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Christoph U. Keller
Professor of Experimental Astrophysics

Leiden Observatory
Leiden University, The Netherlands

Calibrating a high-resolution wavefront corrector with a static focal-plane camera

7 Oct 2013

by Korkiakoski, Visa, Doelman, Niek, Codona, Johanan, Kenworthy, Matthew, Otten, Gilles, Keller, Christoph U., is now available here.

Abstract: We present a method to calibrate a high-resolution wavefront-correcting device with a single, static camera, located in the focal plane; no moving of any component is needed. The method is based on a localized diversity and differential optical transfer functions (dOTF) to compute both the phase and amplitude in the pupil plane located upstream of the last imaging optics. An experiment with a spatial light modulator shows that the calibration is sufficient to robustly operate a focal-plane wavefront sensing algorithm controlling a wavefront corrector with ~40 000 degrees of freedom. We estimate that the locations of identical wavefront corrector elements are determined with a spatial resolution of 0.3% compared to the pupil diameter.

Three-dimensional magnetic and abundance mapping of the cool Ap star HD 24712 . I. Spectropolarimetric observations in all four Stokes parameters

7 Oct 2013

by Rusomarov, N., Kochukhov, O., Piskunov, N., Jeffers, S. V., Johns-Krull, C. M., Keller, C. U., Makaganiuk, V., Rodenhuis, M., Snik, F., Stempels, H. C., Valenti, J. A., is now available here.

Abstract: Context. High-resolution spectropolarimetric observations provide simultaneous information about stellar magnetic field topologies and three-dimensional distributions of chemical elements. High-quality spectra in the Stokes IQUV parameters are currently available for very few early-type magnetic chemically peculiar stars. Here we present analysis of a unique full Stokes vector spectropolarimetric data set, acquired for the cool magnetic Ap star HD 24712 with a recently commissioned spectropolarimeter. Aims: The goal of our work is to examine the circular and linear polarization signatures inside spectral lines and to study variation of the stellar spectrum and magnetic observables as a function of rotational phase. Methods: HD 24712 was observed with the HARPSpol instrument at the 3.6-m ESO telescope over a period of 2010-2011. We achieved full rotational phase coverage with 43 individual Stokes parameter observations. The resulting spectra have a signal-to-noise ratio of 300-600 and resolving power exceeding 105. The multiline technique of least-squares deconvolution (LSD) was applied to combine information from the spectral lines of Fe-peak and rare earth elements. Results: We used the HARPSPol spectra of HD 24712 to study the morphology of the Stokes profile shapes in individual spectral lines and in LSD Stokes profiles corresponding to different line masks. From the LSD Stokes V profiles we measured the longitudinal component of the magnetic field, ⟨Bz⟩, with an accuracy of 5-10 G. We also determined the net linear polarization from the LSD Stokes Q and U profiles. Combining previous ⟨Bz⟩ measurements with our data allowed us to determine an improved rotational period of the star, Prot = 12.45812 ± 0.00019 d. We also measured the longitudinal magnetic field from the cores of Hα and Hβ lines. The analysis of ⟨Bz⟩ measurements showed no evidence for a significant radial magnetic field gradient in the atmosphere of HD 24712. We used our ⟨Bz⟩ and net linear polarization measurements to determine parameters of the dipolar magnetic field topology. We found that magnetic observables can be reasonably well reproduced by the dipolar model, although significant discrepancies remain at certain rotational phases. We discovered rotational modulation of the Hα core and related it to a non-uniform surface distribution of rare earth elements. Based on observations collected at the European Southern Observatory, Chile (ESO programs 084.D-0338, 085.D-0296, 086.D-0240).Figure 3 and Appendix A are available in electronic form at http://www.aanda.org

Are there tangled magnetic fields on HgMn stars?

7 Oct 2013

by Kochukhov, O., Makaganiuk, V., Piskunov, N., Jeffers, S. V., Johns-Krull, C. M., Keller, C. U., Rodenhuis, M., Snik, F., Stempels, H. C., Valenti, J. A., is now available here.

Abstract: Context. Several recent spectrophotometric studies failed to detect significant global magnetic fields in late-B HgMn chemically peculiar stars, but some investigations have suggested the presence of strong unstructured or tangled fields in these objects. Aims: We used detailed spectrum synthesis analysis to search for evidence of tangled magnetic fields in high-quality observed spectra of eight slowly rotating HgMn stars and one normal late-B star. We also evaluated recent sporadic detections of weak longitudinal magnetic fields in HgMn stars based on the moment technique. Methods: Our spectrum synthesis code calculated the Zeeman broadening of metal lines in HARPS spectra, assuming an unstructured, turbulent magnetic field. A simple line formation model with a homogeneous radial field distribution was applied to assess compatibility between previous longitudinal field measurements and the observed mean circular polarization signatures. Results: Our analysis of the Zeeman broadening of magnetically sensitive spectral lines reveals no evidence of tangled magnetic fields in any of the studied HgMn or normal stars. We infer upper limits of 200-700 G for the mean magnetic field modulus - much smaller than the field strengths implied by studies based on differential magnetic line intensification and quadratic field diagnostics. The new HARPSpol longitudinal field measurements for the extreme HgMn star HD 65949 and the normal late-B star 21 Peg are consistent with zero at a precision of 3-6 G. Re-analysis of our Stokes V spectra of the spotted HgMn star HD 11753 shows that the recent moment technique measurements retrieved from the same data are incompatible with the lack of circular polarization signatures in the spectrum of this star. Conclusions: We conclude that there is no evidence for substantial tangled magnetic fields on the surfaces of studied HgMn stars. We cannot independently confirm the presence of very strong quadratic or marginal longitudinal fields for these stars, so results from the moment technique are likely to be spurious. Based on observations collected at the European Southern Observatory, Chile (ESO programmes 084.D-0338, 085.D-0296, 086.D-0240).

The color dependent morphology of the post-AGB star HD 161796

7 Oct 2013

by Min, M., Jeffers, S. V., Canovas, H., Rodenhuis, M., Keller, C. U., Waters, L. B. F. M., is now available here.

Abstract: Context. Many protoplanetary nebulae show strong asymmetries in their surrounding shells, pointing to asymmetries during the mass loss phase. Questions concerning the origin and the onset of deviations from spherical symmetry are important for our understanding of the evolution of these objects. Here we focus on the circumstellar shell of the post-AGB star HD 161796. Aims: We aim to detect signatures of an aspherical outflow, and to derive its properties. Methods: We used the imaging polarimeter the Extreme Polarimeter (ExPo), a visitor instrument at the William Herschel Telescope, to accurately image the dust shell surrounding HD 161796 in various wavelength filters. Imaging polarimetry allows us to separate the faint, polarized, light that comes from circumstellar material from the bright, unpolarized, light from the central star. Results: The shell around HD 161796 is highly aspherical. A clear signature of an equatorial density enhancement can be seen. This structure is optically thick at short wavelengths and changes its appearance to optically thin at longer wavelengths. In the classification of the two different appearances of planetary nebulae from HST images it changes from being classified as DUst-Prominent Longitudinally-EXtended (DUPLEX) at short wavelengths to star-obvious low-level-elongated (SOLE) at longer wavelengths. This strengthens the interpretation that these two appearances are manifestations of the same physical structure. Furthermore, we find that the central star is hotter than often assumed and the relatively high observed reddening is a consequence of circumstellar rather than interstellar extinction. Based on observations made with the William Herschel Telescope operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofsicaŋsica de Canarias.

Observing Circumstellar Neighbourhoods with the Extreme Polarimeter

7 Oct 2013

by Rodenhuis, M., Canovas, H., Jeffers, S. V., Min, M., Keller, C. U., is now available here.

Abstract: The study of circumstellar environments at ever higher contrasts has generated considerable interest in recent years. One method to increase the contrast is to observe the linearly polarized light scattered by the circumstellar material while suppressing the unpolarized stellar flux. This paper presents some sample imaging polarimetry results obtained with the Extreme Polarimeter (ExPo). ExPo operates in the visible part of the spectrum, and currently achieves a polarimetric sensitivity of 10-4. Despite the demise of the Utrecht Astronomical Institute, where this instrument was developed, the instrument is still being used and upgraded. It has now moved to the Leiden Observatory.

Sterrekundig Instituut Utrecht: The Last Years

7 Oct 2013

by Keller, C. U., is now available here.

Abstract: I describe the last years of the 370-year long life of the Sterrekundig Instituut Utrecht, which was the second-oldest university observatory in the world and was closed in early 2012 after the Faculty of Science and the Board of Utrecht University decided, without providing qualitative or quantitative arguments, to remove astrophysics from its research and education portfolio.

Astronomical Polarimetry: Polarized Views of Stars and Planets

7 Oct 2013

by Snik, Frans, Keller, Christoph U., is now available here.

Abstract: Polarization is a fundamental property of light from astronomical objects, and measuring that polarization often yields crucial information, which is unobtainable otherwise.This chapter reviews the useful formalisms for describing polarization in the optical regime, the mechanisms for the creation of such polarization, and methods for measuring it. Particular emphasis is given on how to implement a polarimeter within an astronomical facility, and on how to deal with systematic effects that often limit the polarimetric performance.

The color dependent morphology of the post-AGB star HD161796

26 Mar 2013

by Min, M., Jeffers, S. V., Canovas, H., Rodenhuis, M., Keller, C. U., Waters, L. B. F. M., is now available here.

Abstract: Context. Many protoplanetary nebulae show strong asymmetries in their surrounding shell, pointing to asymmetries during the mass loss phase. Questions concerning the origin and the onset of deviations from spherical symmetry are important for our understanding of the evolution of these objects. Here we focus on the circumstellar shell of the post-AGB star HD 161796. Aims. We aim at detecting signatures of an aspherical outflow, as well as to derive the properties of it. Methods. We use the imaging polarimeter ExPo (the extreme polarimeter), a visitor instrument at the William Herschel Telescope, to accurately image the dust shell surrounding HD 161796 in various wavelength filters. Imaging polarimetry allows us to separate the faint, polarized, light from circumstellar material from the bright, unpolarized, light from the central star. Results. The shell around HD 161796 is highly aspherical. A clear signature of an equatorial density enhancement can be seen. This structure is optically thick at short wavelengths and changes its appearance to optically thin at longer wavelengths. In the classification of the two different appearances of planetary nebulae from HST images it changes from being classified as DUPLEX at short wavelengths to SOLE at longer wavelengths. This strengthens the interpretation that these two appearances are manifestations of the same physical structure. Furthermore, we find that the central star is hotter than often assumed and the relatively high observed reddening is due to circumstellar rather than interstellar extinction.

Magnetically Controlled Accretion on the Classical T Tauri Stars GQ Lupi and TW Hydrae

26 Mar 2013

by Johns-Krull, Christopher M., Chen, Wei, Valenti, Jeff A., Jeffers, Sandra V., Piskunov, Nikolai E., Kochukhov, Oleg, Makaganiuk, V., Stempels, H. C., Snik, Frans, Keller, Christoph, Rodenhuis, M., is now available here.

Abstract: We present high spectral resolution (R ≈ 108, 000) Stokes V polarimetry of the classical T Tauri stars (CTTSs) GQ Lup and TW Hya obtained with the polarimetric upgrade to the HARPS spectrometer on the ESO 3.6 m telescope. We present data on both photospheric lines and emission lines, concentrating our discussion on the polarization properties of the He I emission lines at 5876 Å and 6678 Å. The He I lines in these CTTSs contain both narrow emission cores, believed to come from near the accretion shock region on these stars, and broad emission components which may come from either a wind or the large-scale magnetospheric accretion flow. We detect strong polarization in the narrow component of the two He I emission lines in both stars. We observe a maximum implied field strength of 6.05 ± 0.24 kG in the 5876 Å line of GQ Lup, making it the star with the highest field strength measured in this line for a CTTS. We find field strengths in the two He I lines that are consistent with each other, in contrast to what has been reported in the literature on at least one star. We do not detect any polarization in the broad component of the He I lines on these stars, strengthening the conclusion that they form over a substantially different volume relative to the formation region of the narrow component of the He I lines.

HARPS Spectropolarimetry of the Classical T Tauri Stars GQ Lup and TW Hya

26 Mar 2013

by Johns-Krull, Christopher M., Chen, W., Valenti, J. A., Jeffers, S. V., Piskunov, N. E., Kochukhov, O., Makaganiuk, V., Stempels, H. C., Snik, F., Keller, C., Rodenhuis, M., is now available here.

Abstract: We present high spectral resolution Stokes V polarimetery of the Classical T Tauri stars (CTTSs) GQ Lup and TW Hya obtained with the polarimetric upgrade to the HARPS spectrometer on the ESO 3.6 m telescope. We present data on both photospheric lines and emission lines, concentrating our discussion on the polarization properties of the He I emission lines at 5876 A and 6678 A. The He I lines in both these CTTS contain both narrow emission cores, believed to come from near the accretion shock region on these stars, and broad emission components which may come from either a wind or the large scale magnetospheric accretion flow. We detect strong polarization in the narrow component of both the He I emission lines in both stars. We observe a maximum implied field strength of 5.8 +/- 0.3 kG in the 5876 A line of GQ Lup, the highest field strength measured to date in this line for a CTTS. We find field strengths in the two He I lines that are consistent with each other, unlike what has been reported in the literature on at least one star. We do not detect any polarization in the broad component of the He I lines on these stars, strengthening the conclusion that they form over a substantially different volume relative the formation region of the narrow component of the He I lines.

Astronomical Polarimetry: Polarized Views of Stars and Planets

26 Mar 2013

by Snik, Frans, Keller, Christoph U., is now available here.

Abstract: Polarization is a fundamental property of light from astronomical objects, and measuring that polarization often yields crucial information, which is unobtainable otherwise.This chapter reviews the useful formalisms for describing polarization in the optical regime, the mechanisms for the creation of such polarization, and methods for measuring it. Particular emphasis is given on how to implement a polarimeter within an astronomical facility, and on how to deal with systematic effects that often limit the polarimetric performance.

Observing the Earth as an exoplanet with LOUPE, the lunar observatory for unresolved polarimetry of Earth

26 Mar 2013

by Karalidi, T., Stam, D. M., Snik, F., Bagnulo, S., Sparks, W. B., Keller, C. U., is now available here.

Abstract: The detections of small, rocky exoplanets have surged in recent years and will likely continue to do so. To know whether a rocky exoplanet is habitable, we have to characterize its atmosphere and surface. A promising characterization method for rocky exoplanets is direct detection using spectropolarimetry. This method will be based on single pixel signals, because spatially resolving exoplanets is impossible with current and near-future instruments. Well-tested retrieval algorithms are essential to interpret these single pixel signals in terms of atmospheric composition, cloud and surface coverage. Observations of Earth itself provide the obvious benchmark data for testing such algorithms. The observations should provide signals that are integrated over the Earth's disk, that capture day and night variations, and all phase angles. The Moon is a unique platform from where the Earth can be observed as an exoplanet, undisturbed, all of the time. Here, we present LOUPE, the Lunar Observatory for Unresolved Polarimetry of Earth, a small and robust spectropolarimeter to observe our Earth as an exoplanet.

Semidefinite programming for model-based sensorless adaptive optics

26 Mar 2013

by Antonello, Jacopo, Verhaegen, Michel, Fraanje, Rufus, van Werkhoven, Tim, Gerritsen, Hans C., Keller, Christoph U., is now available here.

Abstract: Not Available

Unusual Stokes V profiles during flaring activity of a delta sunspot

26 Mar 2013

by Fischer, C. E., Keller, C. U., Snik, F., Fletcher, L., Socas-Navarro, H., is now available here.

Abstract: Aims: We analyze a set of full Stokes profile observations of the flaring active region NOAA 10808. The region was recorded with the Vector-Spectromagnetograph of the Synoptic Optical Long-term Investigations of the Sun facility. The active region produced several successive X-class flares between 19:00 UT and 24:00 UT on September 13, 2005 and we aim to quantify transient and permanent changes in the magnetic field and velocity field during one of the flares, which has been fully captured. Methods: The Stokes profiles were inverted using the height-dependent inversion code LILIA to analyze magnetic field vector changes at the flaring site. We report multilobed asymmetric Stokes V profiles found in the δ-sunspot umbra. We fit the asymmetric Stokes V profiles assuming an atmosphere consisting of two components (SIR inversions) to interpret the profile shape. The results are put in context with Michelson Doppler Imager (MDI) magnetograms and reconstructed X-ray images from the Reuven Ramaty High Energy Solar Spectroscopic Imager. Results: We obtain the magnetic field vector and find signs of restructuring of the photospheric magnetic field during the flare close to the polarity inversion line at the flaring site. At two locations in the umbra we encounter strong fields (~3 kG), as inferred from the Stokes I profiles, which, however, exhibit a low polarization signal. During the flare we observe in addition asymmetric Stokes V profiles at one of these sites. The asymmetric Stokes V profiles appear co-spatial and co-temporal with a strong apparent polarity reversal observed in MDI-magnetograms and a chromospheric hard X-ray source. The two-component atmosphere fits of the asymmetric Stokes profiles result in line-of-sight velocity differences in the range of ~12 km s-1 to 14 km s-1 between the two components in the photosphere. Another possibility is that local atmospheric heating is causing the observed asymmetric Stokes V profile shape. In either case our analysis shows that a very localized patch of ~5″ in the photospheric umbra, co-spatial with a flare footpoint, exhibits a subresolution fine structure.

Potential of phase-diversity for metrology of active instruments

26 Mar 2013

by Korkiakoski, Visa, Venema, Lars, Agocs, Tibor, Keller, Christoph U., Doelman, Niek, Fraanje, Rufus, Andrei, Raluca, Verhaegen, Michel, is now available here.

Abstract: We investigate the potential of phase-diversity (PD) and Gerchberg-Saxton (GS) algorithms in the calibration of active instruments. A set of images is recorded with the focal-plane scientific camera, each image having a known and unique defocus. The phase-retrieval algorithms are used, with those images, to estimate the non-common path aberration that needs to be compensated by correct alignment of the instrument. We demonstrate by numerical simulations that such algorithms, in particular GS, are sufficient detection methods to fully correct wavefronts with an rms error at least up to 6 rad — but this requires several iterative correction stages.

Modeling the instrumental polarization of the VLT and E-ELT telescopes with the M&m's code

26 Mar 2013

by de Juan Ovelar, M., Diamantopoulou, S., Roelfsema, R., van Werkhoven, T., Snik, F., Pragt, Johan, Keller, C., is now available here.

Abstract: Polarimetry is a particularly powerful technique when imaging circumstellar environments. Currently most telescopes include more or less advanced polarimetric facilities and large telescopes count on it for their planet-finder instruments like SPHERE-ZIMPOL on the VLT or EPICS on the future E-ELT. One of the biggest limitations of this technique is the instrumental polarization (IP) generated in the telescope optical path, which can often be larger than the signal to be measured. In most cases this instrumental polarization changes over time and is dependent on the errors affecting the optical elements of the system. We have modeled the VLT and E-ELT telescope layouts to characterize the instrumental polarization generated on their optical paths using the M&m's code, an error budget and performance simulator for polarimetric systems. In this study we present the realistic Mueller matrices calculated with M&m's for both systems, with and without the setups to correct for the IP, showing that correction can be achieved, allowing for an accurate polarimetric performance.

A spectro-polarimetric integral field spectrograph for EPICS-EPOL

26 Mar 2013

by Rodenhuis, M., Sprenger, B., Keller, C. U., is now available here.

Abstract: Imaging polarimetry offers a way to increase the contrast of light scattered from circumstellar material, enabling direct observation of exoplanets -possibly rocky- with the E-ELT. To actually characterize these planets, some spectral resolution is essential. With sufficient resolution -both spectral and spatial- the spectral differential imaging technique can be used in addition to the polarimetry to detect circumstellar point sources. We present the concept for a spectro-polarimetric integral field spectrograph for the EPICS-EPOL instrument and our current efforts to demonstrate this concept with our existing imaging polarimeter ExPo.

The extreme polarimeter: design, performance, first results and upgrades

26 Mar 2013

by Rodenhuis, M., Canovas, H., Jeffers, S. V., de Juan Ovelar, Maria, Min, M., Homs, L., Keller, C. U., is now available here.

Abstract: Well over 700 exoplanets have been detected to date. Only a handful of these have been observed directly. Direct observation is extremely challenging due to the small separation and very large contrast involved. Imaging polarimetry offers a way to decrease the contrast between the unpolarized starlight and the light that has become linearly polarized after scattering by circumstellar material. This material can be the dust and debris found in circumstellar disks, but also the atmosphere or surface of an exoplanet. We present the design, calibration approach, polarimetric performance and sample observation results of the Extreme Polarimeter, an imaging polarimeter for the study of circumstellar environments in scattered light at visible wavelengths. The polarimeter uses the beam-exchange technique, in which the two orthogonal polarization states are imaged simultaneously and a polarization modulator is swaps the polarization states of the two beams before the next image is taken. The instrument currently operates without the aid of Adaptive Optics. To reduce the effects of atmospheric seeing on the polarimetry, the images are taken at a frame rate of 35 fps, and large numbers of frames are combined to obtain the polarization images. Four successful observing runs have been performed using this instrument at the 4.2 m William Herschel Telescope on La Palma, targeting young stars with protoplanetary disks as well as evolved stars surrounded by dusty envelopes. In terms of fractional polarization, the instrument sensitivity is better than 10-4. The contrast achieved between the central star and the circumstellar source is of the order 10-6. We show that our calibration approach yields absolute polarization errors below 1%.

Searching for signs of habitability with LOUPE, the Lunar Observatory of Unresolved Polarimetry of Earth

26 Mar 2013

by Karalidi, T., Stam, D. M., Snik, F., Bagnulo, S., Sparks, W. B., Keller, C. U., is now available here.

Abstract: We present LOUPE, a novel type of spectropolarimeter to measure the flux and state of polarization of sunlight that is reflected by the Earth from 0.4 to 0.8 μm. LOUPE has been designed as payload of a lunar lander. From the moon, the Earth can be observed as a whole, during its daily rotation and at all phase angles, just as if it were an exoplanet. LOUPE will provide benchmark data for the development of instruments for Earth-like exoplanet characterization, and for the testing of numerical retrieval algorithms.

Evidence for the disintegration of KIC 12557548 b

26 Mar 2013

by Brogi, M., Keller, C. U., de Juan Ovelar, M., Kenworthy, M. A., de Kok, R. J., Min, M., Snellen, I. A. G., is now available here.

Abstract: Context. The Kepler object KIC 12557548 b is peculiar. It exhibits transit-like features every 15.7 h that vary in depth between 0.2% and 1.2%. Rappaport et al. (2012, ApJ, 752, 1) explain the observations in terms of a disintegrating, rocky planet that has a trailing cloud of dust created and constantly replenished by thermal surface erosion. The variability of the transit depth is then a consequence of changes in the cloud optical depth. Aims: We aim to validate the disintegrating-planet scenario by modeling the detailed shape of the observed light curve, and thereby constrain the cloud particle properties to better understand the nature of this intriguing object. Methods: We analyzed the six publicly-available quarters of raw Kepler data, phase-folded the light curve and fitted it to a model for the trailing dust cloud. Constraints on the particle properties were investigated with a light-scattering code. Results: The light curve exhibits clear signatures of light scattering and absorption by dust, including a brightening in flux just before ingress correlated with the transit depth and explained by forward scattering, and an asymmetry in the transit light curve shape, which is easily reproduced by an exponentially decaying distribution of optically thin dust, with a typical grain size of 0.1 μm. Conclusions: Our quantitative analysis supports the hypothesis that the transit signal of KIC 12557548 b is due to a variable cloud of dust, most likely originating from a disintegrating object.

Sterrekundig Instituut Utrecht: The Last Years

26 Mar 2013

by Keller, Christoph U., is now available here.

Abstract: I describe the last years of the 370-year long life of the Sterrekundig Instituut Utrecht, which was the second-oldest university observatory in the world and was closed in early 2012 after the Faculty of Science and the Board of Utrecht University decided, without providing qualitative or quantitative arguments, to remove astrophysics from its research and education portfolio.

Linear analytical solution to the phase diversity problem for extended objects based on the Born approximation

26 Mar 2013

by Andrei, Raluca M., Smith, Carlas S., Fraanje, Rufus, Verhaegen, Michel, Korkiakoski, Visa A., Keller, Christoph U., Doelman, Niek, is now available here.

Abstract: In this paper we give a new wavefront estimation technique that overcomes the main disadvantages of the phase diversity (PD) algorithms, namely the large computational complexity and the fact that the solutions can get stuck in a local minima. Our approach gives a good starting point for an iterative algorithm based on solving a linear system, but it can also be used as a new wavefront estimation method. The method is based on the Born approximation of the wavefront for small phase aberrations which leads to a quadratic point-spread function (PSF), and it requires two diversity images. First we take the differences between the focal plane image and each of the two diversity images, and then we eliminate the constant object, element-wise, from the two equations. The result is an overdetermined set of linear equations for which we give three solutions using linear least squares (LS), truncated total least squares (TTLS) and bounded data uncertainty (BDU). The last two approaches are suited when considering measurements affected by noise. Simulation results show that the estimation is faster than conventional PD algorithms.

Bilinear solution to the phase diversity problem for extended objects based on the Born approximation

26 Mar 2013

by Andrei, Raluca M., Fraanje, Rufus, Verhaegen, Michel, Korkiakoski, Visa A., Keller, Christoph U., Doelman, Niek, is now available here.

Abstract: We propose a new approach for the joint estimation of aberration parameters and unknown object from diversity images with applications in imaging systems with extended objects as astronomical ground-based observations or solar telescopes. The motivation behind our idea is to decrease the computational complexity of the conventional phase diversity (PD) algorithm and avoid the convergence to local minima due to the use of nonlinear estimation algorithms. Our approach is able to give a good starting point for an iterative algorithm or it can be used as a new wavefront estimation method. When the wavefront aberrations are small, the wavefront can be approximated with a linear term which leads to a quadratic point-spread function (PSF) in the aberration parameters. The presented approach involves recording two or more diversity images and, based on the before mentioned approximation estimates the aberration parameters and the object by solving a system of bilinear equations, which is obtained by subtracting from each diversity image the focal plane image. Moreover, using the quadratic PSFs gives improved performance to the conventional PD algorithm through the fact that the gradients of the PSFs have simple analytical formulas.

Experimental validation of optimization concepts for focal-plane image processing with adaptive optics

26 Mar 2013

by Korkiakoski, Visa, Keller, Christoph U., Doelman, Niek, Fraanje, Rufus, Andrei, Raluca, Verhaegen, Michel, is now available here.

Abstract: We show experimental results demonstrating the feasibility of an extremely fast sequential phase-diversity (SPD) algorithm for point sources. The algorithm can be implemented on a typical adaptive optics (AO) system to improve the wavefront reconstruction beyond the capabilities of a wavefront sensor by using the information from the imaging camera. The algorithm is based on a small-phase approximation enabling fast numerical implementation, and it finds the optimal wavefront correction by iteratively updating the deformable mirror. Our experiments were made at an AO-setup with a 37 actuator membrane mirror, and the results show that the algorithm finds an optimal image quality in 5-10 iterations, when the initial wavefront errors are typical non-common path aberrations having a magnitude of 1-1.5 rad rms. The results are in excellent agreement with corresponding numerical simulations.

FOAM: the modular adaptive optics framework

26 Mar 2013

by van Werkhoven, T. I. M., Homs, L., Sliepen, G., Rodenhuis, M., Keller, C. U., is now available here.

Abstract: Control software for adaptive optics systems is mostly custom built and very specific in nature. We have developed FOAM, a modular adaptive optics framework for controlling and simulating adaptive optics systems in various environments. Portability is provided both for different control hardware and adaptive optics setups. To achieve this, FOAM is written in C++ and runs on standard CPUs. Furthermore we use standard Unix libraries and compilation procedures and implemented a hardware abstraction layer in FOAM. We have successfully implemented FOAM on the adaptive optics system of ExPo - a high-contrast imaging polarimeter developed at our institute - in the lab and will test it on-sky late June 2012. We also plan to implement FOAM on adaptive optics systems for microscopy and solar adaptive optics. FOAM is available* under the GNU GPL license and is free to be used by anyone.

Extremely fast focal-plane wavefront sensing for extreme adaptive optics

26 Mar 2013

by Keller, Christoph U., Korkiakoski, Visa, Doelman, Niek, Fraanje, Rufus, Andrei, Raluca, Verhaegen, Michel, is now available here.

Abstract: We present a promising approach to the extremely fast sensing and correction of small wavefront errors in adaptive optics systems. As our algorithm's computational complexity is roughly proportional to the number of actuators, it is particularly suitable to systems with 10,000 to 100,000 actuators. Our approach is based on sequential phase diversity and simple relations between the point-spread function and the wavefront error in the case of small aberrations. The particular choice of phase diversity, introduced by the deformable mirror itself, minimizes the wavefront error as well as the computational complexity. The method is well suited for high­ contrast astronomical imaging of point sources such as the direct detection and characterization of exoplanets around stars, and it works even in the presence of a coronagraph that suppresses the diffraction pattern. The accompanying paper in these proceedings by Korkiakoski et al. describes the performance of the algorithm using numerical simulations and laboratory tests.

News ArchiveCalibrating a high-resolution wavefront corrector with a static focal-plane cameraThree-dimensional magnetic and abundance mapping of the cool Ap star HD 24712 . I. 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