Mark Daehler

All-reflection Michelson interferometer: analysis and test for far ir Fourier spectroscopy

The beam splitter of the all-reflection Michelson interferometer consists of a combination of three parallel diffraction gratings. This paper extends the analysis of the instrument to include the effects of lateral errors in the grating adjustment (i.e., displacements parallel to the grating faces and perpendicular to the grooves). Such errors are shown to introduce a phase shift independent of wavenumber and proportional to grating order number. Tests of an instrument designed for Fourier transform spectroscopy in the 500-1000-microm spectral range are reported and shown to be in agreement with the analysis. For wavenumbers which pass through the instrument in 2 or more orders, cross-order interference effects are expected to occur which cause rapid variations in the efficiency vs wavenumber curve. This possibility should be eliminated in the design of a practical instrument. The resolution of the test instrument (1.6 cm(-1)) was insufficient to reveal this effect.

Michelson interferometer with frustrated-total-internal-reflection beam splitter

An efficient beam splitter for a Michelson interferometer can be made from a pair of prisms, using the process of frustrated total internal reflection. Although the phases and irradiances of the beams reflected and transmitted by the beam splitter depend on the polarization, the phase difference between the two interferometer beams vanishes for all polarizations and the transmittance can be made polarization insensitive by suitable design. The interferometer transmits radiant power within a wavelength band approximately two octaves wide, and rejects all other radiant power. The design, laboratory tests, and astronomical applications of an interferometer for the millimeter and submillimeter regions are discussed. An observation of the day-sky spectrum is presented.

Cryogenic Infrared Grating Spectrometer

A liquid-helium-cooled Ebert-Fastie grating spectrometer for use in a sounding rocket is described. Twelve detectors and associated filters separate the 5-70-microm spectral range into twelve intervals, each of which is scanned as the grating is rotated. The instrument was launched into an aurora from Fort Churchill, Canada, but a cryogenic failure occurred early in the flight, and only a small amount of data was obtained.

Far Infrared Photoconductive Detectors For Infrared Astronomy

Photoconductive detectors in the 10-1000μm range have been developed and flown in rocket borne liquid helium cooled telescopes for sky survey and auroral emission studies. Measurements under background limited conditions have been made in the spectral range of 60-300μm using the 30cm telescope aboard the NASA Lear Jet. We present the results of the detector sensitivities used under these two environments.

Sounder Updates for Statistical Model Predictions of Maximum Usable Frequencies on HF Sky Wave Paths

A method is presented for the short-term prediction of maximum usable frequencies (MUFs) in a large communications region. It is shown how ionospheric measurements from a network of ionospherit sounders can be used to update sunspot number or solar 10.7 cm flux inputs to a climatological MUF prediction model, MINIMUF in this case, which is then used to predict MUFs on paths throughout the region. Analysis of mid-latitude oblique-incidence sounder data sets indicates the advantage gained from single-path sounder updates of flux for MUF predictions on adjacent paths. Under specified conditions a further dramatic improvement in MUF prediction accuracy is found from spatial interpolation of sounder-updated flux values. MUF prediction accuracies within 0.5 MHz are obtained for fairly modest sounder network deployments, in which the sounder midpath point distributions and updating frequency satisfy particular requirements.

Global Considerations for Utilization of Real-Time Channel Evaluation Systems in HF Spectrum Management

Real-Time Channel Evaluation (RTCE) devices are coming into increased use in modern management of HF communications systems. The earliest techniques used in channel evaluation exploited vertical incidence pulse sounders (VIS) and these devices are still being used for some applications. An oblique incidence sounding technology has also been developed employing both pulse and chirp waveform approaches. There are a variety of RTCE devices which are identified in the paper but the major emphasis is placed on the OIS chirp sounder. Using this device as a canonical channel evaluator, the implications for global, theater, and local HF resource management may be addressed. As one might suspect, there are both advantages and disadvantages which may accrue from construction of a sounder network. Issues include: data applicability, data collection and dissemination, network size and cost, network ECM vulnerabilities, optimum network architecture, and system component reliabilities to name a few. These issues are outlined in the paper.

A Cold Fabry-Perot Spectrometer

Design considerations are discussed for a ground-based high-resolution scanning spectrometer in which thermal background radiation to the detector is reduced to the level of the detector NEP by cooling the interferometers to liquid nitrogen temperature.