Earth-like extra-solar planets have luminosities which are many orders of magnitude less than those of their parent star. We propose and test a method for identifying molecular spectral bands in light from such a planet by looking at an offcenter part of an infrared Fourier transform interferogram. This results in superior sensitivity to narrow spectral bands, which are expected in the planet’s spectrum, but are absent in the parent star. We support this by astronomical observations to illustrate the method in the visible. The results suggest that this method is applicable to searches for planet biolines, and for differentiating between narrow lines and wide lines in other astronomical scenarios.
The extremely low signal contrast between an Earth-like extra-solar planet (exoplanet) and a parent star is a difficult
obstacle in their detection, imaging and spectroscopic analysis. We suggest a method of using selected parts of the
Fourier interferogram of the combined light sources (both planet and sun) in order to increase the signal to noise ratio
and identify the specific spectral features from the planet in the background of the parent star.
A habitable exoplanet is expected to reflect and emit a luminosity which is many orders of magnitude less than that of
the parent star. However, its spectral features are much different, being much narrower than its sun. Narrower lines are
more coherent, so their Fourier spectrum extends to much larger delays. Thus they can be discriminated for by looking
at an off-center part of a Fourier spectrogram. As the center (with the shorter delay) has all the power from the star's
wider features, these will not affect the result. Now all the power will be distributed at the longer delays (where the
exoplanets lines appear), improving the signal to noise ratio. We support this idea by realistic simulations which
include photon and thermal noise, and show it to be feasible at a luminosity ratio of 10-6 in the infra-red for a Sun-like
star and an Earth-like planet. We also carried out a laboratory experiment to illustrate the method. The results suggest
that this method should be applicable to a very large number of candidate stars.
Interferometry in space has marked advantages: long integration times and observation in spectral bands where the
atmosphere is opaque. When installed on separate spacecraft, it also has extended and flexible baselines for better
filling of the uv plane. Intensity interferometry has an additional advantage, being insensitive to telescope and path
errors, but is unfortunately much less light-sensitive. In planning towards such a mission, we are experimenting with
some fundamental research issues. Towards this end, we constructed a system of three vehicles floating on an air table
in formation flight, with an autonomous orbit control. Each such device holds its own light collector, detector, and
transmitter, to broadcast its intensity signal towards a central receiving station. At this station we implement parallel
radio receivers, analogue to digital converters, and a digital three-way correlator.
Current technology limits us to ~1GHz transmission frequency, which corresponds to a comfortable 0.3m accuracy in
light-bucket shape and in its relative position. Naïve calculations place our limiting magnitude at ~7 in the blue and
ultraviolet, where amplitude interferometers are limited. The correlation signal rides on top of this huge signal with its
own Poisson noise, requiring a very large dynamic range, which needs to be transmitted in full. We are looking at
open questions such as deployable optical collectors and radio antennae of similar size of a few meters, and how they
might influence our data transmission and thus set our flux limit.
Experiments are in progress to prepare for intensity interferometry with arrays of air Cherenkov
telescopes. At the Bonneville Seabase site, near Salt Lake City, a testbed observatory has been set
up with two 3-m air Cherenkov telescopes on a 23-m baseline. Cameras are being constructed, with
control electronics for either off- or online analysis of the data. At the Lund Observatory (Sweden),
in Technion (Israel) and at the University of Utah (USA), laboratory intensity interferometers simulating stellar observations have been set up and experiments are in progress, using various analog and digital correlators, reaching 1.4 ns time resolution, to analyze signals from pairs of laboratory telescopes.
Building on technological developments over the last 35 years, intensity interferometry now appears a feasible option by which to achieve diffraction-limited imaging over a square-kilometer synthetic aperture. Upcoming Atmospheric Cherenkov Telescope projects will consist of up to 100 telescopes, each with ~100m<sup>2</sup> of light gathering area, and distributed over ~1km<sup>2</sup>. These large facilities will offer thousands of baselines from 50m to more than 1km and an unprecedented (u,v) plane coverage. The revival of interest in Intensity Interferometry has recently led to the formation of a IAU working group. Here we report on various ongoing efforts towards implementing modern Stellar Intensity Interferometry.
Recent experiments in high-power lasers show that they modulate of the density of air at long ranges, up to filamentation
in a restricted volume. There are two effects: light concentration and plasma filamentation, depending on the laser
power. Two such laser-heated volumes can scatter stellar light into a central station, where they are made to interfere in
speckled fringes. Usually the density modulations deflect the light only slightly, so the maximum baseline is not
extended. However, if either the modulation of the density of air is strong, or its spatial frequency is high, then the stellar
beam deflection is significant. In such a case, the scattering volumes can be further off to the sides, and baselines of
hundreds of meters can be envisaged.
A European Laser Guide Star (LGS) test facility is proposed for the 4.2m William Herschel Telescope (WHT) on La
Palma. It will test the next-generation Adaptive Optics (AO) LGS technologies to aid risk mitigation of Extremely Large
Telescope (ELT) LGS AO systems. In particular, critical scaling of current LGS AO technologies to ELT dimensions
will be tested. For example, experiments addressing increased spot elongation, cone effect and the order of correction
A pan-European consortium proposes to construct test facility infrastructure on the WHT for a number of risk mitigating
experiments. The infrastructure includes the construction of a Nasmyth platform based controlled environment 'Ground-based
Adaptive optics Innovative Laboratory' (GRAIL), an experimental test environment 'Testbed integration facility'
(TIF) and some common-experiment equipment such as the Common Re-Imaging AO System.
Experiments that are proposed for this facility cover the areas of laser technologies, spot elongation, LGS wavefront
sensing, parallel launch concepts, Multi-Object AO, atmospheric characterisation, co-phasing and real-time control
system risk mitigation.
Stellar amplitude interferometry is limited by the need to have optical distances known to a fraction of the wavelength. We suggest reviving intensity interferometry, which requires far less accurate hardware (~1cm mechanical precision) at the cost of more limited sensitivity. We present an algorithm that uses the very high redundancy of a uniform linear array to increase the sensitivity of the instrument by more than a hundredfold. An array of a hundred ~100m diameter elements can achieve a limiting magnitude of <i>m</i><sub>b</sub>=14.4. Off-line processing of the data will enable such a ground-based facility to transform a two-dimensional field of point-like sources to a three-dimensional distribution of micro-arcsec resolved systems, each imaged in several optical bands. Each system will also have its high resolution residual timing, high quality (inside each band) spectra and light curve, emergent flux, effective temperature, polarization effects and perhaps some thermodynamic properties, all directly measured in a single observation run of such a dedicated facility. Coronagraphy, selectively suppressing large scale structures of the sources, can also be achieved by specific aperture shapes. We conclude that after three decades of abandonment optical intensity interferometry deserves another review, also as a ground-based alternative to the science goals of space interferometers.
One of the concepts of radio interferometry which is very difficult to apply to the visible domain is phase closure. The main difficulty is the spatial requirement, namely that all pencil beams will interfere with all other beams on a flat detector. We use a pair-wise combination method using anamorphic stretching of the beams. All beams are lined up, imaged through a cylindrical lens into a square where each beam is now spread into a parallel line. The comb of lines is made to interfere with a copy of itself rotated at 90°. A rotation shear interferometer is employed for that stage, and the cross pattern of apertures is imaged on the detector. The diagonal shows interference of each beam with itself, for intensity calibration purposes. An extended source clearly reduces contrast on some off-axis patterns, in a symmetric manner. We have already tested two designs in the laboratory using lasers and white light.
Perspective imaging of laser guide stars leads to their elongation. This effect is significant in future large telescopes, where many such beacons are necessary. Solutions to these problems include mechanical or electronic movement at the sensing devices, optical separation of the reference sources and more. Another solution is to shift the burden of analysis to the lasers and create a pattern which suffers less from perspective elongation when scattered back onto the detectors. Finally, different analysis methods can then be employed to solve for the wave front tomographically from this projected pattern.
A new observation mode, a survey of temporal coherence in large fields, is described. Longitudinal coherence can arise in many astrophysical scenarios and at different wave lengths, from plasma effects, scattering of electrons on periodic electromagnetic volumes, to Einstein rings and even extraterrestrial unintended signaling. There is a plurality of coherence seeking devices, developed for military purposes, which can be easily adopted for this task. Many are based on unbalanced interferometers, with a path difference that exceeds the white light envelope. They were shown to be able to discern very weak coherent sources in a heavily cluttered environment. For searches where the coherent wave length was unknown, a signal to background ratio of 1:10,000 was demonstrated. At a known wave length (e.g. molecular lines) one can even expect 1:1,000,000 detection ratio. Once such sources are found, they can be better monitored by most astronomical interferometers, whose field of view is usually rather narrow.
Recently we developed and tested different algorithms for wave front reconstruction from dense Hartmann-Shack patterns. All depend on the recognition of a main frequency in the patterns, whose distortion from wave aberrations can be construed as slight phase changes in the pattern. An alternative description of these aberrations is a slight frequency change in Fourier domain. The slopes can thus be found by demodulation in either the image or the Fourier domain. These slopes can then be integrated in the Fourier domain again for the wave front itself. For smooth slopes both demodulation and integration can be performed in the Fourier domain. In addition, commands for the adaptive optics loop can be taken directly in the Fourier domain, saving on processing time. We modeled and tested these algorithms thoroughly in simulation and in laboratory experiments on two separate adaptive optics systems.
The interference pattern of many beams includes multiple fringe sets, each for a corresponding pair of beams. These fringes must have a limited spectral band, so that they can extend far from the central, white-light fringe. It is possible instead to use a chromatic corrector in order to match the fringe spacing at different wave lengths. As a result, they will overlap over a much larger area, and thus improve the selectivity of the beam pairs.
Adaptive optics systems on single big telescopes correct many modes, allowing imaging in the infra red. At the same time, visible photons can be used as well, especially when infra red light is also employed for wave front sensing. It is argued that pupil-plane interferometry is the most useful application for high-resolution imaging. This is because the isoplanatic patch area and the integration time are larger after correction, and they afford enhanced signal collection in the aperture plane. In contrast, speckle imaging methods only gain indirectly from this enhancement.
It is possible to reduce significantly spot elongation in adaptive optics systems, if the laser creating the artificial beacon is broken up into many weak independent lasers, sent from scattered locations up to the periphery of a large telescope.
In between the star-oriented tomographic measurement of atmospheric layers, and the pure layer-oriented one, there is a simple third option, which measures the turbulence in the layers' images proper by a Hartmann-Shack type sensor. The wide field is achieved by looking at multiple stars in each lenslet. The method is simple to use, but requires a fast, sensitive camera with many pixels for the lenslets' wide fields. Most of the empty pixels (without stellar images) are skipped during read-out.
The World Space Observatory is an unconventional space project proceeding via distributed studies. The present design, verified for feasibility, consists of a 1.7-meter telescope operating at the second Largangian point of the Earth-Sun system. The focal plane instruments consist of three UV spectrometers covering the spectral band from Lyman alpha to the atmospheric cutoff with R~55,000 and offering long-slit capability over the same band with R~1,000. In addition, a number of UV and optical imagers view adjacent fields to that sampled by the spectrometers. Their performance compares well with that of HST/ACS and the spectral capabilities of WSO rival those of HST/COS.
The WSO, as presently conceived, will be constructed and operated with the same distributed philosophy. This will allow as many groups and countries to participate, each contributing as much as feasible but allowing multi-national participation. Although designed originally with a conservative approach, the WSO embodies some innovative ideas and will allow a world-class mission to be realized with a moderate budget.
We propose to modify the solar collector PETAL (Photon Energy Transformation & Astrophysics Laboratory) for astronomy. The mirror is a segmented parabolic dish collector, which has a relatively poor imaging quality. The conversion can be done by either of two principal methods: (1) phasing the surface of the collector itself or significant sections thereof; (2) transforming the structure into an optical interferometer by mounting small telescopes around its rim, and using fiber optics to combine the light at a common focus.
Following the development of a Hartmann-Shack sensor with a variable pitch, we needed a data analysis method that does not require a priori the locations of the foci of the lenslets. Thus we developed a method for retrieving the wave front from a Hartmann-Shack sensor using a two-dimensional Fourier demodulation technique. Digital demodulation is applied twice on the pattern, along two orthogonal axes. This provides both phase gradient components in real space, smoothed over the extent of the array. If necessary, a Fourier reconstruction can be used to obtain the corresponding wave front from the gradient components. The technique is rather robust to Poisson and white noise and to relatively large deviations of the foci. The method was tested successfully on a mirror with known large aberrations.
Adaptive optics is limited toady to correction of turbulence inside a cone extending from the reference source to the telescope aperture. Even when the reference source is a natural star, the measured- and corrected- cylinder does not allow observation of most extended astronomical objects. Hence the search is on for a method to measure turbulence in a conical volume which opens up from the telescope upwards. Various schemes were proposed to widen the field of view by using more artificial or natural guide stars, and by processing the measured data in different ways. It has been shown experimentally that the existence of three natural stars around the rim of the required cone is sufficient. Using multiple laser guide stars, schemes varying from separation of measured volumes and stitching of their edges, to integrated methods were suggested. It was also proposed to infer the turbulence form the shape of the beams as they propagate up in the atmosphere. Structured light above the turbulence is another option that was raised. Such a grid is created by interference of laser beams or by interference of powerful radio beams that break down the air into visible plasma. It can be shown that these fringes, either from a laser of from radio, can be analyzed optically, reducing the power requirements significantly. This field of atmospheric tomography is likely to produce soon corrected images of extended astronomical objects. In addition, being able to separate the contribution of the atmospheric layers, we will acquire better knowledge of atmospheric turbulence.
Curvature sensing is a non-interferometric wave front sensing method. Its resolution is only limited by the sensor resolution, and it only requires one pixel for each point. In order to understand its applicability it was tested in a controlled laboratory environment. We tried various optical configurations and different data processing methods, such as Projection on convex sets and finite elements. In edition it is presented that a simple silicon wafer, on the back of which porous silicon is etched, can serve as deformable mirror. This is the first report, to our knowledge, of the piezoelectric and piezo-optic response of porous silicon.
We present a novel technique to alleviate the problem of the global tilt in artificial guide stars for adaptive optics. This technique is based on the registration of trails of radio-excited plasma spots caused by the atmospheric tilt. Following the time trace of the trails one can find and measure the tilt produced by atmospheric turbulent layers. Different methods were applied to estimate the extent of the trails. We describe results of computer simulations, showing the performance of the proposed approach.
The problem of global tilt arises when the downgoing light from an artificial guide star traces in reverse the upgoing laser beam. The problem also exists if the upgoing beam is in the radio, since we still cannot determine its absolute position to the required accuracy. We propose a way to solve this problem by tracing the tails of the artificial guide star. Radio pulses breaking down the air into visible plasma create the beacon, and each pulse uses the residue plasma of its predecessor to restart the break down. As the radio beams wander on their way up, the new spot will not overlap perfectly with the previous one. Thus, the spot will have a small trail, which can help trace the history of the tilt. Comparison of the measurement of the previous pulse to the tail of the new one will provide the differential movement of the downgoing beam. Integrating this movement will yield the absolute pulse location. Cumulative errors are reduced by comparison to weak nearby natural guide stars. We ran simulations of the process, where we include random atmospheric tilts for the radio beam. We also checked the effect of intentional nutations of the upgoing beam and the effects of atmospheric winds on the plasma spot, as well as detection noise. We fitted the measured signal to the head and tail of each spot, and found their difference, which amounts to the change in tilts since the last pulse. We integrated this difference to find the absolute tilt. We show that indeed the tail trail can be traced to yield information about the tilt.
While developing deformable mirrors for adaptive optics, and while studying scintillations, we also tested the method of curvature sensing and different variants of it. By very carefully adjusting the optics we were able to discern variations on the scale of one nanometer. The limited dynamic range of the detectors and various optical artifacts caused systematic and random errors of 5 - 8 percent. Calibration of the measurements turned out to be a difficult issue as well. One of the main problems with non-atmospheric measurements was vignetting. We suspect that strong atmospheric scintillation might cause similar problems in curvature sensing, because of light scattered outside the measurement aperture, leading to errors in the estimated wave front phase. We looked into measuring turbulence along the optical path, by comparing field data of both Hartmann- Shack and intensity sensors collected under similar conditions. It seems that some of the turbulence can be tracked back to its range, but this is still being tested. If so, it might be possible to correct it using multi- conjugate optics and reduce significantly scintillation effects. Scintillation can also be removed artificially by correcting a scaled-up version of the turbulence at a scaled-down conjugate distance and vice versa.
Measurements of atmospheric turbulence, mainly for adaptive optical correction, are carried out today by using a reference source near the astronomical object. Laser-created guide stars are being tested as alternatives to scarce natural stars. It might be easier to obtain radio-created guide stars at high elevations. By interference of radio beams, visible plasma is created or modified in fine fringes, and their observation at multiple angles is used for tomography of the turbulence. Such guide stars might also be used for phasing radio-telescopes.
In addition to natural and laser guide stars for adaptive optics, it is proposed to use radio-created guide stars or fringes. Heating by intense radio beams either modulators sodium lamp illumination, or creates and modifies plasma in different altitudes. The plasma relaxes by artificial air glow, which is concentrated in few lines, mainly nitrogen and oxygen. Fringes between intense radio beams create plasma fringes, also visible from the telescope. Ionospheric heating was also shown to create patchy artificial aurora. In all cases, the multiplicity of sources or fringes allows multi-conjugate adaptive optics of wide fields of view, and shows promise for long baseline optical interferometry.
A new treatment is presented for light propagation through multilayer turbulence. Equations for the intensity and phase of an observed wavefront re derived together with their validity conditions for both single and multiple layer systems. A method for finding the statistics of observed scintillations is presented together with a detailed calculation for a single layer system.
Stellar scintillations provide statistical information about the higher atmosphere (7 - 12 km). Since each realization of scintillation is the Fresnel diffraction off high altitude turbulence, it can be inverted separately. Sensors for adaptive optics integrate the wave front error over all layers of turbulence. They measure scintillation for calibration. But this discarded information yields the high wave fronts. Separate correction for low and/or high turbulence widens the repaired field of view. The method requires that the reference star is bright and small, that the middle turbulence (2 - 7 km) is negligible, and that the sensor has good spatio-temporal resolution. Simulations show that the turbulence can be retrieved, with lowest and highest frequencies lost first.
Wavefront sensors for adaptive optics measure the phase error produced by all layers of atmospheric turbulence. In doing so, they also measure the scintillation pattern in the aperture plane of the telescope for calibration purposes. However, scintillation can provide information about wavefronts in the higher atmosphere (7 - 10 km). This is because it is produced as Fresnel diffraction of high elevation turbulence. Scintillation can also be viewed as a laplacian of the same high altitude perturbations, when there is no middle level turbulence. Thus it can be inverted to yield these perturbations. Using this additional information, adaptive optics systems could correct for either low- or high-elevation turbulence (or both), and increase the field of view available for observation. The method is limited to high intensity and small size of the reference star, absence of middle level turbulence, and narrow spectral response by the detector.
We describe the applicability of wave front correction by using a bimorph mirror in conjunction with a curvature sensor. We use Zernike polynomials to describe analytically the quality of the correction for atmospheric turbulence. The match is limited by boundary conditions of the mirror and by the discreteness of the electrodes. The correction is limited by coupling of lower and higher order Zernike polynomials and necessitates an interfacing computer between the wave front sensor and the bimorph mirror.
We identify stellar wave fronts that passed through turbulence as fractals from the Fractional Brownian motion family. Fractal behavior manifests itself both in space and in time. We propose predicting wave fronts, based on earlier measurements at each pixel and at neighboring pixels. If the wind direction is known, mainly upward measurements are needed. The accuracy of the method is improved if atmospheric layers can be measured separately.
In order to increase the corrected field of view in multi-conjugate adaptive optics we suggest a method for measuring and separating the contribution of atmospheric turbulent layers. The sodium layer serves as a huge screen, on which we project a wide fringe pattern from a single laser on the ground. A modified Hartmann-Shack sensor is employed to detect deformations in the pattern: sections of the fringe pattern are imaged by a lenslet array onto a large-format camera. Low layer turbulence causes overall shift of the fringe pattern in each lenslet, while high altitude turbulence results in internal deformations in the pattern, which are detected by that section of the camera which is behind each lenslet. Parallel Fourier analyses for the different lenslets allows separation of the atmospheric layers: the periodicity of the fringes lends itself to digital demodulation, which yields the deformations in the fringe. We present a statistical error analysis and simulations, showing good performance over a field of view of 80', compared to the performance expected of a conventional single sodium beacon, single adaptive mirror system.
Atmospheric turbulence degraded wave fronts can be described as fractal surfaces from the Fractional Brownian motion family. Fractal character can be ascribed both to the spatial and temporal behavior. Implications of this description with regard to computer simulation and prediction are presented.
A thin bimorph adaptive mirror can correct corrupted wave fronts when measured by a wave front curvature sensor. The correction is given analytically in terms of Zernike polynomials. Boundary conditions of the mirror, discreteness of the electrodes, and coupling between low and high order Zernike polynomials limits the fit, especially for atmospherically corrupted wave fronts.
Both the theoretical and the experimental problems of backgrounds are examined. The authors show why the current definitions of correlation length should be used with care, with attention paid to the intensity histogram of a scene. Different effects of the sub-pixel features in a measured scene on the clutter for imaging and scanning systems are also explained. The two- dimensional polarization of a scene is measured and found to compare favorably with the theoretical predictions. Finally, the authors show how to simulate backgrounds whose power spectrum is given, together with constraints on the image proper. This is achieved by iteratively transforming between the image plane and its Fourier conjugate, while imposing the appropriate constraints in both planes.
We have used a laser interferometer to measure the relative optical pathlength variations from the focal plane
to the six individual secondary telescopes of the Multiple Mirror Telescope (MMT). These pathlengths vary as func-
Lions of elevation due to variable gravitational loading, drive acceleration and velocity, temperature variations, and
wind loading. Vibrations induced by wind loading and telescope drives, including building vibrations transmitted to
the mount, cause high-frequency variations in the otherwise slowly varying optical pathlengths. This experiment was
designed to evaluate the effects of these high-frequency perturbations on optical interferometry at various tracking
rates, including those relevant to Earth Satellite observation. We find effects which can strongly affect the contrast of