Manuel A. Quijada

Exchange interaction effects on the optical properties of LuMnO3.

We have measured the optical conductivity of single crystal LuMnO3 from 10 to 45000 cm(-1) at temperatures between 4 and 300 K. A symmetry allowed on-site Mn d-d transition near 1.7 eV is observed to blueshift ( approximately 0.1 eV) in the antiferromagnetic state due to Mn-Mn superexchange interactions. Similar anomalies are observed in the temperature dependence of the TO phonon frequencies which arise from spin-phonon interaction. We find that the known anomaly in the temperature dependence of the quasistatic dielectric constant epsilon(0) below T(N) approximately 90 K is overwhelmingly dominated by the phonon contributions.

Photometry using the Infrared Array Camera on the Spitzer Space Telescope

We present several corrections for point-source photometry to be applied to data from the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. These corrections are necessary because of characteristics of the IRAC arrays and optics and the way the instrument is calibrated in flight. When these corrections are applied, it is possible to achieve a ~2% relative photometric accuracy for sources of adequate signal-to-noise ratio in an IRAC image.

Multiwalled carbon nanotubes for stray light suppression in space flight instruments

Observations of the Earth are extremely challenging; its large angular extent floods scientific instruments with high flux within and adjacent to the desired field of view. This bright light diffracts from instrument structures, rattles around and invariably contaminates measurements. Astrophysical observations also are impacted by stray light that obscures very dim objects and degrades signal to noise in spectroscopic measurements. Stray light is controlled by utilizing low reflectance structural surface treatments and by using baffles and stops to limit this background noise. In 2007 GSFC researchers discovered that Multiwalled Carbon Nanotubes (MWCNTs) are exceptionally good absorbers, with potential to provide order-of-magnitude improvement over current surface treatments and a resulting factor of 10,000 reduction in stray light when applied to an entire optical train. Development of this technology will provide numerous benefits including: a.) simplification of instrument stray light controls to achieve equivalent performance, b.) increasing observational efficiencies by recovering currently unusable scenes in high contrast regions, and c.) enabling low-noise observations that are beyond current capabilities. Our objective was to develop and apply MWCNTs to instrument components to realize these benefits. We have addressed the technical challenges to advance the technology by tuning the MWCNT geometry using a variety of methods to provide a factor of 10 improvement over current surface treatments used in space flight hardware. Techniques are being developed to apply the optimized geometry to typical instrument components such as spiders, baffles and tubes. Application of the nanostructures to alternate materials (or by contact transfer) is also being investigated. In addition, candidate geometries have been tested and optimized for robustness to survive integration, testing, launch and operations associated with space flight hardware. The benefits of this technology extend to space science where observations of extremely dim objects require suppression of stray light.

Performance and prospects of far ultraviolet aluminum mirrors protected by atomic layer deposition

Abstract. Metallic aluminum mirrors remain the best choice for high reflectance applications at ultraviolet wavelengths (90 to 320 nm) and maintain good performance through optical and infrared wavelengths. Transparent protective coatings are required to prevent the formation of an oxide layer, which severely degrades reflectance at wavelengths below 250 nm. We report on the development of atomic layer deposition (ALD) processes for thin protective films of aluminum fluoride that are viable for application at substrate temperatures <200°C. Reflectance measurements of aluminum films evaporated in ultrahigh vacuum conditions, and protected mirrors encapsulated with ALD AlF3 are used to evaluate the far ultraviolet (90 to 190 nm) and near ultraviolet (190 to 320 nm) performance of both the ALD material and the underlying metal. Optical modeling is used to predict the performance of optimized structures for future astronomical mirror applications.

Technology gap assessment for a future large-aperture ultraviolet-optical-infrared space telescope

Abstract. The Advanced Technology Large Aperture Space Telescope (ATLAST) team identified five key technology areas to enable candidate architectures for a future large-aperture ultraviolet/optical/infrared (LUVOIR) space observatory envisioned by the NASA Astrophysics 30-year roadmap, “Enduring Quests, Daring Visions.” The science goals of ATLAST address a broad range of astrophysical questions from early galaxy and star formation to the processes that contributed to the formation of life on Earth, combining general astrophysics with direct-imaging and spectroscopy of habitable exoplanets. The key technology areas are internal coronagraphs, starshades (or external occulters), ultra-stable large-aperture telescope systems, detectors, and mirror coatings. For each technology area, we define best estimates of required capabilities, current state-of-the-art performance, and current technology readiness level (TRL), thus identifying the current technology gap. We also report on current, planned, or recommended efforts to develop each technology to TRL 5.

Angle-of-incidence effects in the spectral performance of the infrared array camera of the Spitzer Space Telescope

The Infrared Array Camera (IRAC) on board the Spitzer Space Telescope uses two dichroic beamsplitters, four interference filters, and four detector arrays to acquire images in four different channels with nominal wavelengths of 3.6, 4.5, 5.8, and 8 μm for channels 1 through 4 respectively. A ray-tracing analysis of the IRAC optical system indicates a distribution of angles that is position-dependent at each optical element and the focal-plane arrays. For the band-pass filters in channels 1 and 2, the angle distribution relative to the filter surface normal is 0-28°, whereas for channels 3 and 4, the distribution is from 30° to 58°. Since these angle variations will cause changes in the center-band wavelengths for these interference filters that needed to be accounted for, we performed spectral performance measurements as a function of the angle of incidence on witness samples corresponding to each of the four filters and the two beamsplitters in the IRAC instrument. These measurements were done in the 2-10 μm wavelength range and at the temperature of 5 K, which is near the operating temperature of IRAC. Based on these filter measurements, we also performed an analysis of the pass-band wavelength distributions as a function of position on the instrument focal-plane array detectors. This information is necessary to attain the highest possible photometric accuracy when using IRAC for astronomical observations.

Technology Development for the Advanced Technology Large Aperture Space Telescope (ATLAST) as a Candidate Large UV-Optical-Infrared (LUVOIR) Surveyor

The Advanced Technology Large Aperture Space Telescope (ATLAST) team has identified five key technologies to enable candidate architectures for the future large-aperture ultraviolet/optical/infrared (LUVOIR) space observatory envisioned by the NASA Astrophysics 30-year roadmap, Enduring Quests, Daring Visions. The science goals of ATLAST address a broad range of astrophysical questions from early galaxy and star formation to the processes that contributed to the formation of life on Earth, combining general astrophysics with direct-imaging and spectroscopy of habitable exoplanets. The key technologies are: internal coronagraphs, starshades (or external occulters), ultra-stable large-aperture telescopes, detectors, and mirror coatings. Selected technology performance goals include: 1x10-10 raw contrast at an inner working angle of 35 milli-arcseconds, wavefront error stability on the order of 10 pm RMS per wavefront control step, autonomous on-board sensing and control, and zero-read-noise single-photon detectors spanning the exoplanet science bandpass between 400 nm and 1.8 μm. Development of these technologies will provide significant advances over current and planned observatories in terms of sensitivity, angular resolution, stability, and high-contrast imaging. The science goals of ATLAST are presented and flowed down to top-level telescope and instrument performance requirements in the context of a reference architecture: a 10-meter-class, segmented aperture telescope operating at room temperature (~290 K) at the sun-Earth Lagrange-2 point. For each technology area, we define best estimates of required capabilities, current state-of-the-art performance, and current Technology Readiness Level (TRL) – thus identifying the current technology gap. We report on current, planned, or recommended efforts to develop each technology to TRL 5.

Hemispherical reflectance and emittance properties of carbon nanotubes coatings at infrared wavelengths

Recent visible wavelength observations of Multiwalled Carbon Nanotubes (MWCNT) coatings have revealed that they represent the blackest materials known in nature with a Total Hemispherical Reflectance (THR) of less than 0.25%. This makes them exceptionally good as absorbers, with the potential to provide order-ofmagnitude improvement in stray-light suppression over current black surface treatments when used in an optical system. Here we extend the characterization of this class of materials into the infrared spectral region to further evaluate their potential for use on instrument baffles for stray-light suppression and to manage spacecraft thermal properties through radiant heat transfer process. These characterizations will include the wavelength-dependent Total Hemispherical Reflectance (THR) properties in the mid- and far-infrared spectral regions (2-110 μm). Determination of the temperature-dependent emittance will be investigated in the temperature range of 40 to 300 K. These results will be compared with other more conventional black coatings such as Acktar Fractal Black or Z306 coatings among others.

Aluminum Mirror Coatings for UVOIR Telescope Optics Including the Far UV

NASA Cosmic Origins (COR) Program identified the development of high reflectivity mirror coatings for large astronomical telescopes particularly for the far ultra violet (FUV) part of the spectrum as a key technology requiring significant materials research and process development. In this paper we describe the challenges and accomplishments in producing stable high reflectance aluminum mirror coatings with conventional evaporation and advanced Atomic Layer Deposition (ALD) techniques. We present the current status of process development with reflectance of ~ 55 to 80% in the FUV achieved with little or no degradation over a year.

Characterization of infrared filters for the wide-field camera 3 of Hubble Space Telescope

The Wide Field Camera 3 (WFC3) is a panchromatic imager that will be deployed in the Hubble Space Telescope (HST) in 2004. The mission of the WFC3 is to enhance HSTs imaging capability in the ultraviolet, visible and near-infrared spectral regions. Together with a wavelength coverage spanning 2000A to 1.7 microns, the WFC3 high sensitivity, high spatial resolution, and large field-of-view provide the astronomer with an unprecedented set of tools for exploring all types of exciting astrophysical terrain and for addressing many key questions is astronomy today. The filter compliment, which includes broad, medium, and narrow band filters, naturally reflects the diversity of astronomical programs to be targeted with WFC3. The WFC3 holds 61 UVIS filters elements, 14 IR filters, and 3 dispersive elements. Accurate and comprehensive knowledge of the optical performance of these components including its pass-band and out-of-band rejection behavior are necessary to verify that the instrument will meet its scientific objectives. The measured throughput curves are essential components in instrument performance models used to plan observations, and in calibration algorithms for removing the instrument signature from in-flight data. We will report on the normal incidence in-band and out-of-band transmittance of the IR filters measured near the operating temperature of -30 degree(s)C and additional tests used to characterizes the filters performance. Details of the characterization apparatus, that include an optical cryostat, and a grating spectrometer are discussed.