High-resolution heterodyne spectrometers operating at above 2 THz are crucial for detecting, e.g., the HD line at 2.7
THz and oxygen OI line at 4.7 THz in astronomy. The potential receiver technology is a combination of a hot electron
bolometer (HEB) mixer and a THz quantum cascade laser (QCL) local oscillator (LO).Here we report the first highresolution
heterodyne spectroscopy measurement of a gas cell using such a HEB-QCL receiver. The receiver employs a
2.9 THz free-running QCL as local oscillator and a NbN HEB as a mixer. By using methanol (CH3OH) gas as a signal
source, we successfully recorded the methanol emission line at 2.92195 THz. Spectral lines at IF frequency at different
pressures were measured using a FFTS and well fitted with a Lorentzian profile. Our gas cell measurement is a crucial
demonstration of the QCL as LO for practical heterodyne instruments. Together with our other experimental
demonstrations, such as using a QCL at 70 K to operate a HEB mixer and the phase locking of a QCL such a receiver is
in principle ready for a next step, which is to build a real instrument for any balloon-, air-, and space-borne observatory.
We report on developments of submillimeter heterodyne arrays for high resolution spectroscopy with APEX. Shortly, we will operate
state-of-the-art instruments in all major atmospheric windows accessible from Llano de Chajnantor. CHAMP+, a dual-color 2×7 element heterodyne array for operation in the 450 μm and 350 μm atmospheric windows is in operation since late 2007. With its
state-of-the-art SIS detectors and wide tunable local oscillators, its cold optics with single sideband filters and with 3 GHz of processed IF bandwidth per pixel, CHAMP+ does provide outstanding observing capabilities. The Large APEX sub-Millimeter Array (LAsMA) is in the final design phase, with an installation goal in 2009. The receiver will operate 7 and 19 pixels in the lower submillimeter windows, 285-375 GHz and 385-520 GHz, respectively. The front-ends are served by an array of digital wideband Fast Fourier Transform spectrometers currently processing up to 32×1.5 (optionally 1.8) GHz of bandwidth. For CHAMP+, we process 2.8 GHz of instantaneous bandwidth (in 16.4 k channels) for each of the 14 pixels.
We present a new generation of very flexible and sensitive spectrometers for radio astronomical applications:
Fast Fourier Transform Spectrometer (FFTS). The rapid increase in the sampling rate of commercially available
analog-to-digital converters (ADCs) and the increasing power of field programmable gate array (FPGA) chips has
led to the technical possibility to directly digitize the down-converted intermediate-frequency signal of coherent
radio receivers and to Fourier transform the digital data stream into a power spectrum in continuous real-time
with no gaps in the data. In the last years FPGAs have become very popular for building fast and reconfigurable hardware. State-of-the-art chips include several hundred dedicated 18 bit×18 bit multipliers, which allow up
to 80 billion multiplication and nearly 500 billion 36-bit additions per second. This extremely high computing
power makes it now possible to implement real-time FFTs to decompose a 1GHz frequency band into 16384
spectral channels. In this paper we present the technological concept and results of our novel broad-band 1GHz
FFTS with 16k frequency channels, which is installed at the APEX telescope. This backend can be considered
prototypical for spectrometer developments for future radio astronomical applications.
CHAMP+, a dual-color 2 × 7 element heterodyne array for operation in the 450 μm and 350 μm atmospheric windows is under development. The instrument, which is currently undergoing final evaluation in the laboratories, will be deployed for commissioning at the APEX telescope in August this year.
With its state-of-the-art SIS detectors and wide tunable local oscillators, its cold optics with SSB filters and with 2 GHz of usable IF bandwidth per pixel, CHAMP+ will provide unmatched observing capabilities for the APEX community. The optics allows for simultaneous observations in both colors. For both sub-arrays a hexagonal arrangement with closest feasible spacing of the pixels on sky (2×Θmb) was chosen, which, in scanning mode, will provide data sampled with half-beam spacing. The front-end is connected to a flexible autocorrelator array with a total bandwidth of 32 GHz and 32768 spectral channels, subdivided into 32 IF bands of 1 GHz and 1024 channels each.
APEX, the Atacama Pathfinder Experiment, has been successfully commissioned and is in operation now. This novel submillimeter telescope is located at 5107 m altitude on Llano de Chajnantor in the Chilean High Andes, on what is considered one of the world's outstanding sites for submillimeter astronomy. The primary reflector with 12 m diameter has been carefully adjusted by means of holography. Its surface smoothness of 17-18 μm makes APEX suitable for observations up to 200 μm, through all atmospheric submm windows accessible from the ground.