We describe versatile turbulence simulator for testing and calibration of new techniques for high-resolution imaging of objects outside the Earth’s atmosphere using ground-based instrumentation. Examples here include: dynamic aperture diversity, wave front sensing using multi-aperture phase retrieval, and free-space beam propagation for rapidly re-configurable interferometers. Used in the testing of all of these, the simulator uses a high resolution spatial light modulator in tandem with a lower resolution deformable mirror to simulate atmospheric phase distortions over a wide range of turbulence conditions.
Synoptic telescopes are fundamental tools in Solar Physics and Space Weather. Their typical high cadence full-disk observations are pivotal to assess the physical conditions on the Sun and to forecast the evolution in time of those conditions. The TSST (Tor vergata Synoptic Solar Telescope) is a synoptic telescope composed of two main full-disk instruments: an H-alpha Daystar SR-127 telescope and a Magneto Optical Filter (MOF)-based telescope in the Potassium KI at 769.90 nm. The MOF consists in a glass cell containing a Potassium vapor where a longitudinal magnetic field is applied. The MOF-based channel produces full disk Line-of-Sight magnetic field and velocity maps of the solar photosphere at 300 km above the solar surface. In this work, we present the optical setup, the spectral characterization of the MOF-based telescope, and details on the spectral characterization of the MOFs cells which is a required test to obtain calibrated magnetograms and dopplergrams.
We describe a new concept for future interferometric observations. Our laboratory experiment simulates an interferometer with two telescopes observing through different volumes of atmospheric turbulence. We simulate both vertical and horizontal propagation through the atmosphere; the latter mimics free-space beam propagation without the need for vacuum pipes or fiber optics. Practically, we simulate the effects of atmospheric turbulence using numerically calculated Kolmogorov phase screens injected onto a spatial light modulator. We correct this distorted wavefront using an adaptive optics system to determine the range of turbulence conditions over which we can detect fringes from the two telescopes. Our experiment lays the groundwork for investigating the potential of having movable telescopes in which light propagates from the telescopes to the beam combiner through free space.
High-resolution imagery of satellites from ground-based telescopes plays an important role in space situational awareness (SSA). Current approaches generally rely on large aperture telescopes equipped with adaptive optics. The cost and operational demands of such systems place severe limitations on worldwide surveillance capability. Coverage of the sky is restricted to the regions that happen to be above the small number of geographical locations where facilities exist. However, recent advances in carbon fiber reinforced polymer (CFRP) mirrors offer the potential for field-deployable telescopes with an aperture diameter of 1 m or larger for ground-based imaging of space objects. Concave CFRP mirrors are made from convex mandrels, which are challenging to fabricate. Therefore, a major cost driver of a CFRP telescope is the optical quality of the mandrel. Here we show, using both numerical simulations and real data, that a telescope with significant optical aberration can be used for high-resolution imaging if the telescope is equipped with a wavefront sensor (WFS) and the recorded images and WFS data are processed with an appropriate image restoration algorithm. We anticipate that the maturation of the technology to manufacture telescopes at this size from light-weight replicated components, at a cost considerably lower than conventional telescopes, will address a growing demand for SSA data.
Recurrence Quantification Analysis (RQA) is a non-linear time series analysis technique widely employed in many different research fields. Among the many applications of this method, it has been shown that it can be successfully employed in the detection of small signals embedded into noise. In this work we explore the possibility of using the RQA in astronomical high contrast imaging, for the detection of faint objects nearby bright sources in very high frame rate (1 KHz) data series. For this purpose, we used a real 1 kHz image sequence of a bright star, acquired with the SHARK-VIS forerunner at LBT. Our results show excellent performances in terms of detection contrasts even with a very short data sequence (a few seconds). The use of RQA in astronomical high contrast imaging is discussed in light of the possible science applications and with respect to other techniques like, for example, the angular differential imaging (ADI) or the Speckle-Free ADI (SFADI).
Adaptive optics or numerical restoration algorithms that restore high resolution imaging through atmospheric turbulence are subject to isoplanatic wave-front errors. Mitigating those errors requires that the wave-front aberrations be estimated within the 3D volume of the atmosphere. Present techniques rely on multiple beacons, either natural stars or laser guide stars, to probe the atmospheric aberration along different lines of sight, followed by tomographic projection of the measurements onto layers at defined ranges. In this paper we show that a three-dimensional estimate of the wave-front aberration can be recovered from measurements by a single guide star in the case where the aberration is stratified, provided that the telescope tracks across the sky with non-uniform angular velocity. This is generally the case for observations of artificial earth-orbiting satellites, and the new method is likely to find application in ground-based telescopes used for space situational awareness.
We report photometric measurements of a sodium resonance guide star against the daylight sky when observed through a tuned magneto-optical filter (MOF). The MOF comprises a sodium vapor cell in a kilogauss-level magnetic field between crossed polarizers and has a very narrow transmission profile at the sodium D2 resonance of approximately 0.008 nm. Our observations were made with the 1.5 m Kuiper telescope on Mt. Bigelow, AZ, which has a separately mounted guide star laser projecting a circularly polarized single-frequency beam of approximately 6.5 W at 589.16 nm. Both the beam projector and the 1.5 m telescope were pointed close to zenith; the baseline between them is approximately 5 m. Measurements of the guide star were made on the morning of 2016 March 24 using an imaging camera focused on the beacon and looking through the full aperture of the telescope. The guide star flux was estimated at 1.20×106 photon/m2/s while at approximately 45 minutes after sunrise, the sky background through the MOF was 1100 photon/m2/s/arcsec2. We interpret our results in terms of thermal infrared observations with adaptive optics on the next generation of extremely large telescopes now being built.
Advanced Astronomy for Heliophysics Plus (ADAHELI+) is a project concept for a small solar and space weather mission with a budget compatible with an European Space Agency (ESA) S-class mission, including launch, and a fast development cycle. ADAHELI+ was submitted to the European Space Agency by a European-wide consortium of solar physics research institutes in response to the “Call for a small mission opportunity for a launch in 2017,” of March 9, 2012. The ADAHELI+ project builds on the heritage of the former ADAHELI mission, which had successfully completed its phase-A study under the Italian Space Agency 2007 Small Mission Programme, thus proving the soundness and feasibility of its innovative low-budget design. ADAHELI+ is a solar space mission with two main instruments: ISODY+: an imager, based on Fabry–Pérot interferometers, whose design is optimized to the acquisition of highest cadence, long-duration, multiline spectropolarimetric images in the visible/near-infrared region of the solar spectrum. XSPO: an x-ray polarimeter for solar flares in x-rays with energies in the 15 to 35 keV range. ADAHELI+ is capable of performing observations that cannot be addressed by other currently planned solar space missions, due to their limited telemetry, or by ground-based facilities, due to the problematic effect of the terrestrial atmosphere.
We propose the use of an aperture diverse imaging system for high-resolution imaging through strong atmospheric
turbulence. The system has two channels. One channel partitions the aperture into a set of annular apertures that provide
a set of images of the target at different spatial resolutions. The other channel feeds an imaging Shack-Hartmann wavefront
sensor with a small number of sub-apertures. The combined imagery from this setup is processed using a blind
restoration algorithm that captures the inherent temporal correlations in the observed atmospheric wave fronts. This
approach shows significant promise for providing high-fidelity imagery for observations acquired through strong
atmospheric turbulence. The approach also allows for the separation of the phase perturbations from different layers of
the atmosphere. This characteristic offers potential for the accurate restoration of images with fields of view substantially
larger than the isoplanatic angle.
We used a laser system for determining the bandpasses of the two vapour cells, the Magneto-Optical Filter (MOF)
and the Wing Selector (WS), which are the core of solar narrow-band filters based on the MOF technology. A
new result, which we called the Intensity Effect, was found: the MOF and WS bandpasses depend not only on
the temperature at which the cell is heated and the external magnetic field in which the cell is embedded, but
also on the radiation intensity entering the cell. A theoretical interpretation of the Intensity Effect is proposed
in terms of the kinetic equilibrium of the potassium atomic populations inside the vapour cell. We need to take
the Intensity Effect into account for setting-up MOF based instruments for solar and stellar observations as well
as for modelling the MOF and WS spectral transmissions.
The Doppler-Intensity-Magnetograms with a Magneto-optical filter Instrument at two heights (DIMMI-2h) is a
double channel imager using Magneto Optical Filters (MOF) in the potassium 770 nm and sodium 589 nm lines.
The instrument will provide simultaneous dopplergrams (velocity fields), continuum intensity and longitudinal
magnetic flux images at two heights in the solar atmosphere corresponding to low and high photosphere. Dimmi-
2h is the possible piggy-back payload on ADAHELI satellite. The spatial resolution (approximately 4 arcsec) and
the high temporal cadence (15 s) will permit to investigate low and medium oscillating modes (from 0 to below
1000) up to approximately 32 mHz in the frequency spectrum. The acquisition of long-term simultaneous velocity,
intensity and magnetic information up to these high frequencies will permit also the study of the propagation
and excitation of the waves with a frequency resolution never obtained before.
We describe results from new computational techniques to extend the reach of large ground-based optical telescopes,
enabling high resolution imaging of space objects under daylight conditions. Current state-of-the-art systems, even those
employing adaptive optics, dramatically underperform in such conditions because of strong turbulence generated by
diurnal solar heating of the atmosphere, characterized by a ratio of telescope diameter to Fried parameter as high as 70.
Our approach extends previous advances in multi-frame blind deconvolution (MFBD) by exploiting measurements from
a wavefront sensor recorded simultaneously with high-cadence image data. We describe early results with the new
algorithm which may be used with seeing-limited image data or as an adjunct to partial compensation with adaptive
optics to restore imaging to the diffraction limit even under the extreme observing conditions found in daylight.
Random fluctuations in the index of refraction, caused by differential heating and cooling of the atmosphere, can
severely limit the quality of ground-based observations of space objects. Techniques such as adaptive optics can help
compensate for the deleterious effects that turbulence has on the images by deforming the telescope mirror and thus
correcting the wave-front. However, when imaging through strong turbulence such techniques may not adequately
correct the wave-front. In such cases blind restoration techniques - which estimate both the atmospheric turbulence
characterized by the atmospheric point-spread-function and the object that is being observed - must be used. We
demonstrate high quality blind restorations of object scenes, obtained when observing through strong turbulence, by
using a sequence of images obtained simultaneously at different wavelengths and prior information on the distribution of
the sources of regions of low spectral power in the data.
How to obtain sharp images when viewing through a turbid medium is a problem that arises in a number of applications, including optical biomedical imaging and optical surveillance in the presence of clouds. The main problem with this type of imagery is that it is difficult to accurately characterize the turbid medium sufficiently well to generate a point spread function that can be used to deconvolve the blurred data (and thus increase the resolution). We discuss the use of blind deconvolution as a means of estimating both the blur-free target and the system point spread function. We compare restorations obtained using a non-linear blind deconvolution algorithm with those obtained using a linear backpropagation algorithm. Preliminary results indicate that the blind deconvolution algorithm produces the more visually pleasing restorations. Moreover, it does so without requiring any prior knowledge of the characteristics of the turbid medium, or of what the blur-free target should look like: an important advance over the backpropagation algorithm.
We present analysis and numerical simulations of a new method to sense atmospheric wavefront distortion in real time with Rayleigh beacons. Multiple range-gated images of a single pulse from the laser are used to determine each phase map, providing an advantage over other methods in that photon noise is substantially reduced for a given brightness of the beacon. A laser at about 350 nm projects collimated pulses of light adjacent to the telescope. Rayleigh-scattered light from each pulse is recorded through the full telescope aperture in a sequence of video frames, each a few microseconds long. Images are captured as the pulse approaches and passes through the height at which the camera is focused. Phase diversity is thus naturally introduced between the frames. An iterative algorithm is used to extract the pupil-plane phases from the recorded intensity distributions. We anticipate that such beacons are likely to be valuable in future advanced systems for adaptive optics on very large telescopes with multiple laser beacons and deformable mirrors that aim to provide a large corrected field of view by tomography of the atmospheric turbulence.
We demonstrate the recovery, without a priori object knowledge, of the unknown object and point spread functions (PSFs) from multiframe focal-plane data. By modeling the object Fourier spectrum as an unprejudiced linear combination of the cross-spectra of the measurements and the PSFs, we significantly reduce the number of degrees-of- freedom for the blind deconvolution problem.
Adaptive Optics produces diffraction-limited images but does not fully compensate for the atmospheric degradation of the incoming signal. Post processing is important to fully restore the image. The results of applying a physically constrained iterative deconvolution algorithm to adaptive optics data are presented here for different types of simulated data with different signal-to-noise ratios.
The Navy Prototype Optical Lnterferometer, NPOI, is routinely used to measure visibility amplitudes and closure phase for stellar objects at optical wavelengths (e.g. , Benson et al. ,' Hajian et al.2) . In this poster we describe the fringe data collection aspects and the real time algorithm that enables us to actively track fringes with the
instrument. For a detailed description of the overall instrument see Armstrong et al. .
We present an application of an iterative deconvolution algorithm to speckle interferometric data. This blind deconvolution algorithm permits the recovery of the target distribution when the point spread function is either unknown or poorly known. The algorithm is applied to specklegrams of the multiple star systems, and the results for (zetz) UMa are compared to shift-and-add results for the same data. The linearity of the algorithm is demonstrated and the signal-to-noise ratio of the reconstruction is shown to grow as the square root of the number of specklegrams used. This algorithm does not require the use of an unresolved target for point spread function calibration.
We present applications of a recently developed iterative blind deconvolution algorithm to both simulated and real data. The applications demonstrate the algorithm's performance for a wide range of astronomical imaging. We demonstrate the effectiveness of using multiple observations of the same object convolved with different point spread functions. We also show the extension of the algorithm to phase retrieval when the object Fourier amplitude is available.