Shahram Shiri

Restorer: a visualization technique for handling missing data

Pseudocoloring is a frequently used technique in scientific visualization for mapping a color to a data value. When using pseudocolor and animation to visualize data that contain missing regions displayed as black or transparent, the missing regions popping in and out can distract the viewer from the more relevant information. Filling these gaps with interpolated data could lead to a misinterpretation of the data. The paper presents a method for combining pseudocoloring and grayscale in the same colormap. Valid data are mapped to colors in the colormap. The luminance values of the colors bounding areas of missing data are used in interpolating over these regions. The missing data are mapped to the grayscale portion of the colormap. This approach has the advantages of eliminating distracting gaps caused by missing data and distinguishing between those areas that represent valid data and those areas that do not. This approach was inspired by a technique used in the restoration of paintings.<<ETX>>

Poisson-Spot Intensity Reduction with a Partially-Transparent Petal-Shaped Optical Mask

The presence of a Poisson spot inside a shadow region can best be described as the consequence of constructive interference of light waves diffracted on the edge of the obstruction where its central position can be determined by the symmetry of the object. More recently, the elimination of this spot has received attention in the fields of particle physics, high-energy lasers, astronomy, and lithography. The desired level of intensity suppression is dependent on the type of light source and the field of application. In this paper, we introduce a novel, partially transparent petaled mask shape that suppresses the bright spot by up to ten orders of magnitude in intensity at optical ranges, with potential powerful applications in many of the above fields. The optimization technique formulated in this design can identify mask shapes having partial transparency only near the petal tips.

Fabrication and Test of an Optical Magnetic Mirror

Traditional mirrors at optical wavelengths use thin metalized or dielectric layers of uniform thickness to approximate a perfect electric field boundary condition. The electron gas in such a mirror configuration oscillates in response to the incident photons and subsequently re-emit fields where the propagation and electric field vectors have been inverted and the phase of the incident magnetic field is preserved. We proposed fabrication of sub-wavelength-scale conductive structures that could be used to interact with light at a nano-scale and enable synthesis of the desired perfect magneticfield boundary condition. In a magnetic mirror, the interaction of light with the nanowires, dielectric layer and ground plate, inverts the magnetic field vector resulting in a 0 degree phase shift upon reflection. Geometries such as split ring resonators and sinusoidal conductive strips were shown to demonstrate magnetic mirror behavior in the microwave [1] and then in the visible [2]. Work to design, fabricate and test a magnetic mirror began in 2007 at the NASA Goddard Space Flight Center (GSFC) under an Internal Research and Development (IRAD) award. Our initial nanowire geometry was sinusoidal but orthogonally asymmetric in spatial frequency, which allowed clear indications of its behavior by polarization. We report on the fabrication steps and testing of magnetic mirrors using a phase shifting interferometer and the first far-field imaging of an optical magnetic mirror.

Telescopes for space-based gravitational wave missions

Abstract. Space-based observation of gravitational waves promises to enable the study of a rich variety of high energy astrophysical sources in the 0.0001 to 1 Hz band using signals complementary to traditional electromagnetic waves. Gravitational waves represent the first new tool for studying the sky since gamma ray telescopes debuted in the 1970s, and we expect compelling science to be the result. The fundamental measurement is to monitor the path length difference between pairs of freely falling test masses with laser interferometry to a precision of picometers over gigameter baselines. The test masses are arranged in an equilateral triangle to allow simultaneous measurement of both gravitational wave polarizations. The heliocentric orbital space environment enables the test masses to be shielded from large ground motions at low frequencies, and allows the construction of long measurement baselines that are well matched to the signal wavelengths. Optical telescopes play an important role in the measurement because they deliver laser light efficiently from one spacecraft to another. The telescopes are directly in the measurement path, so there are additional performance requirements to support precision metrology beyond the usual requirements for good image formation.

An update on the role of systems modeling in the design and verification of the James Webb Space Telescope

The James Web Space Telescope (JWST) is a large, infrared-optimized space telescope scheduled for launch in 2014. The imaging performance of the telescope will be diffraction limited at 2μm, defined as having a Strehl ratio >0.8. System-level verification of critical performance requirements will rely on integrated observatory models that predict the wavefront error accurately enough to verify that allocated top-level wavefront error of 150 nm root-mean-squared (rms) through to the wave-front sensor focal plane is met. Furthermore, responses in several key disciplines are strongly crosscoupled. The size of the lightweight observatory structure, coupled with the need to test at cryogenic temperatures, effectively precludes validation of the models and verification of optical performance with a single test in 1-g. Rather, a complex series of incremental tests and measurements are used to anchor components of the end-to-end models at various levels of subassembly, with the ultimate verification of optical performance is by analysis using the assembled models. The assembled models themselves are complex and require the insight of technical experts to assess their ability to meet their objectives. This paper describes the modeling approach used on the JWST through the detailed design phase.

A vector diffraction model of wave propagation in a coronagraphic terrestrial planet finder

In this work we study vector electromagnetic wave propagation in a visible-light coronagraph for applications to the design and analysis of Terrestrial Planet Finder (TPF). A visible light coronagraph in TPF requires detection of a terrestrial planet which is ~1010 dimmer than the central stellar source. Consequently, any theory used to design and analyze TPF requires accuracy better than 10-10 in intensity or 10-5 in electric field. Current coronagraphic approaches to TPF have relied on scalar diffraction theory. However, the vector nature of light requires a vector approach to the problem. In this study we employ a time-harmonic vector theory to study the electromagnetic field propagation through metallic focal plane occulting mask on dielectric substrate. We use parallelized edge-based vector finite element model to compute the wave propagation in a three-dimensional tetrahedral grid representing the geometry of the coronagraph. The edge-based finite element method overcomes the problem of modal propagation and rigorously enforces the field divergence to be zero. The reflectivity and transmittivity in the geometry are computed through the gold metal in various shapes using a planar incident beam. Subsequently, the near-field beam diffraction around the mask is investigated.