Pulsed lidar provides a simple direct measure of range that can be scanned to produce accurate 3D point clouds for modest computational cost. A simple implementation, however, can be subject to point-cloud artifacts particularly when sensing scenes with high dynamic range.
We discuss the origins of some of these artifacts and some possible mitigations. Of particular interest is range aliasing. By creating multiple hypotheses for the range of any given return a practical system may accurately and precisely classify aliased returns..
The 1-meter Swedish solar telescope is a new solar telescope that was put in operation on the island of La Palma in the Canary Islands at the end of May 2002. The goal of this telescope is to reach its diffraction limited resolution of 0.1 arcsec in blue light. This has already been achieved by use of a low-order adaptive optics (AO)system. This paper describes the AO system initially developed for the former 50-cm Swedish Vacuum Solar Telescope (SVST) and further improved for the new telescope. Both systems use a combination of bimorph modal mirrors and Shack-Hartmann wavefront sensors. Unique to these systems are that they rely on a single workstation or a PC to do all the computations required to extract and pre-process the images, measure their positions using cross correlation techniques and for controlling the deformable mirror. This is in the present system possible by using the PERR instruction available on Compaq's Alpha architecture and in the new system using the PSADDBW instruction, available on Pentium 4 and Athlon processors. We describe both these systems with an emphasis on the performance, the ease of support and upgrades of performance.
We also describe the optimization of the electrode geometry for the new 37-electrode bimorph mirror, supplied by AOPTIX Technologies, Inc., for controlling Karhunen--Loeve modes. Expected performance, based on closed-loop simulations, is discussed.
The microprocessors used in off-the-shelf workstations double in performance every eighteen months. The Swedish Vacuum Solar Tower (SVST) uses off-the-shelf workstations for all aspects of its on-line telescope control and data acquisition. Since 1995 workstation performance has been adequate for a correlation tracker of solar granulation controlling a tip- tilt corrector. In 2000 workstation performance permits the construction of a 20 - 50 subimage Shack-Hartmann based low- latency adaptive optics system. It is argued that workstations provide a cost-effective, upgradable, low-risk and flexible means of construction of stellar and solar adaptive optics systems. We give an overview of the adaptive optics system installed at the SVST in May 1999. The system uses a bimorph modal mirror with 19 electrodes from Laplacian Optics. For use with extended targets, such as solar fine structure, cross- correlations with 16 X 16-pixel sub-images are used. For use with point sources, a centroiding algorithm is implemented. The work station used is capable of completing all processing required by the adaptive optics system in 0.5 ms (cross-correlations) or 0.3 ms (centroiding), with potential for significant performance improvements.
We describe the use of a reconfigurable interface board based on FPGAs and a UNIX workstation to implement a correlation tracker with 3.8ms latency. The correlation tracker is part of an active mirror system in use at the Swedish Vacuum Solar Telescope, La Palma, Canary Islands. The reconfigurable interface is used to leverage the workstation CPU, relieving it of tasks that it performs poorly such as rapid context switching and low-level bit manipulation. The reconfigurable interface handles control of external devices, high- performance input (16 MB/s) and data preformatting. The workstation CPU, a 64-bit microprocessor, performs the bulk of the computation. For the key computations of the correlation tracker we are able to treat 8 pixels in parallel in the CPU's 64-bit integer datapath. We present the structure of the CCD interface configuration and the implementations of the key algorithms on the workstation CPU. We describe the design trade-offs that arose during the development of the system, and demonstrate the symbiosis between components implemented in software and configurable hardware.