Using NASA Goddard Space Flight Center’s Cryogenic High Accuracy Refraction Measuring System (CHARMS), we
measured absolute refractive indices for three infrared glasses from Ohara for lens designs for instruments at two of the
world’s largest, ground-based, astronomical observatories – the present W.M. Keck Observatory and the future Giant
Magellan Telescope (GMT). MOSFIRE (Keck), a near-infrared multi-object spectrograph and wide-field camera, has
demonstrated diffraction limited performance at 120 K in part owing to our absolute refractive index measurements of
Ohara S-FPL51 and S-FTM16 covering wavelengths and temperatures from 0.5 to 2.6 μm and 30 to 300 K, respectively.
Measured index uncertainties range from 0.7–3.5E-5 and 0.7–2.9E-5 for S-FPL51 and S-FTM16, respectively,
depending on wavelength and temperature, and for the latter on which test prism. NIRMOS (GMT), a near infrared
multiple object imager/spectrograph, uses S-TIM28 in its imaging lens design. We measured S-TIM28’s indices for
wavelengths and temperatures from 0.40 to 2.8 μm and 25 to 300 K, respectively with uncertainties ranging from 1.6–
3.0E-5. Absolute indices and their wavelength and temperature derivatives for these infrared glasses are reported along
with coefficients for temperature-dependent Sellmeier fits of the measured index data to enable accurate computation of
index to other wavelengths and temperatures. We compare our measurements to those in the literature.
This paper summarizes a comparison of pre- and post-flight optical performance on optical components (mostly
mirrors) from the Corrective Optics Space Telescope Axial Replacement (COSTAR) instrument and the Wide
Field Planetary Camera 2 (WF/PC 2) pickoff mirror. These measurements were carried out after both the
COSTAR and WF/PC 2 were retrieved from the Hubble Space Telescope in May of 2009 and returned to GSFC
in July of 2009. Both of these instruments had a highly UV-reflecting coating of Al with a MgF2 layer on
top for protection on their reflecting optics. We studied these in order to document the aging process on these
coatings while in space for more than 15.5 years. When compared to data before flight and witness coupons
kept on the ground, we find a severely degraded UV performance for the coatings that flew in space, particularly
at the Lyman-α wavelength. Based on similar observations seen earlier on the WF/PC1 POM, the current
degradation, of the latest optical components removed from HST, are a result of outgassing of substances such
as hydrocarbons and silicone from nearby hardware on the spacecraft and UV light that photo-polymerize those
materials on the mirror surfaces.
We have designed, fabricated, and tested compact radiative control structures, including antireflection coatings and
resonant absorbers, for millimeter through submillimeter wave astronomy. The antireflection coatings consist of micromachined
single crystal silicon dielectric sub-wavelength honeycombs. The effective dielectric constant of the structures
is set by the honeycomb cell geometry. The resonant absorbers consist of pieces of solid single crystal silicon substrate
and thin phosphorus implanted regions whose sheet resistance is tailored to maximize absorption by the structure. We
present an implantation model that can be used to predict the ion energy and dose required for obtaining a target implant
layer sheet resistance. A neutral density filter, a hybrid of a silicon dielectric honeycomb with an implanted region, has
also been fabricated with this basic approach. These radiative control structures are scalable and compatible for use
large focal plane detector arrays.
Astronomical dust is observed in a variety of astrophysical environments and plays an important role in radiative
processes and chemical evolution in the galaxy. Depending upon the environment, dust can be either carbon-rich or
oxygen-rich (silicate grains). Both astronomical observations and ground-based data show that the optical properties of
silicates can change dramatically with the crystallinity of the material, and recent laboratory research provides evidence
that the optical properties of silicate dust vary as a function of temperature as well. Therefore, correct interpretation of a
vast array of astronomical data relies on the understanding of the properties of silicate dust as functions of wavelength,
temperature, and crystallinity. The OPASI-T (Optical Properties of Astronomical Silicates with Infrared Techniques)
project addresses the need for high quality optical characterization of metal-enriched silicate condensates using a variety
of techniques. A combination of both new and established experiments are used to measure the extinction, reflection,
and emission properties of amorphous silicates across the infrared (near infrared to millimeter wavelengths), providing a
comprehensive data set characterizing the optical parameters of dust samples. We present room temperature
measurements and the experimental apparatus to be used to investigate and characterize additional metal-silicate
materials.
Correct interpretation of a vast array of astronomical data relies heavily on understanding the properties of silicate dust
as a function of wavelength, temperature, and crystallinity. We introduce the OPASI-T (Optical Properties of
Astronomical Silicates with Infrared Techniques) project to address the need for high fidelity optical characterization
data on the various forms of astronomical dust. We use two spectrometers to provide extinction data for silicate samples
across a wide wavelength range (from the near infrared to the millimeter). New experiments are in development that will
provide complementary information on the emissivity of our samples, allowing us to complete the optical
characterization of these dust materials. In this paper, we present initial results from several materials including
amorphous iron silicate, magnesium silicate and silica smokes, over a wide range of temperatures, and discuss the design
and operation of our new experiments.
These studies consist of measuring the frequency dependent transmittance (T(ω)) and reflectance (R(ω)) above
and below the optical band-gap in the UV/Visible and infrared frequency ranges for Cd1-xZnxTe materials for
x=0 and x=0.04. Measurements were also done in the temperature range from 5 to 300 K. The results show that
the optical gap near 1.49 eV at 300 K increases to 1.62 eV at 5 K. Finally, we observe sharp absorption peaks near
this gap energy at low temperatures for the x=0.04 sample. The close proximity of these peaks to the optical
transition threshold suggests that they originate from the creation of bound electron-hole pairs or excitons. The
decay of these excitonic absorptions may contribute to a photoluminescence and transient background response
of these back-illuminated HgCdTe CCD detectors.
We discuss a new type of direct detector, a silicon hot-electron bolometer, for measurements in the far-infrared and submillimeter spectral ranges. High performance bolometers can be made using the electron-phonon conductance in heavily doped silicon to provide thermal isolation from the cryogenic bath. Noise performance is expected to be near thermodynamic limits, allowing background limited performance for many far infrared and submillimeter photometric and spectroscopic applications. We report measurements of device I-V characteristics and terahertz surface impedance.
There currently exists a great void in high quality, cryogenic, infrared (IR) refractive index data, even for the most common IR optical materials. Meanwhile, as the designs of many future refractive IR optical systems and instruments will rely critically on very accurate knowledge of the indices of refraction of their constituent optical components at design operating temperatures, there has been increasing demand for such data within the IR community. We present our progress to date in the design and construction of a Cryogenic, High-Accuracy Refraction Measuring System (CHARMS), which will measure absolute indices of refraction accurate to better than ±1 x 10-5. We will operate at wavelengths from 0.105 μm in the far ultraviolet to 20 μm in the mid-IR for sample temperatures ranging from near absolute zero to somewhat above room temperature. Technical challenges, accomplishments, and component developments necessary for successful implementation of the refractometer are discussed. We also present component level accuracy measurements and initial ambient index of refraction measurements for fused silica.
D. Mott, Richard Barclay, Alexander Bier, Tina Chen, Barbara DiCamillo, Drake Deming, Matthew Greenhouse, Ross Henry, Tilak Hewagama, Mindy Jacobson, Manuel Quijada, Shobita Satyapal, D. Schwinger
Micromachined Fabry-Perot tunable filters with a large clear aperture (12.5 to 40 mm) are being developed as an optical component for wide-field imaging spectroscopy. This program applies silicon micromachining fabrication techniques to miniaturize Fabry-Perot filters for astronomical science instruments. The filter assembly consists of two reflector plates that form a tunable Fabry-Perot etalon. One plate is fixed and the second plate is free to move along the optical axis on silicon springs. The moving plate is actuated electrostatically by capacitance pads on the stationary and moving plates. To reduce mass, both reflectors are fabricated by applying optical coatings to a thin freestanding silicon nitride film held flat in drumhead tension. In this paper, we discuss the etalon design, electromechanical modeling, fabrication, and initial results. In the current design, the transmission aperture is 11.0 mm in diameter, the moving plate is 26.3 mm in diameter, and the stationary plate is 32.6 mm in diameter. The plates and springs are nominally 350 μm thick, the electrical and mechanical spacing between plates is 18 μm, and the uncoated optical spacing is 15 μm.
Tilak Hewagama, Richard Barclay, Tina Chen, Drake Deming, Cedric Goukenleuque, Matthew Greenhouse, Ross Henry, Mindy Jacobson, Brent Mott, Shobita Satyapal, David Schwinger
We use analytical models to characterize the optical response of
observing systems and explore spectroscopic techniques that exploit
the planetary spectral signature to enhance the planet-to-star
contrast in imaging. Radiative transfer model calculations of
gas-giant extrasolar planets are employed in selecting spectral
regions where the planetary signal shows enhancement with respect to
the stellar spectrum. We specifically discuss the use of frequency switching, in conjunction with pupil plane techniques (shaping, apodizing), to optimize the detection of the planetary signal of potential planetary systems for both ground-based and space-borne observations. Expected properties of known extrasolar planets motivate the use of the near-IR spectral region. This work was motivated by the focused technology development of a MEMS tunable filter, and the application of such devices to frequency-switched imaging spectroscopy.
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