Thomas Luthi

Stereoscopic passive millimeter-wave imaging and ranging

This paper presents the first stereoscopic range measurements at a wavelength of 3.3 mm and discusses the accuracy of this new method. The synthesis of passive millimeter-wave imaging and stereoscopy combines the advantages of both principles, naturally looking high-contrast images and superior poor-weather performance (compared to visible and infrared wavelengths), as well as the passive ranging capability. Our setup using two antennas with a half-power beamwidth (HPBW) of 0.9/spl deg/ and a stereoscopic baseline of 1.15 m allows ranging with an accuracy of /spl ap/10% up to a distance of /spl ap/300 m. The range resolution improves with increasing stereoscopic baseline, lower radiometer noise, narrower antenna beams, and higher scene contrast. For scenes with sufficient contrast, the directional resolution is considerably better than the antenna HPBW. Thus, massive oversampling of the scene in the plane of the stereoscopic baseline is required. For our setup, an oversampling factor of 36 is optimal. Since additional ranging errors result from nonstationary scenes, fast scanning imagers should be applied.

Expandable fully reflective focal-plane optics for millimeter- and submillimeter-wave array receivers

We describe a focal-plane optics for millimeter- and submillimeter-wave array receivers which are both fully reflective—thus avoiding the absorption and reflection losses of dielectric lenses—and expandable to an arbitrary number of pixels. The optics unit cell consists of two mirrors and a feedhorn optimized for near-field operation. Employing an integrated optics approach the mirror setup consists of only three mechanical parts, independent of the number of pixels, and requires no internal optical alignment. With a 345GHz 3×3-beam prototype a Gaussicity of ⩾98% and a focal-plane beam separation of 3.6 waist radii was obtained. In this article we present the optics design as well as numerical simulations and measured beam patterns.

The Relationship between Synchrotron and Bremsstrahlung Emission of Nonthermal Electrons during a Solar Flare. A detailed study during the August 30, 2002 X1.5 event

Synchrotron emission from nonthermal electrons has a strong dependence on the magnetic field of the medium. On the contrary, Bremsstrahlung emission does not depend on the magnetic field. The simultaneous observations of both forms of radiation may give us clues about local magnetic field configuration. In this report we use the optically thin part of the radio spectrum during the microwave maximum of the flare occurred on August 30, 2002, at 1328 UT to determine different mean magnetic field intensities and nonthermal electron density distributions compatible with the observed data. Assuming that the same electrons emit by coulomb interactions, the obtained distributions are used to compute the photon spectrum of the X‐Ray emission by Bremsstrahlung and the spectra are compared with observations obtained by instruments on board the RHESSI satellite. We discuss the effects of the trapping on the Bremsstrahlung emitted radiation, giving constraints on both magnetic field intensity and trapping time.

Design and simulations of a nulling interferometer using a single Martin-Puplett interferometer for double-sideband operation

We propose a new quasi-optical setup for a nulling interferometer at short millimeter and submillimeter wavelengths employing a single Martin-Puplett interferometer (MPI) for sideband separation. The new design allows the simultaneous use of both sidebands, thus avoiding the loss of sensitivity from conventional sideband filters. Although intended for solar flare observations, the quasi-optical setup can be applied to cancel any extended circular source, while at the same time preserving the full sensitivity toward a point-like source on or near the extended one. Numerical simulations show that the sinusoidal transmission characteristic of the MPI limits the usable relative bandwidth to one-third of the IF. Compared to an instrument employing lossless sideband filters, an increase of the response to a point-like continuum source by a factor of /spl sim/1.6-2 is expected, depending on the source position and relative bandwidth.