Douglas B. Leviton

Advanced camera for the Hubble Space Telescope

The Advanced Camera for the Hubble Space Telescope has three cameras. The first, the Wide Field Camera, will be a high- throughput, wide field, 4096 X 4096 pixel CCD optical and I-band camera that is half-critically sampled at 500 nm. The second, the High Resolution Camera (HRC), is a 1024 X 1024 pixel CCD camera that is critically sampled at 500 nm. The HRC has a 26 inch X 29 inch field of view and 29 percent throughput at 250 nm. The HRC optical path includes a coronagraph that will improve the HST contrast near bright objects by a factor of approximately 10 at 900 nm. The third camera, the solar-blind camera, is a far-UV, pulse-counting array that has a relatively high throughput over a 26 inch X 29 inch field of view. The advanced camera for surveys will increase HSTs capability for surveys and discovery by a factor of approximately 10 at 800 nm.

Temperature-dependent refractive index of silicon and germanium

Silicon and germanium are perhaps the two most well-understood semiconductor materials in the context of solid state device technologies and more recently micromachining and nanotechnology. Meanwhile, these two materials are also important in the field of infrared lens design. Optical instruments designed for the wavelength range where these two materials are transmissive achieve best performance when cooled to cryogenic temperatures to enhance signal from the scene over instrument background radiation. In order to enable high quality lens designs using silicon and germanium at cryogenic temperatures, we have measured the absolute refractive index of multiple prisms of these two materials using the Cryogenic, High-Accuracy Refraction Measuring System (CHARMS) at NASAs Goddard Space Flight Center, as a function of both wavelength and temperature. For silicon, we report absolute refractive index and thermo-optic coefficient (dn/dT) at temperatures ranging from 20 to 300 K at wavelengths from 1.1 to 5.6 μm, while for germanium, we cover temperatures ranging from 20 to 300 K and wavelengths from 1.9 to 5.5 μm. We compare our measurements with others in the literature and provide temperature-dependent Sellmeier coefficients based on our data to allow accurate interpolation of index to other wavelengths and temperatures. Citing the wide variety of values for the refractive indices of these two materials found in the literature, we reiterate the importance of measuring the refractive index of a sample from the same batch of raw material from which final optical components are cut when absolute accuracy greater than ±5 x 10-3 is desired.

Temperature-dependent absolute refractive index measurements of synthetic fused silica

Using the Cryogenic, High-Accuracy Refraction Measuring System (CHARMS) at NASAs Goddard Space Flight Center, we have measured the absolute refractive index of five specimens taken from a very large boule of Corning 7980 fused silica from temperatures ranging from 30 to 310 K at wavelengths from 0.4 to 2.6 microns with an absolute uncertainty of ±1 ×10-5. Statistical variations in derived values of the thermo-optic coefficient (dn/dT) are at the ±2 × 10-8/K level. Graphical and tabulated data for absolute refractive index, dispersion, and thermo-optic coefficient are presented for selected wavelengths and temperatures along with estimates of uncertainty in index. Coefficients for temperature-dependent Sellmeier fits of measured refractive index are also presented to allow accurate interpolation of index to other wavelengths and temperatures. We compare our results to those from an independent investigation (which used an interferometric technique for measuring index changes as a function of temperature) whose samples were prepared from the same slugs of material from which our prisms were prepared in support of the Kepler mission. We also compare our results with sparse cryogenic index data from measurements of this material from the literature.

Wide-field imaging interferometry testbed I: purpose, testbed design, data, and synthesis algorithms

The Wide-field Imaging Interferometry Testbed was designed to validate, experiment with, and refine the technique of wide field mosaic imaging for optical/IR interferometers. We offer motivation for WIIT, present the testbed design, and describe algorithms that can be used to reduce the data from a spatial and spectral Michelson interferometer. A conventional single-detector Michelson interferometer operating with narrow bandwidth at center wavelength lc is limited in its field of view to the primary beam of the individual telescope apertures, or ~λc/dtel radians, where dtel is the telescope diameter. Such a field is too small for many applications; often one wishes to image extended sources. We are developing and testing techniques analogous to the mosaicing method employed in millimeter and radio astronomy, but applicable to optical/IR Michelson interferometers, in which beam combination is done in the pupil plane. An Npix × Npix array detector placed in the image plane of the interferometer is used to record simultaneously the fringe patterns from many contiguous telescope fields, effectively multiplying the field size by Npix/2, where the factor 2 allows for Nyquist sampling. This technique will be especially valuable for interferometric space observatories, such as the Space Infrared Interferometric Telescope and the Submillimeter Probe of the Evolution of Cosmic Structure.

Design of a cryogenic high-accuracy absolute prism refractometer for infrared through far-ultraviolet optical materials

The next generation of cryogenic, infrared (IR), space optical instrumentation (for NGST and other missions) will require a knowledge of refractive indices for constituent optical materials to a level of accuracy which is not currently attainable. The rationale for and design of a broadband, absolute, prism refractometer for measuring refractive index at cryogenic temperatures to very high absolute accuracy is discussed. The refractometer design also permits similar measurements through the far ultraviolet where accurate refractive index data are scarce for most UV optical materials. The technical challenges in achieving high accuracy in these wavelength regions and at extremely cold temperatures are presented, along with novel solutions under development to meet those challenges.

Performance of ion-figured silicon carbide SUMER telescope mirror in the vacuum ultraviolet

Measured and theoretical encircled energy and small-angle scatter of the telescope mirror (SST) of the solar ultraviolet measurements of emitted radiation (SUMER) instrument are compared at the wavelength of 123.6 nm. Mirror performance modeling was accomplished with the Optical Surface Analysis Code software package. The modeling is based on measured mirror-surface figure error data and roughness characteristics covering all important spatial frequencies that affect imaging in the vacuum ultraviolet wavelength region. Mirror-surface errors were measured with a Zygo Mark IV interferometer, Bauer Model 200 Profiler, and WYKO Topo 2-D (two-dimensional) interferometer. Performance of the SST mirror, including encircled energy and small-angle scatter, was also directly measured. A good agreement is found between measured and theoretical encircled energy within 6 arcsec and small-angle scatter up to ~50 arcmin from the peak. The 80% encircled energy diameter of the SST mirror is ~1.9 arcsec, and the amount of scattered light drops to approximately 1.0 × 10(-10) of peak irradiance (normalized to 1 arcsec(2) in the focal plane) 50 arcmin from the peak. Vacuum ultraviolet performance of the mirror is degraded primarily by midfrequency errors.

Temperature-dependent refractive index of CaF2 and Infrasil 301

In order to enable high quality lens designs using calcium fluoride (CaF2) and Heraeus Infrasil 301 (Infrasil) for cryogenic operating temperatures, we have measured the absolute refractive index of these two materials as a function of both wavelength and temperature using the Cryogenic, High-Accuracy Refraction Measuring System (CHARMS) at NASAs Goddard Space Flight Center. For CaF2, we report absolute refractive index and thermo-optic coefficient (dn/dT) at temperatures ranging from 25 to 300 K at wavelengths from 0.4 to 5.6 μm, while for Infrasil, we cover temperatures ranging from 35 to 300 K and wavelengths from 0.4 to 3.6 μm. For CaF2, we compare our index measurements to measurements of other investigators. For Infrasil, we compare our measurements to the material manufacturers data at room temperature and to cryogenic measurements for fused silica from previous investigations including one of our own. Finally, we provide temperature-dependent Sellmeier coefficients based on our measured data to allow accurate interpolation of index to other wavelengths and temperatures.

The wide-field imaging interferometry testbed

We are developing a Wide-Field Imaging Interferometry Testbed (WIIT) in support of design studies for NASAs future space interferometry missions, in particular the SPIRIT and SPECS far-infrared/submillimeter interferometers. WIIT operates at optical wavelengths and uses Michelson beam combination to achieve both wide-field imaging and high-resolution spectroscopy. It will be used chiefly to test the feasibility of using a large-format detector array at the image plane of the sky to obtain wide-field interferometry images through mosaicing techniques. In this setup each detector pixel records interferograms corresponding to averaging a particular pointing range on the sky as the optical path length is scanned and as the baseline separation and orientation is varied. The final image is constructed through spatial and spectral Fourier transforms of the recorded interferograms for each pixel, followed by a mosaic/joint-deconvolution procedure of all the pixels. In this manner the image within the pointing range of each detector pixel is further resolved to an angular resolution corresponding to the maximum baseline separation for fringe measurements. We present the motivation for building the testbed, show the optical, mechanical, control and data system design, and describe the image processing requirements and algorithms. WITT is presently under construction at NASAs Goddard Space Flight Center.

Laboratory studies of petal-shaped occulters

We present laboratory studies of scaled occulting starshades for the New Worlds Observer (NWO). A deep reactive ion etched silicon starshade has been fabricated by NIST, designed to cover the same number of Fresnel zones as in the proposed mission. The broadband shadow is mapped with a photometer in a dark vacuum tunnel fed by a heliostat at HAO. CCD images provide direct contrast measurements of different features around the starshade. Preliminary measurements reach 5x10-6 suppression in the center of the shadow at the focal plane. The two-dimensional structure of the starshade diffraction pattern is compared to that produced by the Fresnel integral.

Automation, operation, and data analysis in the cryogenic, high accuracy, refraction measuring system (CHARMS)

The Cryogenic High Accuracy Refraction Measuring System (CHARMS) at NASAs Goddard Space Flight Center has been enhanced in a number of ways in the last year to allow the system to accurately collect refracted beam deviation readings automatically over a range of temperatures from 15 K to well beyond room temperature with high sampling density in both wavelength and temperature. The engineering details which make this possible are presented. The methods by which the most accurate angular measurements are made and the corresponding data reduction methods used to reduce thousands of observed angles to a handful of refractive index values are also discussed.