We report the development and initial results of two Terahertz imaging systems based on monochromatic sources at 0.2 and 2.52 THz. The first is based on a microwave oscillator, whose frequency is multiplied to 0.2 THz, used in conjunction with a zero-bias detector. The sample is scanned across the beam, and transmission images are obtained after processing. The second system allows real-time images, and consists of a methanol gas laser emitting at 119 microns (2.52 THz) and a commercial camera based on a microbolometer array. We describe the construction and performance of the methanol laser and a tunable CO2 laser, which emits 20 W at the 9P(36) pump line. Due to the high coherence of the laser, this system is particularly suited for diffraction and interference imaging. We have measured the absorption coefficients of a few samples assuming the Beer law.
The solar submillimeter-wave telescope (SST) is the only one of its kind dedicated to solar continuous observations.
Two radiometers at 0.740 mm (405 GHz), and four at 1.415 mm (212 GHz) are placed in the Cassegrain focal plane of
the 1.5-m dish at El Leoncito high altitude site, San Juan, Argentina. The aperture efficiencies are close to design
predictions: 20% and 35% for 2 and 4 arcminutes beam sizes at 405 and 212 GHz, respectively. The positioner absolute
pointing accuracy is 10 arcseconds. Spectral coverage is complemented by ground-based mid-infrared telescopes
developed for high cadence observations in the continuum 10 micron band (30 THz), using small apertures and room-temperature
microbolometer cameras. Using the system, a new solar burst spectral component was discovered,
exhibiting fluxes increasing for smaller wavelengths, separated from the well known microwave component. Rapid sub-second
pulsations are common for all bursts. The pulsations onset times of appear to be connected to the launch times of
CMEs. Active regions are brighter for shorter submillimeter-waves. Mid-IR bright regions are found closely associated
with calcium plages and magnetic structures near the solar photosphere. Intense and rapid 10 micron brightening was
detected on active centers in association with weak flares. These results raise challenging difficulties for interpretation.
The concept of partially overlapping multiple beams, produced by focal plane arrays in large antennas, has been used successfully at mm-waves to detect instantaneously spatial positions of rapid spikes produced by solar flares. The technique has been used at mm-waves and was recently applied to the Solar Submillimeter-wave Telescope, which operates at 212 and 405 GHz. We present the basic description of the concept and the results obtained. New applications are being considered for shorter submm-IR wavelengths, with the use of focal plane arrays of bolometers, which spatial angular accuracy will strongly depend on the knowledge of the beamshapes of the individual beams produced.
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