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Telescope Mount

The telescope is situated atop the inner of two concentric towers attached to the Martin A. Pomerantz Observatory (MAPO) at the South Pole station (see Figure 1); the inner tower is mechanically isolated from the outer tower, so that no vibration can be transmitted from the building to the telescope. A room beneath the telescope, vibrationally isolated from the telescope, houses helium compressors, drive amplifiers, and an air handling unit for managing waste heat from the telescope and helium compressors. The interior of the telescope opens directly onto the compressor room, providing access to drive systems, receivers and electronics even in mid-winter, when the darkness and extreme cold (as low as $-80\deg$ C ambient) severely restrict outside activity. An insulated fabric bellows allows motion in the elevation axis while keeping the interior of the telescope and drive assemblies at room temperature.

Figure: The DASI telescope (left) being lifted to the 35-ft tower attached to the MAPO building at the Amundsen-Scott South Pole Station 3 December 1999 and (right) operating at sunset March 2000.
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The telescope consists of an altitude-azimuth mount, employing a counterbalanced gear and pinion elevation drive, yielding excellent tracking and pointing stability. Heavy box steel-plate construction lends the mount extreme rigidity and immunity to flexure. The interferometer has 13 primary antenna elements, arranged in a three-fold symmetric pattern on a rigid faceplate, which attaches to the elevation cradle.

The faceplate can also be rotated about its axis; in combination with the three-fold symmetry, this feature provides important diagnostic capabilities, for instance permitting discrimination of spurious signals due to cross-talk between the antenna elements, a concern with any compact array in which the elements are nearly touching. Since the antenna pattern repeats with every $120\deg$ of rotation, any signal in the far field will be unchanged by the rotation, while any signal due to cross-talk will rotate with the faceplate. For purposes of imaging, the rotation also allows dense sampling of the u-vplane.

In addition, the rigid faceplate greatly simplifies the design of an interferometer. Unlike conventional tracking arrays, projected baseline lengths for a co-planar array are independent of the pointing center, and IF tracking delays are not required.

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Erik Leitch