High-contrast Imaging Data Research Articles

A novel encryption scheme for high-contrast image data in the Fresnelet domain.

In this paper, a unique and more distinctive encryption algorithm is proposed. This is based on the complexity of highly nonlinear S box in Flesnelet domain. The nonlinear pattern is transformed further to enhance the confusion in the dummy data using Fresnelet technique. The security level of the encrypted image boosts using the algebra of Galois field in Fresnelet domain. At first level, the Fresnelet transform is used to propagate the given information with desired wavelength at specified distance. It decomposes given secret data into four complex subbands. These complex sub-bands are separated into two components of real subband data and imaginary subband data. At second level, the net subband data, produced at the first level, is deteriorated to non-linear diffused pattern using the unique S-box defined on the Galois field . In the diffusion process, the permuted image is substituted via dynamic algebraic S-box substitution. We prove through various analysis techniques that the proposed scheme enhances the cipher security level, extensively.

PCA-based approach for subtracting thermal background emission in high-contrast imaging data

Aims.Ground-based observations at thermal infrared wavelengths suffer from large background radiation due to the sky, telescope and warm surfaces in the instrument. This significantly limits the sensitivity of ground-based observations at wavelengths longer than ~3 μm. The main purpose of this work is to analyse this background emission in infrared high-contrast imaging data as illustrative of the problem, show how it can be modelled and subtracted and demonstrate that it can improve the detection of faint sources, such as exoplanets. Methods. We used principal component analysis (PCA) to model and subtract the thermal background emission in three archival high-contrast angular differential imaging datasets in the M′ and L′ filter. We used an M′ dataset of β Pic to describe in detail how the algorithm works and explain how it can be applied. The results of the background subtraction are compared to the results from a conventional mean background subtraction scheme applied to the same dataset. Finally, both methods for background subtraction are compared by performing complete data reductions. We analysed the results from the M′ dataset of HD 100546 only qualitatively. For the M′ band dataset of β Pic and the L′ band dataset of HD 169142, which was obtained with an angular groove phase mask vortex vector coronagraph, we also calculated and analysed the achieved signal-to-noise ratio (S/N). Results. We show that applying PCA is an effective way to remove spatially and temporarily varying thermal background emission down to close to the background limit. The procedure also proves to be very successful at reconstructing the background that is hidden behind the point spread function. In the complete data reductions, we find at least qualitative improvements for HD 100546 and HD 169142, however, we fail to find a significant increase in S/N of β Pic b. We discuss these findings and argue that in particular datasets with strongly varying observing conditions or infrequently sampled sky background will benefit from the new approach.

Quantitative assessment of image motion blur in diffraction images of moving biological cells

Motion blur (MB) presents a significant challenge for obtaining high-contrast image data from biological cells with a polarization diffraction imaging flow cytometry (p-DIFC) method. A new p-DIFC experimental system has been developed to evaluate the MB and its effect on image analysis using a time-delay-integration (TDI) CCD camera. Diffraction images of MCF-7 and K562 cells have been acquired with different speed-mismatch ratios and compared to characterize MB quantitatively. Frequency analysis of the diffraction images shows that the degree of MB can be quantified by bandwidth variations of the diffraction images along the motion direction. The analytical results were confirmed by the p-DIFC image data acquired at different speed-mismatch ratios and used to validate a method of numerical simulation of MB on blur-free diffraction images, which provides a useful tool to examine the blurring effect on diffraction images acquired from the same cell. These results provide insights on the dependence of diffraction image on MB and allow significant improvement on rapid biological cell assay with the p-DIFC method.

NEW TECHNIQUES FOR HIGH-CONTRAST IMAGING WITH ADI: THE ACORNS-ADI SEEDS DATA REDUCTION PIPELINE

We describe Algorithms for Calibration, Optimized Registration, and Nulling the Star in Angular Differential Imaging (ACORNS-ADI), a new, parallelized software package to reduce high-contrast imaging data, and its application to data from the SEEDS survey. We implement several new algorithms, including a method to register saturated images, a trimmed mean for combining an image sequence that reduces noise by up to ~20%, and a robust and computationally fast method to compute the sensitivity of a high-contrast observation everywhere on the field-of-view without introducing artificial sources. We also include a description of image processing steps to remove electronic artifacts specific to Hawaii2-RG detectors like the one used for SEEDS, and a detailed analysis of the Locally Optimized Combination of Images (LOCI) algorithm commonly used to reduce high-contrast imaging data. ACORNS-ADI is written in python. It is efficient and open-source, and includes several optional features which may improve performance on data from other instruments. ACORNS-ADI requires minimal modification to reduce data from instruments other than HiCIAO. It is freely available for download at www.github.com/t-brandt/acorns-adi under a BSD license.

Pynpoint: an image processing package for finding exoplanets

We present the scientific performance results of pynpoint, our Python-based software package that uses principal component analysis to detect and estimate the flux of exoplanets in two-dimensional imaging data. Recent advances in adaptive optics and imaging technology at visible and infrared wavelengths have opened the door to direct detections of planetary companions to nearby stars, but image processing techniques have yet to be optimized. We show that the performance of our approach gives a marked improvement over what is presently possible using existing methods such as loci. To test our approach, we use real angular differential imaging (ADI) data taken with the adaptive optics-assisted high resolution near-infrared camera NACO at the VLT. These data were taken during the commissioning of the apodizing phase plate (APP) coronagraph. By inserting simulated planets into these data, we test the performance of our method as a function of planet brightness for different positions on the image. We find that in all cases pynpoint has a detection threshold that is superior to that given by our loci analysis when assessed in a common statistical framework. We obtain our best improvements for smaller inner working angles (IWAs). For an IWA of ∼0.29 arcsec we find that we achieve a detection sensitivity that is a factor of 5 better than loci. We also investigate our ability to correctly measure the flux of planets. Again, we find improvements over loci, with pynpoint giving more stable results. Finally, we apply our package to a non-APP data set of the exoplanet β Pictoris b and reveal the planet with high signal-to-noise. This confirms that pynpoint can potentially be applied with high fidelity to a wide range of high-contrast imaging data sets.

New observations of the PZ Tel system, its substellar companion and debris disc

We present follow-up high-contrast imaging data of PZ Tel B, the substellar companion of a solar analogue pre-main-sequence star and member of the approximately 12-Myr-old β Pic moving group. Between 2010 October and 2011 June, we observed PZ Tel in three observing epochs with NACO/ESO-VLT, which proves the companionship of PZ Tel B on a very high significance level and shows that the photometry of the PZ Tel system is stable on the 10 per cent level. The orbital motion of PZ Tel B relative to its primary is clearly detected between all observing epochs, and we even find evidence for deceleration of its orbital motion, as expected for an object on a Keplerian orbit moving towards its apastron. We also present new photometric measurements of PZ Tel just recently obtained by WISE, which clearly confirm that there is no excess emission in the spectral energy distribution (SED) of the star up to 24 μ m, while a prominent excess is detected at 70 μm with MIPS/Spitzer. To quantify the orbit elements of the PZ Tel system, we applied a least-squares Monte Carlo (LSMC) fit on all astrometric measures taken with NACO. The results of the performed LSMC fit are that one cannot yet determine a single ‘best'-fitting orbital solution, due to the poor astrometric coverage of the orbit. However, the obtained good LSMC solutions () give an overview of the distributions and correlations of all orbit elements. According to our LSMC fit, PZ Tel is an eccentric binary (e ≳ 0.6), and the orbit elements degenerate with growing eccentricity. For e < 0.95, the PZ Tel system is seen nearly edge-on (92° ≲ i ≲ 110°), has a longitude of the ascending node of 50° ≲ Ω ≲ 65°, an argument of periastron 170° ≲ ω ≲ 290° and exhibits a semimajor axis a larger than about 20 au, with the highest peak in the a-histogram located at about 25 au (or about 110 yr of orbital period), up to a few tens of au, as orbital solutions with a significantly larger semimajor axis would not be able to harbour a circumbinary disc which excites the observed excess in the SED of PZ Tel A.

HIGH-CONTRAST 3.8 μm IMAGING OF THE BROWN DWARF/PLANET-MASS COMPANION TO GJ 758

We present L' band (3.8 $\mu m$) MMT/Clio high-contrast imaging data for the nearby star GJ 758, which was recently reported by Thalmann et al. (2009) to have one -- possibly two-- faint comoving companions (GJ 758B and ``C", respectively). GJ 758B is detected in two distinct datasets. Additionally, we report a \textit{possible} detection of the object identified by Thalmann et al as ``GJ 758C" in our more sensitive dataset, though it is likely a residual speckle. However, if it is the same object as that reported by Thalmann et al. it cannot be a companion in a bound orbit. GJ 758B has a H-L' color redder than nearly all known L--T8 dwarfs. Based on comparisons with the COND evolutionary models, GJ 758B has T$_{e}$ $\sim$ 560 K$^{^{+150 K}_{-90K}}$ and a mass ranging from $\sim$ 10--20 M$_{J}$ if it is $\sim$ 1 Gyr old to $\sim$ 25--40 M$_{J}$ if it is 8.7 Gyr old. GJ 758B is likely in a highly eccentric orbit, e $\sim$ 0.73$^{^{+0.12}_{-0.21}}$, with a semimajor axis of $\sim$ 44 AU$^{^{+32 AU}_{-14 AU}}$. Though GJ 758B is sometimes discussed within the context of exoplanet direct imaging, its mass is likely greater than the deuterium-burning limit and its formation may resemble that of binary stars rather than that of jovian-mass planets.

Characterizing the mesencephalon using susceptibility-weighted imaging.

The mesencephalon is involved in a number of human neurodegenerative disorders and has been typically imaged with T1-, T2- and T2*-weighted methods. Our aim was to collect high-contrast susceptibility-weighted imaging (SWI) data to differentiate among and within the basic mesencephalic structures: namely, the red nucleus, substantia nigra, and crus cerebri. High-resolution SWI, 3D T1-weighted, and T2-weighted data were collected to study contrast in the mesencephalon at 1.5T and 4T. Contrast between structures was calculated for SWI high-pass (HP)-filtered-phase, T1 gradient-echo, and spin-echo T2-weighted data. SWI HP-filtered-phase data revealed similar contrast for the red nucleus and substantia nigra when compared with T2-weighted imaging. However, SWI was able to show structures within the red nucleus, substantia nigra, and medial geniculate body that were invisible on T2-weighted imaging. T1-weighted imaging, on the other hand, did not reveal measurable contrast for any of the structures of interest. SWI HP-filtered-phase data at 4T agreed well with india ink-stained cadaver brain studies, which appear to correlate with capillary density. With SWI, it is possible to create better anatomic images of the mesencephalon, with improved contrast compared with conventional T1- or T2-weighted sequences.

NACO polarimetric differential imaging of TW Hya

We present high-contrast imaging data on the disk of the classical T Tauri star TW Hya. The images were obtained through the polarimetric differential imaging technique with the adaptive optics system NACO. Our commissioning data show the presence of polarized disk emission between 0.1´´ and 1.4´´ from the star. We derive the first Ks-band radial polarized intensity distribution. We show that the polarized intensity compares well to shorter wavelengths surface brightness observations and confirm the previously reported gradual slope change around 0.8´´. These results show the potential of the new polarimetric differential imaging technique at 8 m-class telescopes to map the inner regions of protoplanetary disks.