The overall architecture of the observation system is presented in Figure 15. The main objective is to offload observation/measurement tasks across multiple industry-standard PCs running the freely available Linux operating system. To attain high time accuracy common station time is distributed with 5 MHz and 1pps signals to all participating computers. Hardware clock counters are used to make precise common sub-second time easily available in all PC Linux software, and higher-level common UTC time is distributed from GPS monitoring subsystem using the ``xntp'' Network Time Protocol software package.
Heidenhain RON806 optical encoders have been in use as the main Az-El
axis encoders since 1996 and they have given us the benefit of a (at
least) tenfold improvement in encoder accuracy when compared to old
Inductosyns. RON806s exhibit very little periodic/geometric
distortions and they deliver noise-free and repeatable sub-millidegree
accuracy. Much of this accuracy can also be attributed to careful
mechanical fitting and alignment of the encoders; also the
Heidenhain-supplied IK121 PC ISA encoder interface board provides
excellent results when it interpolates the 
A sinusoidal current
signals for 1024-fold resolution improvement over the optical
36000 lines per revolution grating line count.
