Developers guide

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3 APT software
3.1 Introduction
As stated in 2.3, the GNU Linux operating system is an integral part of the APT system. It provides a
centralised platform for the APT control software. Fundamental services such as the file system,
networking and device drivers provide the basis for control software to be built.
3.2 Portable Telescope Control System (PTCS)
PTCS is a generic telescope control system designed to support multiple telescopes in multiple
operating environments. It was written by Jeremy Bailey for the AAO
i
in an attempt to promote
reuse of a collection of well known and trusted algorithms for pointing and tracking a telescope.
PTCS is implemented in the DRAMA software environment and consists of a set of DRAMA tasks.
PTCS is currently used at two telescopes, the APT and the 15m James Clerk Maxwell submillimeter
Telescope (JCMT) on Mauna Kea, Hawaii.
Currently the JAS server interfaces to PTCS via a Java interface to DRAMA.
3.2.1 DRAMA
The DRAMA system is a portable environment designed for writing distributed instrumentation
software [ 14 ]. Developed by the AAO, it is essentially a platform independent system of
interprocess communication. Messages are sent and received between DRAMA Tasks.
3.2.2 THI
The Telescope Hardware Interface, like PTCS is also a DRAMA task. It serves to abstract the
specific architecture of the telescope control hardware. The THI provides access to universal
time (UT) information, sending of demand positions to the servos, and reading the encoder
values for each axis.
The APT implementation of the THI interfaces to the PMAC device driver, which in turn provides
access to servo drives and encoders. UT information is calculated from values obtained from the
PC system clock
ii
.
i
Anglo-Australian Observatory
ii
Accuracy of the system clock is maintained by ntpd, the network time protocol daemon