Stanley J. Wellard

Far-infrared spectroscopy of the troposphere (FIRST): sensor development and performance drivers

The radiative balance of the troposphere, and hence global climate, is dominated by the infrared absorption and emission of water vapor, particularly at far-infrared (far-IR) wavelengths from 15-50 μm. Water vapor is the principle absorber and emitter in this region. The distribution of water vapor and associated far-IR radiative forcings and feedbacks are widely recognized as major uncertainties in our understanding of current and the prediction of future climate. Cirrus clouds modulate the outgoing longwave radiation (OLR) in the far-IR, and up to half of the OLR from the Earth occurs beyond 15.4 μm (650 cm-1). Current and planned operational and research satellites observe the mid-infrared to only about 15.4 μm, leaving space or airborne spectral measurement of the far-IR region unsupported. NASA is now developing the technology required to make regular far-IR measurements of the Earth’s atmosphere possible. Far InfraRed Spectroscopy of the Troposphere (FIRST) is being developed for NASA’s Instrument Incubator Program under the direction of the Langley Research Center. The objective of FIRST is to provide a balloon-based demonstration of the key technologies required for a space-based sensor. We discuss the FIRST Fourier transform spectrometer system (0.6 cm-1 unapodized resolution), along with radiometric calibration techniques in the spectral range from 10 to 100 μm (1000 to 100 cm-1). FIRST will incorporate a broad bandpass beamsplitter, a cooled (~180 K) high throughput optical system, and an image type detector system. The FIRST performance goal is a NEΔT of 0.2 K from 10 to 100 μm.

Far infrared spectroscopy of the troposphere (FIRST): flight performance and data processing

The radiative balance of the troposphere, and hence global climate, is dominated by the infrared absorption and emission of water vapor, particularly at far-infrared (far-IR) wavelengths from 15-50 μm. Current and planned satellites observe the infrared region to about 15.4 μm, ignoring spectral measurement of the far-IR region from 15 to 100μm. The far-infrared spectroscopy of the troposphere (FIRST) project, flown in June 2005, provided a balloon-based demonstration of the two key technologies required for a space-based far-IR spectral sensor. We discuss the FIRST Fourier transform spectrometer system (0.6 cm-1 unapodized resolution), its radiometric calibration in the spectral range from 10 to 100 μm, and its performance and science data from the flight. Two primary and two secondary goals are given and data presented to show the goals were achieved by the FIRST flight.

Cryogenic Michelson Interferometer Spectrometer For Space Shuttle Application

A Michelson interferometer spectrometer using a flexural pivot suspension for the moving mirror was fabricated for use at 20° K as part of the CIRRIS 1A experiment. The spectral range 2.5 to 25 μm is achieved using a potassium bromide beamsplitter. The softness of the beamsplitter material required special mounting care to preserve beamsplitter flatness while undergoing the strain of being cooled and the shock and vibration of the shuttle launch. Five various sized elements in the arsenic-doped-silicon focal plane provide for tradeoff of sensitivity, spatial and spectral resolution capabilities. A redundant position reference system uses optical fibers to couple optical power from HeNe lasers through the vacuum/cryogenic interface into the interferometer optics where it travels antiparallel to the infrared signal. An alignment system using geared stepper motors provides capability of realignment in space. An eight-position filter wheel is used to enhance the out-of-band signal rejection to enhance high sensitivity in the presence of strong infrared emitters. The interferometer is mounted inside of a high-off-axis rejection telescope to enable measurement of the earth limb emissions. The telescope is cooled to 20° K using supercritical liquid helium.

A field-widened spectrometer-interferometer: back from the past to measure ionospheric-thermospheric energetics

Recent broadband observations by the SABER sensor aboard the TIMED satellite hint at intriguing new vibrationrotation excitation and loss processes that occur in the energy dissipation of the ionosphere-thermosphere as it responds to solar storms. To address the questions exposed by the SABER data, SDLs field-widened interferometer has been brought back after three decades to again fly into or above aurorally disturbed atmosphere to gain the data needed to better understand the different processes of ionosphere-thermosphere energetics. The paper discusses the evaluation and design phases (laboratory evaluation, a rocket flight, and a satellite flight) needed to prepare this elegant and unique interferometer to reach its goal of making high resolution (0.5 cm-1) and wide bandwidth (1300- 8000 cm-1) measurements of the ionosphere-thermosphere world-wide. Design details of interferometer will be presented along with comparisons between a standard Michelson interferometer and the field-widened sensor to illustrate just how the Bounchareine and Connes field-widened form provides the enhanced performance needed for the new missions. The paper also describes how the improved Inferometer design will leverage advances in modern electronics, detectors, bearing design and software to gain significant improvements in the performance of the upgraded field-widened interferometer-spectrometer when compared to the heritage instrument.

Far-infrared Spectroscopy of the Troposphere (FIRST): sensor calibration performance

The radiative balance of the troposphere, and hence global climate, is dominated by the infrared absorption and emission of water vapor, particularly at far-infrared (far-IR) wavelengths from 15-50 μm. Water vapor is the principal absorber and emitter in this region. The distribution of water vapor and associated far-IR radiative forcings and feedbacks are widely recognized as major uncertainties in our understanding of current and the prediction of future climate. Cirrus clouds modulate the outgoing longwave radiation (OLR) in the far-IR. Up to half of the OLR from the Earth occurs beyond 15.4 μm (650 cm-1). Current and planned operational and research satellites observe the midinfrared to only about 15.4 μm, leaving space or airborne spectral measurement of the far-IR region unsupported. NASA has now developed the sensor required to make regular far-IR measurements of the Earths atmosphere possible. Far InfraRed Spectroscopy of the Troposphere (FIRST) was developed for NASAs Instrument Incubator Program under the direction of the Langley Research Center. The objective of FIRST is to provide a balloon-based demonstration of the key technologies required for a space-based sensor. The FIRST payload will also be proposed for science flights in support of validation of the various experiments on the Earth Observing System (EOS). We discuss the FIRST Fourier transform spectrometer system (0.6 cm-1 unapodized resolution), along with its radiometric calibration in the spectral range from 10 to 100 µm (1000 to 100 cm-1). FIRST incorporates a broad bandpass beamsplitter, a cooled (~180 K) high throughput optical system, and an image type detector system. We also discuss the actual performance of the FIRST instrument relative to its performance goal of a NE(delta)T of 0.2 K from 10 to 100 μm.

Cryogenic Michelson interferometer on the space shuttle

A helium-cooled interferometer was flown aboard shuttle flight STS-39. This interferometer, along with its sister radiometer, set new benchmarks for the quantity and quality of data collected. The interferometer generated approximately 150,000 interferograms during the course of the flight. Data was collected at tangent heights from the earths surface to celestial targets. The interferograms encoded spectral data from aurora, earth limb, and earth terminator scenes. The interferometer collected data at resolutions of 8, 4, and 1 wavenumbers over a spectral range of 2 to 25 micrometers. The interferometers optics, detectors and preamps, laser reference system, realignment system, and eight-position optical filter wheel are described as they performed on-orbit.

If EM waves don't interfere, what causes interferograms?

Photonics engineers involved in designing and operating Fourier transform spectrometers (FTS) often rely on Maxwell’s wave equations and time-frequency (distance-wavenumber) Fourier theory as models to understand and predict the conversion of optical energy to electrical signals in their instruments. Dr. Chandrasekhar Roychoudhuri and his colleagues, at last year’s conference, presented three significant concepts that might completely change the way we comprehend the interaction of light and matter and the way interference information is generated. The first concept is his non-interaction of waves (NIW) formulation, which puts in place an optical wave description that more accurately describe the properties of the finite time and spatial signals of an optical system. The second is a new description for the cosmic EM environment that recognizes that space is really filled with the ether of classical electromagnetics. The third concept is a new metaphysics or metaphotonics that compares the photon as a particle in a void against the photon as a wave in a medium to see which best explain the twelve different aspects of light. Dr. Henry Lindner presents a compelling case that photons are waves in a medium and particles (electrons, protons, atoms) are wave-structures embedded in the new ether. Discussion of the three new principles is intended to increase the curiosity of photonics engineers to investigate these changes in the nature of light and matter.

Far infrared spectroscopy of the troposphere (FIRST): sensor concept

FIRST (a NASA Instrument Incubator Program) is a balloon-based demonstration of a space-based sensor to measure the Earth’s thermal infrared at high spatial and spectral resolution. The radiative balance of the troposphere, and hence climate, is dominated by the infrared absorption and emission of water vapor, particularly at far-infrared (far-IR) wavelengths longer than 15 µm (650 cm-1), the distribution of water vapor and its far-IR radiative forcings and feedbacks are major uncertainties in understanding and predicting future climate. However, far-IR emission (spectra of band-integrated) has rarely been directly measured from space platforms. FIRST will be a Fourier Transform Spectrometer (FTS) with radiometric calibration in the spectral range from 10 to 100 µm (1000 to 100 cm-1) at 0.6 cm-1 unapodized resolution. It will incorporate a broad bandpass beamsplitters and a high-throughput optical and detector system. FIRST has a NEΔT performance goal of 0.2K from 10 to 100 µm. The spectral resolution will allow simultaneous retrievals of temperature and water vapor profiles. A 10 × 10 array of 10 km IFOVs is desired isolate clear and cloudy fields of view, while providing daily global coverage capability.