Stray-light contamination and spatial deconvolution of slit-spectrograph observations

Stray light caused by scattering on optical surfaces and in the Earth's atmosphere degrades the spatial resolution of observations. We study the contribution of stray light to the two channels of POLIS. We test the performance of different methods of stray-light correction and spatial deconvolution to improve the spatial resolution post-facto. We model the stray light as having two components: a spectrally dispersed component and a component of parasitic light caused by scattering inside the spectrograph. We use several measurements to estimate the two contributions: observations with a (partly) blocked FOV, a convolution of the FTS spectral atlas, imaging in the pupil plane, umbral profiles, and spurious polarization signal in telluric lines. The measurements allow us to estimate the spatial PSF of POLIS and the main spectrograph of the German VTT. We use the PSF for a deconvolution of both spectropolarimetric data and investigate the effect on the spectra. The parasitic contribution can be directly and accurately determined for POLIS, amounting to about 5%. We estimate a lower limit of about 10% across the full FOV for the dispersed stray light. In quiet Sun regions, the stray-light level from the close surroundings (d< 2") of a given spatial point is about 20%. The stray light reduces to below 2% at a distance of 20" from a lit area for both POLIS and the main spectrograph. A two-component model of the stray-light contributions seems to be sufficient for a basic correction of observed spectra. The instrumental PSF obtained can be used to model the off-limb stray light, to determine the stray-light contamination accurately for observation targets with large spatial intensity gradients such as sunspots, and also allows one to improve the spatial resolution of observations post-facto.Comment: 14 pages, 16 figures, accepted by A&A. Version V2 revised for language editin

Point spread functions for the Solar Optical Telescope onboard Hinode

The combined PSF of the BFI and the SOT onboard the Hinode spacecraft is investigated. Observations of the Mercury transit from November 2006 and the solar eclipse(s) from 2007 are used to determine the PSFs of SOT for the blue, green, and red continuum channels of the BFI. For each channel large grids of theoretical point spread functions are calculated by convolution of the ideal diffraction-limited PSF and Voigt profiles. These PSFs are applied to artificial images of an eclipse and a Mercury transit. The comparison of the resulting artificial intensity profiles across the terminator and the corresponding observed profiles yields a quality measure for each case. The optimum PSF for each observed image is indicated by the best fit. The observed images of the Mercury transit and the eclipses exhibit a clear proportional relation between the residual intensity and the overall light level in the telescope. In addition there is a anisotropic stray-light contribution. ... BFI/SOT operate close to the diffraction limit and have only a rather small stray-light contribution. The FWHM of the PSF is broadened by only ~1% with respect to the diffraction-limited case, while the overall Strehl ratio is ~ 0.8. In view of the large variations -- best seen in the residual intensities of eclipse images -- and the dependence on the overall light level and position in the FOV, a range of PSFs should be considered instead of a single PSF per wavelength. The individual PSFs of that range allow then the determination of error margins for the quantity under investigation. Nevertheless the stray-light contributions are here found to be best matched with Voigt functions with the parameters sigma = 0."008 and gamma = 0."004, 0."005, and 0."006 for the blue, green, and red continuum channels, respectively.Comment: 14 pages, 9 figures, accepted by A&

Spatially coupled inversion of spectro-polarimetric image data I: Method and first results

When inverting solar spectra, image degradation effects that are present in the data are usually approximated or not considered. We develop a data reduction method that takes these issues into account and minimizes the resulting errors. By accounting for the diffraction PSF of the telescope during the inversions, we can produce a self-consistent solution that best fits the observed data, while simultaneously requiring fewer free parameters than conventional approaches. Simulations using realistic MHD data indicate that the method is stable for all resolutions, including those with pixel scales well beyond those that can be resolved with a 0.5m telescope, such as the Hinode SOT. Application of the presented method to reduce full Stokes data from the Hinode spectro-polarimeter results in dramatically increased image contrast and an increase in the resolution of the data to the diffraction limit of the telescope in almost all Stokes and fit parameters. The resulting data allow for detecting and interpreting solar features that have so far only been observed with 1m class ground-based telescopes. The new inversion method allows for accurate fitting of solar spectro-polarimetric imaging data over a large field of view, while simultaneously improving the noise statistics and spatial resolution of the results significantly.Comment: A&A, accepte

Observation of anisotropic diffusion of light in compacted granular porous materials

It is known that compaction of granular matter can lead to anisotropic mechanical properties. Recent work has confirmed the link to pore space anisotropy, but the relation between compression, mechanical properties and material microstructure remains poorly understood and new diagnostic tools are needed. By studying the temporal and spatial characteristics of short optical pulses diffusively transmitted through compacted granular materials, we show that powder compaction can also give rise to strongly anisotropic diffusion of light. Investigating technologically important materials such as microcrystalline cellulose, lactose and calcium phosphate, we report increasing optical anisotropy with compaction force and radial diffusion constants being up to 1.7 times the longitudinal. This open new and attractive routes to material characterization and investigation of compression-induced structural anisotropy. In addition, by revealing inadequacy of isotropic diffusion models, our observations also have important implications for quantitative spectroscopy of powder compacts (e.g., pharmaceutical tablets).Comment: New version with significantly improved presentation. Data and argumentation identical to previous versio

Stray Light Compensation in Optical Systems

All optical equipment suffers from a phenomenon called stray light, which is defined as unwanted light in an optical system. Images contaminated by stray light tend to have lower contrast and reduced detail, which motivates the need for reducing it in many applications. This master thesis considers computational stray light compensation in digital cameras. In particular, the purpose is to reduce stray light in surveillance cameras developed by Axis Communications. We follow in the spirit of other digital stray light compensation approaches, in which measurements are fit to a parametric shift-variant point spread function (PSF) describing the stray light characteristics of the optical system. The observed contaminated image is modelled as an underlying ideal image convolved with the PSF. Once the PSF has been determined, a deconvolution is performed to obtain a restored image. We provide comparisons of a few deconvolution strategies and their performances regarding the restoration of images. Also, we discuss different techniques for decreasing the computational cost of the compensation. An experiment in which the images are compared to a ground-truth is proposed to objectively measure performance. The results indicate that the restored images are closer to the ground-truth compared to the observed image, which implies that the stray light compensation is successful.se bilaga

Methodology for the Integration of Optomechanical System Software Models with a Radiative Transfer Image Simulation Model

Stray light, any unwanted radiation that reaches the focal plane of an optical system, reduces image contrast, creates false signals or obscures faint ones, and ultimately degrades radiometric accuracy. These detrimental effects can have a profound impact on the usability of collected Earth-observing remote sensing data, which must be radiometrically calibrated to be useful for scientific applications. Understanding the full impact of stray light on data scientific utility is of particular concern for lower cost, more compact imaging systems, which inherently provide fewer opportunities for stray light control. To address these concerns, this research presents a general methodology for integrating point spread function (PSF) and stray light performance data from optomechanical system models in optical engineering software with a radiative transfer image simulation model. This integration method effectively emulates the PSF and stray light performance of a detailed system model within a high-fidelity scene, thus producing realistic simulated imagery. This novel capability enables system trade studies and sensitivity analyses to be conducted on parameters of interest, particularly those that influence stray light, by analyzing their quantitative impact on user applications when imaging realistic operational scenes. For Earth science applications, this method is useful in assessing the impact of stray light performance on retrieving surface temperature, ocean color products such as chlorophyll concentration or dissolved organic matter, etc. The knowledge gained from this model integration also provides insight into how specific stray light requirements translate to user application impact, which can be leveraged in writing more informed stray light requirements. In addition to detailing the methodology\u27s radiometric framework, we describe the collection of necessary raytrace data from an optomechanical system model (in this case, using FRED Optical Engineering Software), and present PSF and stray light component validation tests through imaging Digital Imaging and Remote Sensing Image Generation (DIRSIG) model test scenes. We then demonstrate the integration method\u27s ability to produce quantitative metrics to assess the impact of stray light-focused system trade studies on user applications using a Cassegrain telescope model and a stray light-stressing coastal scene under various system and scene conditions. This case study showcases the stray light images and other detailed performance data produced by the integration method that take into account both a system\u27s stray light susceptibility and a scene\u27s at-aperture radiance profile to determine the stray light contribution of specific system components or stray light paths. The innovative contributions provided by this work represent substantial improvements over current stray light modeling and simulation techniques, where the scene image formation is decoupled from the physical system stray light modeling, and can aid in the design of future Earth-observing imaging systems. This work ultimately establishes an integrated-systems approach that combines the effects of scene content and the optomechanical components, resulting in a more realistic and higher fidelity system performance prediction

Non-parametric PSF estimation from celestial transit solar images using blind deconvolution

Context: Characterization of instrumental effects in astronomical imaging is important in order to extract accurate physical information from the observations. The measured image in a real optical instrument is usually represented by the convolution of an ideal image with a Point Spread Function (PSF). Additionally, the image acquisition process is also contaminated by other sources of noise (read-out, photon-counting). The problem of estimating both the PSF and a denoised image is called blind deconvolution and is ill-posed. Aims: We propose a blind deconvolution scheme that relies on image regularization. Contrarily to most methods presented in the literature, our method does not assume a parametric model of the PSF and can thus be applied to any telescope. Methods: Our scheme uses a wavelet analysis prior model on the image and weak assumptions on the PSF. We use observations from a celestial transit, where the occulting body can be assumed to be a black disk. These constraints allow us to retain meaningful solutions for the filter and the image, eliminating trivial, translated and interchanged solutions. Under an additive Gaussian noise assumption, they also enforce noise canceling and avoid reconstruction artifacts by promoting the whiteness of the residual between the blurred observations and the cleaned data. Results: Our method is applied to synthetic and experimental data. The PSF is estimated for the SECCHI/EUVI instrument using the 2007 Lunar transit, and for SDO/AIA using the 2012 Venus transit. Results show that the proposed non-parametric blind deconvolution method is able to estimate the core of the PSF with a similar quality to parametric methods proposed in the literature. We also show that, if these parametric estimations are incorporated in the acquisition model, the resulting PSF outperforms both the parametric and non-parametric methods.Comment: 31 pages, 47 figure

A Characterisation of the Micromagnetic Properties of Thin Film Magnetic Recording Media by Lorentz Electron Microscopy

This thesis describes the electron microscope based analysis of two thin film magnetic recording media: iron cobalt chromium and cobalt nickel chromium. In particular, Lorentz electron microscopy was employed to characterise the micromagnetic domain structures of square wave written tracks recorded on to these two media. From the images obtained from this examination of the medium a computational technique was developed to calculate the transition noise power spectra for written tracks from Lorentz images of their domain structure. These Lorentz derived spectra can then be directly compared to the spectra measured electronically from the readback signal measured from the hard disk. As an introduction chapter one presents the basic energy considerations associated with ferromagnetic materials. The second half of the chapter briefly reviews the relevant aspects of longitudinal magnetic recording. Electron microscopy provides the means through which the media can be both physically and micromagnetically characterised. The appropriate techniques are therefore described in chapter two, with particular emphasis on qualitatively describing the imaging methods of Lorentz electron microscopy. To obtain thin films of a recording medium on which tracks have been written three stages of preparation are required. Chapter three describes these processes in detail. The bulk of the electron microscope analysis is contained in chapters four and five. Chapter four begins by reporting the characteristics of the physical structure of the iron cobalt chromium thin film and proceeds to describe the micromagnetic characterisation of tracks written on to the medium both along and off the easy axis. A breakthrough in the preparation of the isotropic cobalt nickel chromium thin film medium from hard disk came in the latter stages of this Ph.D. study. The success of the technique on what is a new medium for analysis was thought worthy of inclusion; hence chapter five reviews the first images of the physical and micromagnetic structures of this medium. There has been little or no work done to try and quantitatively relate the micromagnetic domain structures observed in Lorentz images of a recording medium with the corresponding noise performance of the medium. Chapter six describes estimation of the noise spectrum from a.c. erased iron cobalt chromium from Lorentz images and how this spectrum compares favourably with that measured electronically for the a.c. erased medium. The successful estimation of the a. c. erased noise enabled the same method of noise estimation to be employed to characterise the transition noise spectra from square wave written tracks. Chapter seven discusses the limited success of analysing sections of track (10 periods in length) and describes the evolution of a second method based on analysing individual magnetisation reversals. Chapter eight contains the conclusions drawn from the results of chapters four through seven along with an indication of possible future work

The Imaging Magnetograph eXperiment (IMaX) for the Sunrise balloon-borne solar observatory

The Imaging Magnetograph eXperiment (IMaX) is a spectropolarimeter built by four institutions in Spain that flew on board the Sunrise balloon-borne telesocope in June 2009 for almost six days over the Arctic Circle. As a polarimeter IMaX uses fast polarization modulation (based on the use of two liquid crystal retarders), real-time image accumulation, and dual beam polarimetry to reach polarization sensitivities of 0.1%. As a spectrograph, the instrument uses a LiNbO3 etalon in double pass and a narrow band pre-filter to achieve a spectral resolution of 85 mAA. IMaX uses the high Zeeman sensitive line of Fe I at 5250.2 AA and observes all four Stokes parameters at various points inside the spectral line. This allows vector magnetograms, Dopplergrams, and intensity frames to be produced that, after reconstruction, reach spatial resolutions in the 0.15-0.18 arcsec range over a 50x50 arcsec FOV. Time cadences vary between ten and 33 seconds, although the shortest one only includes longitudinal polarimetry. The spectral line is sampled in various ways depending on the applied observing mode, from just two points inside the line to 11 of them. All observing modes include one extra wavelength point in the nearby continuum. Gauss equivalent sensitivities are four Gauss for longitudinal fields and 80 Gauss for transverse fields per wavelength sample. The LOS velocities are estimated with statistical errors of the order of 5-40 m/s. The design, calibration and integration phases of the instrument, together with the implemented data reduction scheme are described in some detail.Comment: 17 figure