The 10-m class Southern African Large Telescope (SALT) at Sutherland, South Africa, was inaugurated in November 2005, following completion of all its major sub-systems. It is the largest single optical telescope in the southern hemisphere. The SAMS (Segment Alignment Measurement System) is a unique capacitive edge sensing solution for the active alignment of the SALT primary mirror. Twelve thin film edge sensors are bonded directly onto the edges of each of the 91 segments, with heat-generating control electronics housed remotely in temperature-controlled enclosures. The SAMS is capable of measuring the tip/tilt and piston of each segment, as well as the change in global radius of curvature, a mode normally undetected by such a system. The primary objective was to build a system that offered an excellent cost-to-performance ratio without sacrificing measurement accuracy, a very necessary requirement because of the scale and number of sensors required for large segmented mirrors. This paper describes the results obtained during the commissioning and calibration of the completed system.
On completion by the end of 2004 the Southern African Large Telescope (SALT) being erected in Sutherland, South Africa, will be the largest single optical telescope in the southern hemisphere. This paper addresses the process of designing, building and demonstrating a high performance primary mirror system for SALT that meets the overall telescope requirements. Throughout the process consideration was given to the fact that SALT is budget sensitive, which required careful allocation of funds among the various subsystems, innovative designs, and using COTS components where possible. The process delivered subsystems with a very high cost-performance ratio.
The design of the Southern African Large Telescope (SALT), which is based closely on the Hobby-Eberly Telescope (HET) at the University of Texas but includes advances incorporating lessons learned from HET, is briefly reviewed. The flowdown of requirements from the optical error budget to the primary mirror control subsystems is presented. The techniques and algorithms used by the Center of Curvature Alignment Sensor (CCAS) to measure segment tilt and piston and estimate the global radius of curvature of the primary are discussed in detail. The steps in the process that allows CCAS to capture and identify segments misaligned by more than 70 arcsec and bring them into alignment with residual errors less than 50milli-arcsec is fully described. Next, the hardware and software designs of CCAS are presented, as well as the results of laboratory performance testing. CCAS has been installed and integrated with the primary mirror control system. Performance results of the integrated system over a range of environmental conditions will be shown. Finally, the overall results of this project are summarized and suggestions for future improvements presented.
The Southern African Large Telescope (SALT) is a little over 18 months away from completion (in early 2005). It is based on the innovative tilted-Arecibo optical analog, first pioneered by the Hobby-Eberly Telescope (HET). By the end of 2003, all major subsystems, including the verification instrument, will be in place and the commissioning of them begun. Tests of a 7-segment subset of the mirror array, including the Shack-Hartmann alignment instrument, the mirror actuators, capacitive edge sensors and active control system has recently started. The first engineering on-sky tests involving the complete light path, from object to detector, have begun. SALT's primary mirror consists of 91 identical segments mounted on a 9 point whiffle tree mount, using three actuators to control tip and tilt, and a foil-type capacitive edge sensor to detect mirror misalignment. These 480 relatively affordable sensors are permanently attached to the segment edges, and are capable of measuring all misalignment modes, including global radius of curvature. This sensing system, used together with a Shack-Hartman wavefront instrument at the center of curvature, controls the primary mirror array, and could be scaled to an array of the size envisaged for an ELT. SALT has developed some innovative designs improvement over the original HET concept. These include a more effective spherical aberration corrector (SAC), interferometric distance sensing and laser auto-collimation of the prime focus payload, the use of newly developed efficient and durable mirror coatings on the SAC optics, and the use of economical low expansion ceramics for the primary mirror segments. These innovative and cost effective solutions used on SALT have potential applications to ELT designs.
SALT, although similar to the Hobby-Eberly Telescope (HET - the prototype for a fixed altitude optical analog of the Arecibo radio telescope), has some significant differences in the optical design. This paper gives an overall description of the SALT optics and a description of the analyses done in order to develop an optical error budget, which satisfies the overall requirements for total image quality. An optical prescription for SALT is presented, including an optical model of the telescope with a segmented primary mirror (PM) array. The design of the spherical aberration corrector (SAC) is summarised, with particular reference to the effects of non-axisymmetric distortion. The concepts for an atmospheric dispersion corrector (ADC), and guidance and focusing (GF) systems, are also discussed. Finally, the primary mirror alignment system (PMAS) concept is presented and the difficulty in controlling the Global Radius of Curvature (GRoC) discussed.
Methods to limit image degradation due to temperature variations in telescopes include reducing the coefficient of thermal expansion (CTE) of the optical elements. In segmented mirror telescopes, not only does the average CTE have to be as close as possible to zero, but, more importantly, a high level of homogeneity is essential to avoid temperature related changes to mirror figure. The Southern African Large Telescope (SALT) has selected Sitall optical glass-ceramic from the Russian company LZOS for the manufacturing of its 91 primary mirror segments. A detailed specification, including strict requirements for CTE, was developed from basic principles. LZOS, together with the Mendeleev Metrological Institute in St. Petersburg, have developed CTE measuring equipment (dilatometers) to demonstrate that individual segments meet specification. A theoretical analysis of the allowable measurement uncertainty was conducted which accounted for the inaccuracy of the dilatometers, resulting from uncertainties due to sample length, fringe fraction reading and laser wavelength instability. Developments include operating in a dynamical temperature mode to reduce testing time and utilizing computer controlled units to read and process interferograms. Fizeau and heterodyne interferometric methods were implemented in separate dilatometers. CTE measurements of the first batch of SALT segments demonstrate that the material complies with the SALT specification. These results are presented in this paper.
The Southern African Large Telescope (SALT), being erected in Sutherland, South Africa, will be the largest single optical telescope in the southern hemisphere, and the 4th largest telescope in the world, when it is completed in late 2004. The SALT design is based on the Hobby-Eberly Telescope (HET). Deviations from the HET design with respect to the telescope structure, primary mirror truss and tracker beam structure are presented in this paper. Finite element models were generated to perform static, dynamic and thermal analyses on the structures. Despite a significantly heavier payload and increased wind loading requirements, equivalent or improved stiffness characteristics were achieved, without increasing structure mass. Dynamic response analyses were performed to characterize the maximum deflections under dynamic wind loading. Automation of the truss design process, which included the control of the 3-D CAD software, the finite element software, as well as CAM software, with a central computer program, allowed the generation of a full 3-D CAD model, as well as CAM inputs, of a structurally optimized truss, within a few days. Extensive analyses, including Monte Carlo simulations, as well as experimentation, were performed to ensure linear temperature-displacement response of the truss. A methodology was developed, using the complete finite element model, to calculate mirror corrections required to correct for lower order deformations of the primary mirror due to temperature fluctuations in the truss.
Segmented mirror telescopes take advantage of modular design to achieve large apertures at low cost. This paper describes the segment mount developed for the Southern African Large Telescope. The mount provides passive precision support for the optics, kinematic registration to the primary mirror truss, precision tip-tilt and piston adjustments, and interchangeability between segments and mounts. A trial production run of mounts is now in fabrication prior to full production of 91 units needed to populate the SALT primary.