Description of the general instrument setup

The array composes of 2×7 pixels, arranged in a hexagonal configuration, that operate - at orthogonal signal pola­ri­zations - simultaneously in the RF tuning range of 620–720 GHz in the low (LFA) and 780–950 GHz in the high frequency (HFA) sub-array, respectively. The beam-spacing is ~2.15·Θmb for both sub-arrays, where Θmb is the FWHM of the main beam at the respective frequency. The central pixels of the two sub-arrays are spatially co-aligned on the sky (to better than 1"). The design allows for de-rotation of the CHAMP images on sky. For best system sensitivities the design allows for cold optics (20 K) and for single-sideband operation (the image sideband is terminated at 20 K). The Local Oscillator (LO) signal derives from cascading frequency multipliers and is distri­bu­ted by collimating Fourier gratings. Since spring 2008, the front-end is connected to a new-technology FFTS back-end array, processing 3 GHz of IF bandwidth for each detector pixel with 16384 spectral channels.
The complete dewar can be rotated to compensate for image rotation on the sky while tracking an astronomical source. An offset-angle can be added in order to position the dewar with the correct tilt of 19.1 deg for optimal beam spacing for “on-the-fly” mapping. For calibration hot- and cold-loads are provided.

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