Development of a Wide Bandwidth Array Acousto-Optical Spectrometer for the Herschel Satellite Mission

The Herschel satellite observatory will explore the universe at far-infrared, submillimetre, and millimetre wavelengths. This regime of the electromagnetic spectrum is difficult to observe, because water vapour in the Earth's atmosphere absorbs the signals of almost all the astronomical sources. With the 3.5m telescope (the largest ever placed aboard a spacecraft), it will be possible to observe various atomic and molecular lines with exceptionally high spatial resolution. The receiver system employs the heterodyne technique, and the spectral information is obtained by means of real-time spectrometers. Within this thesis, the requirements, specifications, design concept, and development of a wide bandwidth array acousto-optical spectrometer (WBS) is discussed. Both ground-based and satellite applications have demonstrated that acousto-optical spectrometers (AOS) utilize a reliable signal-processing technique. For the Herschel mission, the spectrometers demand a large frequency bandwidth and high resolution. These are achieved by means of hybrid technology. This means that the signal is split into four sub-bands and analyzed by using a four-channel array Bragg cell. The spectra of the four individual channels are co-added by software to create the complete wide bandwidth spectrum of the input signal. Since the channels of the deflector are illuminated by a semi-conductor laser that provides relatively low optical power, the imaging optical system requires careful design to make the instrument efficient. In comparison with earlier AOSs, the Herschel WBS has outstanding optical efficiency due to its diffraction-limited and efficiency-optimized optical design. The first part of the optics that illuminates the Bragg cell (Laser Source Module) comprises two laser diodes (the second laser is needed for redundancy reasons), the collimation and imaging optics, and the specially-designed beam splitter. The great advantage of the prism-based beam splitter design is that it generates the four beams necessary for the illumination of the deflector and makes it possible to couple the light of the redundant laser without applying an additional beam splitter, which would significantly reduce the efficiency of the instrument. The second part of optics images the deflected light of the Bragg cell onto a four-line linear CCD detector. The design of the diffraction limited lens system makes it possible to achieve the required high resolution and high efficiency. At the same time, the optimization of the optical efficiency affects the mechanical and thermal sensitivity of the spectrometer. Since the high mechanical stability is an important concern for the spectroscopic stability performance of the instrument, it is essential to use materials that have high stiffness and low thermal expansion. Specifically, the mounting of the laser diode requires a thermally compensated design to reduce the deformations caused by thermal changes. In addition to the high stiffness, the weight of satellite-borne instruments must be minimized. Thus, most of the components are light weighted and made of a special aluminium alloy. The imaging optical system of the spectrometer is designed to be diffraction limited. However, the deflector considerably reduces the resolution, since it has a small aperture, and the acousto-optical material has poor optical quality. For the first time, theoretical investigations of the resolution involving the Bragg cell were carried out. The diffraction phenomenon of the acoustic aperture of the Bragg cell is studied by using the Fraunhofer diffraction theory. The model of the deflector includes the frequency-dependent dimension of the acoustic zone and the attenuation of the acousto-optical material. Furthermore, the model makes it possible to explore the impact of the illuminating laser beam position change relative to the active zone of the Bragg cell on the resolution of the spectrometer. For investigations of alignment errors, this is of particular importance. An additional highlight of this theoretical investigation is that lens aberration errors and crystal imperfections that adversely affect the wavefront aberration, i.e. the resolution, can be also examined by diffraction analysis. As part of the qualification procedure, thorough thermal, deformation, and resonance investigations are made with the spectrometer assembled with dummy loads. During the development phase, the early tests of mechanical deformations caused by thermal variations show the mechanical stability and thermal sensitivity of the instrument and therefore provide detailed information for further optical and mechanical development phases. Further resonance search tests are conducted to measure the mechanical stability of the unit. The power and frequency stability of the spectrometer is partly dependent on the performance of the laser diode and the stability of its power supply. Thus, the laser diodes selected from the batch of the flight lasers were investigated to determine their single mode behaviour, mode jump characteristics, wavelength hysteresis, and wavelength stability. Because the laser light feedback from the surfaces of the collimating optics might also appreciably reduce the power stability of the instrument, investigations are performed with the special collimator design of the WBS. In order to determine the reliability of the lasers, accelerated lifetime tests are performed, and conclusions are drawn relating to lifetime statistics. The WBS must meet strict performance standards before it can be flown on Herschel. The space-qualification procedure comprises diverse tests that simulate the extreme environment of the spectrometer in orbit. The results of these tests indicate the quality of the mechanical and optical design and confirm the adjustment precision of the instrument. Within the operating temperature range of the spectrometer, the efficiency, bandpass, and resolution variations are characterized. Moreover, the noise performance is investigated in detail. For prolonged astronomical measurements it is of crucial importance that the backend system does not significantly contribute to the radiometric noise. Due to the meticulous design of the WBS, the outstanding noise performance makes it possible to efficiently perform integrations of hundreds of seconds. All these tests indicate that the array acousto-optical spectrometer designed for the Herschel satellite will operate successfully

The Antarctic Submillimeter Telescope and Remote Observatory (AST/RO)

AST/RO, a 1.7 m diameter telescope for astronomy and aeronomy studies at wavelengths between 200 and 2000 microns, was installed at the South Pole during the 1994-1995 Austral summer. The telescope operates continuously through the Austral winter, and is being used primarily for spectroscopic studies of neutral atomic carbon and carbon monoxide in the interstellar medium of the Milky Way and the Magellanic Clouds. The South Pole environment is unique among observatory sites for unusually low wind speeds, low absolute humidity, and the consistent clarity of the submillimeter sky. Four heterodyne receivers, an array receiver, three acousto-optical spectrometers, and an array spectrometer are installed. A Fabry-Perot spectrometer using a bolometric array and a Terahertz receiver are in development. Telescope pointing, focus, and calibration methods as well as the unique working environment and logistical requirements of the South Pole are described.Comment: 57 pages, 15 figures. Submitted to PAS

Spaceborne sensors (1983-2000 AD): A forecast of technology

A technical review and forecast of space technology as it applies to spaceborne sensors for future NASA missions is presented. A format for categorization of sensor systems covering the entire electromagnetic spectrum, including particles and fields is developed. Major generic sensor systems are related to their subsystems, components, and to basic research and development. General supporting technologies such as cryogenics, optical design, and data processing electronics are addressed where appropriate. The dependence of many classes of instruments on common components, basic R&D and support technologies is also illustrated. A forecast of important system designs and instrument and component performance parameters is provided for the 1983-2000 AD time frame. Some insight into the scientific and applications capabilities and goals of the sensor systems is also given

Stratospheric Water Vapour in the Tropics: Observations by Ground-Based Microwave Radiometry

This thesis reports on observations of tropical stratospheric water vapour by the ground-based microwave radiometer/spectrometer WaRAM2 in 2007. The 22GHz receiver is set up at Mérida Atmospheric Research Station on top of Pico Espejo, Venezuela (8°32'N, 71°03'W, 4765m above sea level). It is the only such sensor that continuously operates at tropical latitudes. The high altitude site is ideally suitable for microwave observations, because most tropospheric water vapour is located below the sensor. Water vapour plays a key role in middle atmospheric processes. Because of its large infrared resonance, it strongly participates in the radiative budget, both in terms of a greenhouse effect at lower altitudes and radiative cooling at higher altitudes. It is a source gas for the highly reactive hydroxyl radical, and exerts indirect effects on ozone destruction in the formation of polar stratospheric clouds. Due to its long lifetime, water vapour also serves as a dynamical tracer

Validation of Stratospheric and Mesospheric Ozone Observed by SMILES from International Space Station

We observed ozone O3 in the vertical region between 250 and 0.0005 hPa (~ 12-96 km) using the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) on the Japanese Experiment Module (JEM) of the International Space Station (ISS) between 12 October 2009 and 21 April 2010. The new 4K superconducting heterodyne receiver technology of SMILES allowed us to obtain a one order of magnitude better signal-to-noise ratio for the O3 line observation compared to past spaceborne microwave instruments. The non-sun-synchronous orbit of the ISS allowed us to observe O3 at various local times. We assessed the quality of the vertical profiles of O3 in the 100-0.001 hPa (~ 16-90 km) region for the SMILES NICT Level 2 product version 2.1.5. The evaluation is based on four components: error analysis; internal comparisons of observations targeting three different instrumental setups for the same O3 625.371 GHz transition; internal comparisons of two different retrieval algorithms; and external comparisons for various local times with ozonesonde, satellite and balloon observations (ENVISAT/MIPAS, SCISAT/ACE-FTS, Odin/OSIRIS, Odin/SMR, Aura/MLS, TELIS). SMILES O3 data have an estimated absolute accuracy of better than 0.3 ppmv (3%) with a vertical resolution of 3-4 km over the 60 to 8 hPa range. The random error for a single measurement is better than the estimated systematic error, being less than 1, 2, and 7%, in the 40-1, 80-0.1, and 100-0.004 hPa pressure regions, respectively. SMILES O-3 abundance was 10-20% lower than all other satellite measurements at 8-0.1 hPa due to an error arising from uncertainties of the tangent point information and the gain calibration for the intensity of the spectrum. SMILES O3 from observation frequency Band-B had better accuracy than that from Band-A. A two month period is required to accumulate measurements covering 24 h in local time of O3 profile. However such a dataset can also contain variation due to dynamical, seasonal, and latitudinal effects

Comparison of SMILES ClO profiles with satellite, balloon-borne and ground-based measurements

We evaluate the quality of ClO profiles derived from the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) on the International Space Station (ISS). Version 2.1.5 of the level-2 product generated by the National Institute of Information and Communications Technology (NICT) is the subject of this study. Based on sensitivity studies, the systematic error was estimated as 5–10 pptv at the pressure range of 80–20 hPa, 35 pptv at the ClO peak altitude (~ 4 hPa), and 5–10 pptv at pressures ≤ 0.5 hPa for daytime mid-latitude conditions. For nighttime measurements, a systematic error of 8 pptv was estimated for the ClO peak altitude (~ 2 hPa). The SMILES NICT v2.1.5 ClO profiles agree with those derived from another level-2 processor developed by the Japan Aerospace Exploration Agency (JAXA) within the bias uncertainties, except for the nighttime measurements in the low and middle latitude regions where the SMILES NICT v2.1.5 profiles have a negative bias of ~ 30 pptv in the lower stratosphere. This bias is considered to be due to the use of a limited spectral bandwidth in the retrieval process of SMILES NICT v2.1.5, which makes it difficult to distinguish between the weak ClO signal and wing contributions of spectral features outside the bandwidth. In the middle and upper stratosphere outside the polar regions, no significant systematic bias was found for the SMILES NICT ClO profile with respect to data sets from other instruments such as the Aura Microwave Limb Sounder (MLS), the Odin Sub-Millimetre Radiometer (SMR), the Envisat Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), and the ground-based radiometer at Mauna Kea, which demonstrates the scientific usability of the SMILES ClO data including the diurnal variations. Inside the chlorine-activated polar vortex, the SMILES NICT v2.1.5 ClO profiles show larger volume mixing ratios by 0.4 ppbv (30%) at 50 hPa compared to those of the JAXA processed profiles. This discrepancy is also considered to be an effect of the limited spectral bandwidth in the retrieval processing. We also compared the SMILES NICT ClO profiles of chlorine-activated polar vortex conditions with those measured by the balloon-borne instruments: Terahertz and submillimeter Limb Sounder (TELIS) and the MIPAS-balloon instrument (MIPAS-B). In conclusion, the SMILES NICT v2.1.5 ClO data can be used at pressures ≤ ~30 hPa for scientific analysis

The Level 2 research product algorithms for the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES)

This paper describes the algorithms of the level-2 research (L2r) processing chain developed for the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES). The chain has been developed in parallel to the operational chain for conducting researches on calibration and retrieval algorithms. L2r chain products are available to the scientific community. The objective of version 2 is the retrieval of the vertical distribution of trace gases in the altitude range of 18–90 km. A theoretical error analysis is conducted to estimate the retrieval feasibility of key parameters of the processing: line-of-sight elevation tangent altitudes (or angles), temperature and ozone profiles. While pointing information is often retrieved from molecular oxygen lines, there is no oxygen line in the SMILES spectra, so the strong ozone line at 625.371 GHz has been chosen. The pointing parameters and the ozone profiles are retrieved from the line wings which are measured with high signal to noise ratio, whereas the temperature profile is retrieved from the optically thick line center. The main systematic component of the retrieval error was found to be the neglect of the non-linearity of the radiometric gain in the calibration procedure. This causes a temperature retrieval error of 5–10 K. Because of these large temperature errors, it is not possible to construct a reliable hydrostatic pressure profile. However, as a consequence of the retrieval of pointing parameters, pressure induced errors are significantly reduced if the retrieved trace gas profiles are represented on pressure levels instead of geometric altitude levels. Further, various setups of trace gas retrievals have been tested. The error analysis for the retrieved HOCl profile demonstrates that best results for inverting weak lines can be obtained by using narrow spectral windows

Experimental and theoretical investigations of the photochemistry of styrene and the creation and characterisation of shaped femtosecond ultraviolet laser pulses

This thesis is composed of three projects that are linked by the theme of light-molecule interactions. These are covered separately in Chapters 2, 3, and 4. In Chapter 1 some background to the thesis is described so that the various links between the science in the different chapters, and the motivation for the project as a whole are explained. Elements of photochemistry, both experimental and theoretical, are described in this chapter and some material about the most important experimental tools used in this work, ultrafast lasers, is covered, as well as the methodology of time-resolved spectroscopies, and timeresolved photoelectron spectroscopy in particular; the field of laser control of chemistry is also briefly reviewed. Chapter 2 is an account of the design and use of a UV pulse shaper and characterisation setup. This is an applied optics experiment, whose application is to the control of photochemical reactions with specifically shaped ultrafast laser light. Several demonstrations of the pulse shaping capacity of this new experiment are presented. In Chapter 3, calculation of the excited electronic states of the molecule styrene is described. This project is a computational study of the electronic spectroscopy and ionisation of the styrene molecule. In Chapter 4, the direct observation of internal conversion in styrene using time-resolved photoelectron spectroscopy is reported. This is an experimental laser spectroscopy project in which some of the results from the computations in the theory project, Chapter 3, will be used to analyse the experimental spectra. Chapter 5 summarises the conclusions drawn from Chapters 2, 3 and 4 and provides an outlook for future research based on the work in this thesis. Throughout this thesis, but more particularly in Chapters 1 and 2, there is quite a large volume of literature review and background material. This content reflects the personal perspective from which the thesis was approached. Much of the field of ultrafast optics and spectroscopy was entirely new to the writer at the outset of the PhD programme, and most of the review-based writing about these topics found here was originally written early on in the PhD project, as a means of helping to bridge the gap between work on optical experiment design and an undergraduate training in chemistry

Balloon-borne three-meter telescope for far-infrared and submillimeter astronomy

The scientific objectives, engineering analysis and design, results of technology development, and focal-plane instrumentation for a two-meter balloon-borne telescope for far-infrared and submillimeter astronomy are presented. The unique capabilities of balloon-borne observations are discussed. A program summary emphasizes the development of the two-meter design. The relationship of the Large Deployable Reflector (LDR) is also discussed. Detailed treatment is given to scientific objectives, gondola design, the mirror development program, experiment accommodations, ground support equipment requirements, NSBF design drivers and payload support requirements, the implementation phase summary development plan, and a comparison of three-meter and two-meter gondola concepts