of imaging a partially resolved source with high spatial frequencies present all over the source.
A special case of optical aperture synthesis, homothetic mapping, is the topic of this paper. It allows for a wide field of view for interferometric instruments, interesting for astrometric measurments of wide objects. This paper describes a testbed constructed and tested in TNO-TPD in Delft (the Netherlands). This testbed is intended as a tool to investigate the ins and outs of homothetic mapping. The homothetic mapping approach is explained, the whole setup is specified and results are shown.
We present results of experiments obtained using a new nulling
technique that enables deep nulling without the use of achromatic
phase shifters. The experimental set-up consists of a three-beam
interferometer that should provide a nulling depth of several
thousands over a wavelength range of 500 to 650 nm. The intended
depth of null was not achieved and further experiments on
determining the spectrum of each beam revealed why. We describe a
method of obtaining accurate beam spectra in a multi-beam
interferometer. The insights on the need of spectral shape
control were tested with our nulling theory and proved the
sensitivity of this nulling approach with respect to spectral
The Delft Testbed Interferometer (DTI) will be presented. The
basics of homothetic mapping will be explained together with the
method of fulfilling the requirements as chosen in the DTI setup.
The optical layout incorporates a novel tracking concept enabling
the use of homothetic mapping in real telescope systems for
observations on the sky. The requirements for homothetic mapping
and the choices made in the DTI setup will be discussed. Finally
the first results and the planned experiments will be presented.
Homothetic mapping is an aperture synthesis technique that allows interferometric imaging over a wide field-of-view. A laboratory experiment was set up to demonstrate the feasibility of this technique. Here, we present the first static experiments on homothetic mapping that have been done on the Delft Testbed for Interferometry (DTI). Before a changeable telescope configuration is provided, we first took a fixed telescope configuration and tested the algorithms for their ability to provide an exit pupil configuration before beam combination, that was an exact copy of this telescope configuration. By doing so, we created a homothetic imaging system. This is an imaging system that acts as a masked aperture monolithic telescope, but consists of (in our case) three telescopes of which the light follow their own optical trains.
Homothetic mapping is a technique that combines the images from several telescopes so that it looks like as though they came form a single large telescope. This technique enables a much wider interferometric field of image than current techniques can provide. To investigate the feasibility, a research testbed is build know as Delft Testbed interferometer (DTI). DTI simulates a configuration of three telescopes collecting light of a set of 3 stars. The stars are simulated by coupling light of a Xenon light source into three fibres, which illuminate a parabolic mirror. The light that is used has wavelengths of 500 nm - 800 nm. The light of the three telescopes will be combined in such a way that the beam arrangement in the pupil plane corresponds with the telescope arrangement and the Optical Path Difference (OPD) is minimized for the three beams.
To achieve white light fringes with high visibility, the mechanical testbed that is 2 m x 1 m x 0.5 m in size, requires stable mounting of components. This paper describes the mounting of the diamond turned off-axis parabolic mirrors of 200 mm in diameter and 240 mm flat mirrors; furthermore, it describes components like the telescopes and the active controllable components for repositioning of the beam arrangement.
Mechanisms were developed for alignment of piezo actuators and for delay lines. The delay lines can also be used to compensate pupil rotation.
Test results demonstrate that the test setup is highly stable for temperature as well as for airflow, although the system is placed in a non-thermally controlled lab. This allows measurements of nm, in presence of μm disturbances.