Current observations in the context of exoplanet searches with coronagraphic instruments have
shown that one of the main limitations to high-contrast imaging is due to residual quasi-static
speckles. Speckles look like the image of a planet, but they have a different spectral behavior
and are optically coherent with the star. We present two techniques to distinguish a planet from
speckles. We are assuming that the optical path can be changed enough so that the speckles
will change significantly between each image and therefore our model of each image having an
independent source of aberrations (creating a new speckle pattern) from the other images is a
good model. In the future, we would like to design and build a testbed suitable for coherent
speckle detection studies. There are two techniques we want to apply to create the necessary
multiple images with changing speckle patterns. The first is to use images generated using our
existing deformable mirror (DM) control algorithm and the second is to put deliberate shapes
on the DM to achieve the desired speckle pattern outcome.
The achievable contrast level for space-based detection of exo-planets will be limited by the stability of the optics. As a consequence, active amplitude and phase compensation will be needed. In order to mitigate these wavefront instabilities, we suggested, as an alternative to classical adaptive optics, the use of a Michelson interferometer equipped with two deformable mirrors. Simulations showed that this set up is able to create a symmetric "dark hole" in an appropriate area of the image plane. However, increasing the bandwidth of the incident light critically alters this nulling performance. A quantitative analysis of this effect will first be presented. An alternative to circumvent this problem is to introduce a dispersive element in one of the legs of the interferometer so that the path length difference does not exhibit the one over wavelength dependence. In the case of the insertion of a gaseous cell, the OPD could then be controlled by pressure variations. The last section of this paper will present a simulation-oriented proof of concept relying on the dispersive properties of nitrogen.
Current plans call for the first Terrestrial Planet Finder mission, TPF-C, to be a monolithic space telescope with a coronagraph for achieving high contrast. Our group at Princeton pioneered the concept of shaped pupils for high contrast imaging and planet detection. In previous papers we introduced a number of families of optimal shaped pupils in square, circular, and elliptical apertures. Here, we show our most promising designs and present our laboratory results for the elliptical shaped pupil. We are currently achieving better than 10−7 contrast at 10 λ/D and 10−5 contrast at 4 λ/D, without wavefront control. We describe the deep ion etching manufacturing process to make free standing masks. We also discuss what is limiting contrast in the laboratory and our progress in wavefront correction.
The ultimate limitation of visible light high-dynamic-range imaging systems such as shaped pupil coronagraphs comes from scattering caused by imperfections in the optical surfaces of the collecting system, and from the non-uniform reflectivity of those surfaces. This paper focuses on the correction of these imperfections using two deformable mirrors in a zero path length difference Michelson interferometer. Simulations show the advantages and limitations of introducing such a device into a wavefront control loop. Laboratory work shows progress towards high resolution amplitude control.
This paper summarizes our work designing optimal shaped pupils for high-contrast imaging. We show how any effective apodization can be created using shaped pupils and present a variety of both one-dimensional and azimuthally symmetric pupil shapes. Each pupil has its own performance advantage and we discuss the tradeoffs among various designs. Optimizations are typically performed by maximizing a measure of system throughput under constraints on contrast and inner working angle. We mention the question of sensitivity to aberrations. Controlling aberrations will be critical for any implementation of a planet-finding coronagraph. Finally, we present our first laboratory results testing a shaped pupil coronagraph.
The Princeton University Terrestrial Planet Finder (TPF) has been working on a novel method for direct imaging of extra solar planets using a shaped-pupil coronagraph. The entrance pupil of the coronagraph is optimized to have a point spread function (PSF) that provides the suppression level needed at the angular separation
required for detection of extra solar planets. When integration time is to be minimized, the photon count at the planet location in the image plane is a Poisson distributed random process. The ultimate limitation of these high-dynamic-range imaging systems comes from scattering due to imperfections in the optical surfaces of the collecting system. The first step in correcting the wavefront errors is the estimation of the phase aberrations. The phase aberration caused by these imperfections is assumed to be a sum of two-dimensional sinusoidal functions. Its parameters are estimated using a global search with a genetic algorithm and a local optimization with the BFGS quasi-Newton method with a mixed quadratic and cubic line search procedure.
The Princeton University Terrestrial Planet Finder (TPF) group has been working on a novel method for direct imaging of extra solar planets using a shaped-pupil coronagraph. The entrance pupil of the coronagraph is optimized to have a point spread function (PSF) that provides the suppression level needed at the angular separation required for detection of extra solar planets. When integration time is to be minimized, the photon count at the planet location in the image plane is a Poisson distributed random process. The ultimate limitation of these high-dynamic-range imaging systems comes from scattering due to imperfections in the optical surfaces of the collecting system. The first step in correcting the wavefront errors is the estimation of the phase aberrations. The phase aberration caused by these imperfections is assumed to be a sum of two-dimensional sinusoidal functions. Assuming one uses a deformable mirror to correct these aberrations, we propose an algorithm that
optimally decreases the scattering level in specified localized areas in the image plane independent of the choice of influence function of the deformable mirror.
An experimental proof using two liquid crystal spatial light modulators in conjunction with a white light Michelson interferometer to correct amplitude error in telescopes is presented.The principle is reviewed,and then the experiment for a monochromatic closed loop is detailed.
Recent studies of exosolar planet detection methods with a space-based visible light coronagraph have shown the feasibility of this approach. However, the telescope optical precision requirements are extremely demanding - a few Angstroms residual wavefront error - which is beyond current capabilities for large optical surfaces. Secondly, the coronagraph depends upon use of masks located at either the pupil or a focus to reject the starlight and image the exosolar planet. Effects of diffraction and light scatter place precision requirements mask manufacturing. To increase understanding and optimize performance of the coronagraph, laboratory experiments backed by end-to-end integrated models are used to project on-orbit performance. Of particular importance is the wavefront propagation through the optical system - from simple Fraunhaufer propagation to vector propagators taking into account 3D structures of the masks. Accurate models, which match test data are then used to evolve the initial coronagraph concepts for in-flight performance. In part I, we discuss error sources and model development to meet mission goals. In part II, a paper to be published at a future date, we compare lab experiment and expected residual error sources.
We examine several different approaches to achieving high contrast imaging of extrasolar planets. Rather than controlling the diffracted light by masking the star's image as in a classical coronagraph, we use the pupil's transmission function to focus the starlight. There are two broad classes of pupil coronagraphs examined in this paper: apodized pupils with spatially varying transmision functions and shaped pupils, whose transmission values are either 0 or 1. The latter are much easier to manufacture to the needed tolerances. This paper introduces several new shaped pupils and applies integration time and other metrics to them as well as to apodized pupils. These new designs can achieve nearly as high a throughput as the best
apodized pupils and perform significantly better than the apodized square aperture design. The new shaped pupils enable searches of 50% -100% of thedetectable region, suppress the star's light to below 10-10 of its peak value and have inner working distances as small as 2.8 λ/D.
A new method is presented for amplitude and phase control using two liquid crystal spatial light modulators in conjunction with a white light Michelson interferometer. Preliminary proof-of-concept measurements are given showing the prospect of using this method for correction of amplitude errors in telescopes.
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