PSF Model Research Articles

Photometry and Stellar Structure Analysis of the Central Regions of the M33 galaxy

AbstractThe UK InfraRed Telescope/UFTI instrument was used to analyse stars in the central region (square kpc) of the M33 galaxy over 3 nights in August 2005 with observations in K-band. Photometry was obtained for all stars by fitting appropriate PSF models, using DAOPHOT package (Stetson 1987). Around 18500 stars were detected. A master image was produced by applying an appropriate shift to each image and combining all 30 images. The final catalogue was obtained by assigning unique ID numbers to all stars and calibrating instrumental magnitudes. Comparing the UFTI and the UIST catalogues (Javadi et al. 2011) shows that a larger number of stars were detected with the UFTI, reaching a K-band limiting magnitude of 18.5 mag. Cross correlation methods were employed to cross match our catalogue with a previous monitoring survey in the near-infrared, the UIST catalogue. As a result, we assigned J magnitude and J-K color index to those stars which were found in both catalogues. Using these stars, a color-magnitude diagram was produced which shows populations of RGB, AGB and Carbon stars. Isochrones from Marigo et al. (2008) for solar metallicity and a distance modulus of 24.9 magnitude were used to identify different populations in the central region of the M33 galaxy. We will merge the UFTI catalogue with the UIST and the WFCAM, to get better coverage of stars in the central square kiloparsec.

Modelling and simulation of photosynthetic microorganism growth: random walk vs. finite difference method

The paper deals with photosynthetic microorganism growth modelling and simulation in a distributed parameter system. Main result concerns the development and comparison of two modeling frameworks for photo-bioreactor modelling. The first “classical” approach is based on PDE (reaction–turbulent diffusion system) and finite difference method. The alternative approach is based on random walk model of transport by turbulent diffusion. The complications residing in modelling of multi-scale transport and reaction phenomena in microalgae are clarified and the solution is chosen. It consists on phenomenological state description of microbial culture by the lumped parameter model of photosynthetic factory (PSF model) in the re-parametrized form, published recently in this journal by Papáček, et al. (2010). Obviously both approaches lead to the same simulation results, nevertheless they provide different advantages.

Limits on the size and orbit distribution of main belt comets

The first of a new class of objects now known as main belt comets (MBCs) or “activated asteroids” was identified in 1996. The seven known members of this class have orbital characteristics of main belt asteroids yet exhibit dust ejection like comets. In order to constrain their physical and orbital properties we searched the Thousand Asteroid Light Curve Survey (TALCS; Masiero, J.R., Jedicke, R., Durech, J., Gwyn, S., Denneau, L., Larsen, J. [2009]. Icarus 204, 145–171) for additional candidates using two diagnostics: tail and coma detection. This was the most sensitive MBC survey effort to date, extending the search from MBCs with H ∼ 18 ( D ∼ 1 km) to MBCs as small as H ∼ 21 ( D ∼ 150 m). We fit each of the 924 objects detected by TALCS to a PSF model incorporating both a coma and nuclear component to measure the fractional contribution of the coma to the total surface brightness. We determined the significance of the coma detection using the same algorithm on a sample of null detections of comparable magnitude and rate of motion. We did not identify any MBC candidates with this technique to a sensitivity limit on the order of cometary mass loss rate of about 0.1 kg/s. Our tail detection algorithm relied on identifying statistically significant flux in a segmented annulus around the candidate object. We show that the technique can detect tail activity throughout the asteroid belt to the level of the currently known MBCs. Although we did not identify any MBC candidates with this technique, we find a statistically significant detection of faint activity in the entire ensemble of TALCS asteroids. This suggests that many main belt asteroids are active at very low levels. Our null detection of MBCs allows us to set 90% upper confidence limits on the number distribution of MBCs as a function of absolute magnitude, semi-major axis, eccentricity, and inclination. There are ≲400,000 MBCs in the main belt brighter than H V = 21 (∼150-m in diameter) and the MBC:MBA ratio is ≲1:400. We further comment on the ability of observations to meaningfully constrain the snow line’s location. Under some reasonable and simple assumptions we claim 85% confidence that the contemporary snow line lies beyond 2.5 AU.

Fast, Simultaneous Multiple Fluorophore Fitting in Single Molecule Super-Resolution Imaging

Single molecule localization based super-resolution (SML-SR) imaging techniques require repeated localization of many single fluorophores. If, during analysis, successful localization does not require isolated fluorophores, the performance can be improved in one or more of several metrics that result in higher single-frame density of active fluorophores. For example: Data acquisition time can be reduced; A larger number of fluorophores can be localized; There is a higher tolerance on labeling density; and Dyes with higher minimum duty cycle can be used.We have developed a method that uses the maximum likelihood estimator to localize multiple fluorophores simultaneously within a fitting sub region. We find that for a fitting region of size approximately 6 sigmaPSF (where sigmaPSF parameterizes a 2D Gaussian PSF model) localization of up to 5 fluorophores provides a good compromise between fit accuracy and analysis time. For speed, the algorithm is implemented on Graphics Processing Unit (GPU) architecture in a manner similar to our previous single molecule analysis (Smith, Nat. Methods 7, 373-375 2010) and achieves near real-time analysis speed.We show the performance of multiple fluorophore fitting as a function of (1) maximum allowed number of fitted fluorophores and (2) single-frame active emitter density. We describe the details of the algorithm that allow robust fitting, the details of the GPU implementation, and the other imaging processing steps required for the analysis of (SML-SR) data sets. As a demonstration, we show that our new multi-fluorophore super resolution imaging method reveals actin structure in a HeLa cell under conditions where a high single-frame fluorophore density results in poor reconstructed images using conventional single fluorophore based super resolution imaging techniques.

Parametrizing arbitrary galaxy morphologies: potentials and pitfalls

We demonstrate that morphological observables (e.g. steepness of the radial light profile, ellipticity, asymmetry) are intertwined and cannot be measured independently of each other. We present strong arguments in favour of model-based parametrisation schemes, namely reliability assessment, disentanglement of morphological observables, and PSF modelling. Furthermore, we demonstrate that estimates of the concentration and Sersic index obtained from the Zurich Structure & Morphology catalogue are in excellent agreement with theoretical predictions. We also demonstrate that the incautious use of the concentration index for classification purposes can cause a severe loss of the discriminative information contained in a given data sample. Moreover, we show that, for poorly resolved galaxies, concentration index and M_20 suffer from strong discontinuities, i.e. similar morphologies are not necessarily mapped to neighbouring points in the parameter space. This limits the reliability of these parameters for classification purposes. Two-dimensional Sersic profiles accounting for centroid and ellipticity are identified as the currently most reliable parametrisation scheme in the regime of intermediate signal-to-noise ratios and resolutions, where asymmetries and substructures do not play an important role. We argue that basis functions provide good parametrisation schemes in the regimes of high signal-to-noise ratios and resolutions. Concerning Sersic profiles, we show that scale radii cannot be compared directly for profiles of different Sersic indices. Furthermore, we show that parameter spaces are typically highly nonlinear. This implies that significant caution is required when distance-based classificaton methods are used.

Brightness Analysis by Z-Scan Fluorescence Fluctuation Spectroscopy for the Study of Protein Interactions within Living Cells

Fluorescence fluctuation spectroscopy (FFS) quantifies interactions of fluorescently labeled proteins inside living cells by brightness analysis. Conventional FFS implicitly requires that the sample thickness exceeds the size of the observation volume. This condition is not always fulfilled when measuring cells. Cytoplasmic sections, especially, can be thinner than the axial size of the observation volume. The finite sample thickness introduces a brightness bias which, if not recognized, leads to an erroneous interpretation of the data. To avoid this artifact, we introduce z-scan FFS which consists of a fluorescence intensity z scan through the sample followed by an FFS measurement. To model the experimental z-scan data, a new PSF model had to be introduced. We use the intensity z scan together with the PSF model to determine the geometry of the sample and then extract the brightness from the FFS data. Cells expressing EGFP serve as a model system for testing the experimental approach. We demonstrate that z-scan FFS abolishes the brightness artifact and use the method to determine the oligomerization of cytoplasmic nuclear transport factor 2.

Impact of Time-of-Flight on PET Tumor Detection

Time-of-flight (TOF) PET uses very fast detectors to improve localization of events along coincidence lines-of-response. This information is then utilized to improve the tomographic reconstruction. This work evaluates the effect of TOF upon an observer's performance for detecting and localizing focal warm lesions in noisy PET images. An advanced anthropomorphic lesion-detection phantom was scanned 12 times over 3 days on a prototype TOF PET/CT scanner (Siemens Medical Solutions). The phantom was devised to mimic whole-body oncologic (18)F-FDG PET imaging, and a number of spheric lesions (diameters 6-16 mm) were distributed throughout the phantom. The data were reconstructed with the baseline line-of-response ordered-subsets expectation-maximization algorithm, with the baseline algorithm plus point spread function model (PSF), baseline plus TOF, and with both PSF+TOF. The lesion-detection performance of each reconstruction was compared and ranked using localization receiver operating characteristics (LROC) analysis with both human and numeric observers. The phantom results were then subjectively compared to 2 illustrative patient scans reconstructed with PSF and with PSF+TOF. Inclusion of TOF information provides a significant improvement in the area under the LROC curve compared to the baseline algorithm without TOF data (P = 0.002), providing a degree of improvement similar to that obtained with the PSF model. Use of both PSF+TOF together provided a cumulative benefit in lesion-detection performance, significantly outperforming either PSF or TOF alone (P < 0.002). Example patient images reflected the same image characteristics that gave rise to improved performance in the phantom data. Time-of-flight PET provides a significant improvement in observer performance for detecting focal warm lesions in a noisy background. These improvements in image quality can be expected to improve performance for the clinical tasks of detecting lesions and staging disease. Further study in a large clinical population is warranted to assess the benefit of TOF for various patient sizes and count levels, and to demonstrate effective performance in the clinical environment.

Deconvolution with shapelets

We seek to find a shapelet-based scheme for deconvolving galaxy images from the PSF which leads to unbiased shear measurements. Based on the analytic formulation of convolution in shapelet space, we construct a procedure to recover the unconvolved shapelet coefficients under the assumption that the PSF is perfectly known. Using specific simulations, we test this approach and compare it to other published approaches. We show that convolution in shapelet space leads to a shapelet model of order $n_{max}^h = n_{max}^g + n_{max}^f$ with $n_{max}^f$ and $n_{max}^g$ being the maximum orders of the intrinsic galaxy and the PSF models, respectively. Deconvolution is hence a transformation which maps a certain number of convolved coefficients onto a generally smaller number of deconvolved coefficients. By inferring the latter number from data, we construct the maximum-likelihood solution for this transformation and obtain unbiased shear estimates with a remarkable amount of noise reduction compared to established approaches. This finding is particularly valid for complicated PSF models and low $S/N$ images, which renders our approach suitable for typical weak-lensing conditions.

THE ACS SURVEY OF GLOBULAR CLUSTERS. V. GENERATING A COMPREHENSIVE STAR CATALOG FOR EACH CLUSTER

The ACS Survey of Globular Clusters has used HST's Wide-Field Channel to obtain uniform imaging of 65 of the nearest globular clusters to provide an extensive homogeneous dataset for a broad range of scientific investigations. The survey goals required not only a uniform observing strategy, but also a uniform reduction strategy. To this end, we designed a sophisticated software program to process the cluster data in an automated way. The program identifies stars simultaneously in the multiple dithered exposures for each cluster and measures them using the best available PSF models. We describe here in detail the program's rationale, algorithms, and output. The routine was also designed to perform artificial-star tests, and we run a standard set of ~10^5 tests for each cluster in the survey. The catalog described here will be exploited in a number of upcoming papers and will eventually be made available to the public via the world-wide web.

Galaxy morphology and evolution from SWAN adaptive optics imaging

We present the results from adaptive optics (AO) assisted imaging in the Ks band of an area of 15 arcmin^2 for SWAN (Survey of a Wide Area with NACO). We derive the high resolution near-IR morphology of ~400 galaxies up to Ks~23.5 in the first 21 SWAN fields around bright guide stars, carefully taking into account the survey selection effects and using an accurate treatment of the anisoplanatic AO PSF. The detected galaxies are sorted into two morphological classes according to their Sersic index. The extracted morphological properties and number counts of the galaxies are compared with the predictions of different galaxy formation and evolution models, both for the whole galaxy population and separately for late-type and early-type galaxies. This is one of the first times such a comparison has been done in the near-IR, as AO observations and accurate PSF modeling are needed to obtain reliable morphological classification of faint field galaxies at these wavelengths. For early-type galaxies we find that a pure luminosity evolution model, without evidence for relevant number and size evolution, better reproduces the observed properties of our Ks-selected sample than current semi-analytic models based on the hierarchical picture of galaxy formation. In particular, we find that the observed flattening of elliptical galaxy counts at Ks~20 is quantitatively in good agreement with the prediction of the pure luminosity evolution model that was calculated prior to the observation. For late-type galaxies, while both models are able to reproduce the number counts, we find some hints of a possible size growth. These results demonstrate the unique power of AO observations to derive high resolution details of faint galaxies' morphology in the near-IR and drive studies of galaxy evolution.

Theoretical approach to CMOS APS PSF and MTF modeling - evaluation

In this work, a fully theoretical CMOS active pixel sensor (APS) modulation transfer function model is formulated, evaluated, and compared with practical results. The model is based on a two-dimensional diffusion equation solution and covers the symmetrical photocarriers diffusion effect together with the impact of the pixel active area geometrical shape. Thorough scanning results obtained by means of a unique submicron scanning system (the S-cube system) from various APS chips, implemented in a standard CMOS 0.35-/spl mu/m technology, are compared with our theoretical predictions. The agreement of the presented comparison results indicates that for any potential active area shape, an analytical reliable estimate of image performance is possible.

Weak‐Lensing Analysis of thez ≃ 0.8 Cluster CL 0152−1357 with the Advanced Camera for Surveys

We present a weak lensing analysis of the X-ray luminous cluster CL 0152-1357 at z~0.84 using HST/ACS observations. The unparalleled resolution and sensitivity of ACS enable us to measure weakly distorted, faint background galaxies to the extent that the number density reaches ~175 arcmin^-2. The PSF of ACS has a complicated shape that also varies across the field. We construct a PSF model for ACS from an extensive investigation of 47 Tuc stars in a modestly crowded region. We show that this model PSF excellently describes the PSF variation pattern in the cluster observation when a slight adjustment of ellipticity is applied. The high number density of source galaxies and the accurate removal of the PSF effect through moment-based deconvolution allow us to restore the dark matter distribution of the cluster in great detail. The direct comparison of the mass map with the X-ray morphology from Chandra observations shows that the two peaks of intracluster medium traced by X-ray emission are lagging behind the corresponding dark matter clumps, indicative of an on-going merger. The overall mass profile of the cluster can be well described by an NFW profile with a scale radius of r_s =309+-45 kpc and a concentration parameter of c=3.7+-0.5. The mass estimates from the lensing analysis are consistent with those from X-ray and Sunyaev-Zeldovich analyses. The predicted velocity dispersion is also in good agreement with the spectroscopic measurement from VLT observations. In the adopted WMAP cosmology, the total projected mass and the mass-to-light ratio within 1 Mpc are estimated to be 4.92+-0.44 10^14 solar mass and 95+-8 solar mass/solar luminosity, respectively.

On the fractal scaling of soil data. Particle-size distributions

Fractal geometry concepts have been widely applied as a tool for describing complex natural phenomena, in particular, in mechanics and physics of rocks and soils. The introduction of scaling has allowed a fractal representation of particle-size distributions (PSDs) of geological materials. In many cases, both log-linear and nonlinear regressions on empirical data have showed a clear deviation from the traditional fractal behaviour in terms of R 2 or standard error of estimates. The objectives of this work were (i) to partition PSDs collected from soils into two scaling domains using a piecewise fractal model, (ii) study the relationship between scaling exponents and soil texture (clay and/or sand content), and (iii) compare goodness-of-fit of traditional Weibull and normal distributions, and the proposed piecewise fractal function. A total of 140 PSDs were considered. They corresponded to five textural classes. The clay content ranged from 3% for sandy soils to 93% for very clay soils. Eighty-four PSDs (60%) showed two fractal scaling domains within the range 0–2.0 mm, and only five PSDs (3.6%) were fractal within this conventional domain. Very clayed texture soils were not fractal within the context of the present study ( D 1= D 2≅3), while clay texture soils were fractal only within the domain 0< δ 1≤0.2 mm according to the PSF model developed by Perrier et al. [Geoderma 88 (1999) 137]. Fractal dimensions of the first domain, D 1, correlated significantly well with clay content following a linear trend ( R 2=0.771, P<0.001), while those corresponding to the second domain, D 2, correlated negatively with sand for clay loam, loam, and sandy soils ( R 2=0.792, P<0.001), the constant c 1 correlated well with soil bulk density. In general, PSDs can present different behaviour depending on soil texture.

Comparison of source detection procedures for XMM-Newton images

Procedures based on current methods to detect sources in X-ray images are applied to simulated XMM-Newton images. All significant instrumental effects are taken into account, and two kinds of sources are considered -unresolved sources represented by the telescope PSF and extended ones represented by a β-profile model. Different sets of test cases with controlled and realistic input configurations are constructed in order to analyze the influence of confusion on the source analysis and also to choose the best methods and strategies to resolve the difficulties. In the general case of point-like and extended objects the mixed approach of multiresolution (wavelet) filtering and subsequent detection by SExtractor gives the best results. In ideal cases of isolated sources, flux errors are within 15-20% . The maximum likelihood technique outperforms the others for point-like sources when the PSF model used in the fit is the same as in the images. However, the number of spurious detections is quite large. The classification using the half-light radius and SExtractor stellarity index is successful in more than 98% of the cases. This suggests that average luminosity clusters of galaxies ( erg/s) can be detected at redshifts greater than 1.5 for moderate exposure times in the energy band below 5 keV, provided that there is no confusion or blending by nearby sources. We find also that with the best current available packages, confusion and completeness problems start to appear at fluxes around 6 10-16 erg/s/cm2 in [0.5-2] keV band for XMM-Newton deep surveys.

A Probability Description of the Yule-Nielsen Effect

The Yule-Nielsen effect, also called optical dot gain, has often been modeled based on convolutions between halftone dot patterns and a point spread function, PSF, characteristic of the paper. The form of the PSF is generally assumed or measured empirically. An alternative approach to modeling the Yule-Nielsen effect employs a probability function Pp, which describes the fraction of reflected light emerging between halftone dots and under dots. The probability model is shown to fit experimental data on the Yule-Nielsen effect quite well. Moreover, the model can be implemented with simple algebraic expressions rather than the convolution or Fourier calculations required for PSF models. In addition, the quantitative relationship between Pp and PSF is demonstrated.

CCD star images: On the determination of Moffat’s PSF shape parameters

Among the variety of empirical models of optical Point Spread Function used in the astronomical environment, only the Moffat’s (1969) one is able to describe by means of two parameters (in the circular case) both the inner and the outer star image regions. In view of this very important feature, the problem of the simultaneous estimates of Moffat’s PSF shape parameters, off-centring, and the background level in CCD star images has been investigated. The problem does not seem to be rigorously resolvable, but an approximate way to calculate all the parameters except off-centring is shown. It must be stressed that, the Moffat’s PSF model being a softened power law belonging to the family of modified King and Hubble models, the present discussion can be of aid in many other research fields. Also, the integral equation enabling us to convolve a spherical source with Moffat’s PSF is given and applied for comparison to Multi-Gaussian convolution.