GREAT/SOFIA atmospheric calibration

The GREAT observations need frequency-selective calibration across the passband for the residual atmospheric opacity at flight altitude. At these altitudes the atmospheric opacity has both narrow and broad spectral features. To determine the atmospheric transmission at high spectral resolution, GREAT compares the observed atmospheric emission with atmospheric model predictions, and therefore depends on the validity of the atmospheric models. We discusse the problems identified in this comparison with respect to the observed data and the models, and describe the strategy used to calibrate the science data from GREAT/SOFIA during the first observing periods.Comment: 14 pages, 4 figure

Holographic surface measurement system for the Fred Young Submillimeter Telescope

We describe a system being developed for measuring the shapes of the mirrors of the Fred Young Submillimeter Telescope (FYST), now under construction for the CCAT Observatory. "Holographic" antenna-measuring techniques are an efficient and accurate way of measuring the surfaces of large millimeter-wave telescopes and they have the advantage of measuring the wave-front errors of the whole system under operational conditions, e.g. at night on an exposed site. Applying this to FYST, however, presents significant challenges because of the high accuracy needed, the fact that the telescope consists of two large off-axis mirrors, and a requirement that measurements can be made without personnel present. We use a high-frequency (~300GHz) source which is relatively close to the telescope aperture (<1/100th of the Fresnel distance) to minimize atmospheric effects. The main receiver is in the receiver cabin and can be moved under remote control to different positions, so that the wave-front errors in different parts of the focal plane can be measured. A second receiver placed on the yoke provides a phase reference. The signals are combined in a digital cross-correlation spectrometer. Scanning the telescope provides a map of the complex beam pattern. The surface errors are found by inference, i.e. we make models of the reflectors with errors and calculate the patterns expected, and then iterate to find the best match to the data. To do this we have developed a fast and accurate method for calculating the patterns using the Kirchhoff-Fresnel formulation. This paper presents details of the design and outlines the results from simulations of the measurement and inference process. These indicate that a measurement accuracy of ~3 microns rms is achievable.Comment: Proceedings Volume 11445, Ground-based and Airborne Telescopes VIII SPIE Astronomical Telescopes + Instrumentation, 2020, Online Only Conferenc

First detection of the atomic O18 isotope in the mesosphere and lower thermosphere of Earth

In the lower atmosphere of Earth, oxygen contains a higher fraction of the heavy O18 isotope than ocean water does (Dole effect). This isotopic enrichment is a signature of biological activity, set by the equilibrium between oxygenic photosynthesis and respiratory metabolisms in terrestrial and oceanic ecosystems. While the mixing between stratospheric and tropospheric oxygen leads to a slow isotopic homogenization, little is known about the isotopic oxygen enrichment in the mesosphere and thermosphere of Earth. In situ measurements from rocket-borne air samplers are limited to altitudes below the mesopause, while higher layers have only been accessible through the analysis of the oxidation of ancient cosmic spherules. Here we report the detection of the far-infrared fine-structure lines (3P1<-3P2 and 3P0<-3P1) of O18 in absorption against the Moon, and determine the O16/O18 ratio in atomic oxygen from the mesosphere and lower thermosphere in absorption. After correcting for isotopic exchange between atomic and molecular oxygen, our values for the bulk O16/O18 ratio of 468 and 382 in February and November 2021, respectively, fall significantly below that found in solar wind samples (530±2), and encompass, within uncertainties, the corresponding ratios pertaining to the Dole effect in the troposphere (487), and those found in stratospheric ozone (429 to 466). We show that with existing technology, future, more sensitive measurements will allow us to monitor deviations from isotopic homogeneity in the mesosphere and lower thermosphere of Earth by remote sensing. We demonstrate that the collisional excitation of the fine-structure levels of the P3 ground-state triplet of O18 may compete with isotopic exchange reactions, implying a deviation from the Boltzmann distribution that would be established under local thermodynamic equilibrium

A Compact Beam Measurement Setup

We present the design of a compact measurement device to determine the position of a beam in a radio optical setup. The unit is used to align the Terahertz optics of the GREAT instrument on the airborne astronomical observatory SOFIA

Enhanced diffraction efficiency of two-dimensional phase gratings

Phase gratings are used as beam multiplexers in the submillimeter and terahertz spectral range. Two-dimensional beam arrangements can often be obtained most easily by the superposition of two one-dimensional grating structures. We show that in general this approach does not yield the maximum grating efficiency and propose a method to obtain significantly higher efficiencies for this grating class. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreemen

Terahertz Heterodyne Array Receivers for Astronomy

We review the development of multi-pixel heterodyne receivers for astronomical research in the submillimeter and terahertz spectral domains. We shortly address the historical development, highlighting a few pioneering instruments. A discussion of the design concepts is followed by a presentation of the technologies employed in the various receiver subsystems and of the approaches taken to optimize these for current and future instruments

Direct detection of atomic oxygen on the dayside and nightside of Venus

Atomic oxygen is a key species in the mesosphere and thermosphere of Venus. It peaks in the transition region between the two dominant atmospheric circulation patterns, the retrograde super-rotating zonal flow below 70 km and the subsolar to antisolar flow above 120 km altitude. However, past and current detection methods are indirect and based on measurements of other molecules in combination with photochemical models. Here, we show direct detection of atomic oxygen on the dayside as well as on the nightside of Venus by measuring its ground-state transition at 4.74 THz (63.2 µm). The atomic oxygen is concentrated at altitudes around 100 km with a maximum column density on the dayside where it is generated by photolysis of carbon dioxide and carbon monoxide. This method enables detailed investigations of the Venusian atmosphere in the region between the two atmospheric circulation patterns in support of future space missions to Venus

Waveguide Embedding of a Double-Metal 1.9-THz Quantum Cascade Laser: Design, Manufacturing, and Results

We present the details of a coupling structure to embed a 1.9-THz double-metal quantum cascade lasers (QCLs) into a 120-mu m-wide full-height rectangular waveguide. We describe the split-block manufacturing of the waveguide coupler together with a diagonal feed horn and present power and beam shape measurements of two different devices. The two devices differ in coupling factor, which can be chosen in a wide range by the mounting position of the QCL. Our waveguide embedding allows coupling of a large fraction of the laser power into the waveguide and the subsequent horn antenna emitting with a large Gaussian mode content, enabling efficient integration into a diffraction limited optics setup. This is illustrated by a self-mixing experiment and by using the embedded QCL as a local oscillator in a heterodyne receiver