Phase A Design Study

The proposed telescope has a 10 meter diameter clear aperture, and provides a diffraction-limited field-of-view at least 30 resolution elements across at wavelengths between 200 and 1000 microns. All optical elements are offset to minimize reflected power and thereby prevent the creation of weak resonant cavities between the optics and the receivers. The primary mirror consists of passively-mounted aluminum panels on a carbon-fiber reinforced polymer truss. The truss is homologous, in the sense that gravitational deflections in the primary mirror and primary mirror mount cause distortion into a series of parabolic shapes with different pointings and focal lengths: these changes will be accommodated by active repositioning of the secondary mirror. The secondary support, consisting of a large enclosed beam permits the mounting of either a standard set of gregorian optics, or prime focus instrumentation packages.

A tertiary chopper is located at the exit pupil of the instrument, in a design similar to that used successfully on the AST/RO telescope (Stark et al. 1997a). The tertiary chopper can be repositioned to direct the beam to a prime-focus CMB package similar to that used on the VIPER telescope. The telescope will allow the mounting of large prime focus instrument packages for CMB, and the mounting of large (100 element) array receivers in a non-tilting Nasmyth cabin. All mechanical systems are enclosed to protect them from the elements. Waste heat from compressors and motors will be ducted inside this enclosure.

Primary Mirror Deflection Analysis

An important property of this antenna design is homology, that is, the tendency for gravity-induced primary mirror surface deflections to act as a focus shift. Simply stated the deflections must be parabolic and predictable, permitting their effect to be removed by a pre-calculated re-positioning of the secondary mirror. Homologous primary mirror design is a particularly effective technique for a South Pole telescope, because gravitation rather than wind or temperature change is the dominant cause of structural deflections in the weather conditions prevalent at Pole (cf. Chamberlin, Lane and Stark 1997). We have made a preliminary structural analysis of the baseline primary mirror design. This includes the panels, back-up structure, steel support, secondary, and mounting arrangement. For the preliminary analysis, a single-sized structural component was used throughout the reflector back-up truss. The analysis shows that the deflection in the primary resulting from a pointing change from zenith to an elevation angle of 50 degrees follows a parabola to within 26 microns, or 16 micron RMS, with an absolute maximum deflection of 860 microns. This is surprisingly good, considering that the preliminary design has equal-stiffness truss elements and has not yet been optimized in any way. We are therefore confident that the 10 meter primary can be designed to be diffraction-limited at 200 microns wavelength for elevations between 90 and 50 degrees. The next step in the design is to iteratively modify the stiffness of the carbon fiber truss rods, in order to improve the homology.