The CHAMP+ instrument is mounted to the elevation flange of the Nasmyth cabin B. The telescope beam, re-imaged by the tertiary mirror M1 (not shown), enters the Nasmyth cabin through the elevation bearing. M1 and M2 form a Gaussian telescope with a magnification of 4.8. After the flat folding mirror, a second Gaussian telescope, composed of M3 and a HDPE-lens (the latter already inside the cryostat), guide to beam through the cold optics to the individual sub-arrays. On top of the dewar the Local Oscillator units are located. The Sumitomo cooling machine as well as the two sub-array modules enter from below. For easier handling the complete assembly can be lifted electrically by a build-in lifter mechanism, which can also be used to crane-up the vacuum vessel only, thus providing free access for work on the inner parts of the cryostat.
The CHAMP+ instrument is mounted to the elevation flange of the Nasmyth cabin B. The telescope beam, re-imaged by the tertiary mirror M1 (not shown), enters the Nasmyth cabin through the elevation bearing. M1 and M2 form a Gaussian telescope with a magnification of 4.8. After the flat folding mirror, a second Gaussian telescope, composed of M3 and a HDPE-lens (the latter already inside the cryostat), guide to beam through the cold optics to the individual sub-arrays. On top of the dewar the Local Oscillator units are located. The Sumitomo cooling machine as well as the two sub-array modules enter from below. For easier handling the complete assembly can be lifted electrically by a build-in lifter mechanism, which can also be used to crane-up the vacuum vessel only, thus providing free access for work on the inner parts of the cryostat.
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.
The footprints of the two sub-arrays, based on the positions of the pixels as actually measured on the sky. The half-power beam width Θmb, are 8.8'' (692 GHz) and 7.7'' (806 GHz), respectively. If - e.g., for on-the-fly scanning - the array is rotated by 19.1 degrees, data will be sampled with 0.7×Θmb. With one additional observation (open circles), offset properly, Nyquist sampled maps can be achieved.
The footprints of the two sub-arrays, based on the positions of the pixels as actually measured on the sky. The half-power beam width Θmb, are 8.8'' (692 GHz) and 7.7'' (806 GHz), respectively. If - e.g., for on-the-fly scanning - the array is rotated by 19.1 degrees, data will be sampled with 0.7×Θmb. With one additional observation (open circles), offset properly, Nyquist sampled maps can be achieved.
