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The real-time control and monitoring of the Sloan Digital Sky Survey 2.5 meter telescope is handled by a pair of vxWorks-based real-time computers. Of these, we are primarily concerned with the Telescope Performance Monitor (TPM) that serves out engineering data obtained from the Mount Control Processor (MCP). This data is obtained from a shared memory board that the MCP updates at rates up to 200Hz.
In the original version of the TPM, the only function was to write the an interface to the primary and secondary mirror support systems into a series of files on a NFS-mounted workstation disk. There were no graphical tools to examine either real-time or archived TPM information.
Given the problem of how to examine the data we proposed a utilization of the standard Experimental Physics and Industrial Control System (EPICS) toolkit to provide graphical interfaces to both archived data and to network-based access of real-time TPM channels.
The EPICS toolkit is the product of an on-going collaboration at over 100 international sites including many accelerator facilities and astronomical observatories (EPICS 1999).
The EPICS toolkit creates a client-server architecture on the control system with communication handled by the channel access (CA) protocol.
Channel access maintains a global name space of process variables within the local Internet sub-net. These process variables can be connected to a variety of data sources including a real-time database running on the vxWorks operating system.
Two separate and independent extensions have been created to access SDSS TPM data.
First, in order to provide real-time access to TPM data across the local SDSS Internet we created an EPICS database resident on the TPM IOC (Input-Output Controller) to serve live data using the supplied vxWorks-based channel access server. We can then use a number of EPICS extensions (channel access clients) to create graphical interfaces to the real-time data.
The second solution involved the construction of a data source module for the XARR data archive viewer so it can read the TPM log files that are written on the workstation disk.
The TPM serves EPICS process variables that map onto a real-time database. The key database components within the TPM are:
The real-time application development required only four hours to determine the hooks into the existing TPM software, create the SUBROUTINE record support, construct the database, and the initial displays.
Other EPICS client applications can be installed at Apache Point Observatory including the alarm handler and an interface between channel access and the IDL visualization product.
XARR, the X-Windows based EPICS data archive viewer, is presently used for retrieval and display of archived parameters. XARR also supports printing of displays and exporting into ASCII files. It is common practice for the facility operators at Los Alamos (accelerator physicists) to process these exported files using commercial spreadsheet programs and other off-line tools.
XARR can retrieve data from a variety of data sources, all of which described by classes derived from a fundamental data source base class. It was relatively simple to create a data source class for TPM data that called the existing TPM access library.
Due to the high-bandwidth channels such as mount encoder data, it was necessary to implement a scheme for block data caching. The major challenge was to maintain a collection of cached blocks to allow speed-optimized access to channels such as the 200Hz axis encoder data.
The algorithm used in the management of the cache is:
Each night's log file is over 150MB in length and must be maintained for long periods, perhaps forever, due to the need to assess and maintain science data consistency over the life of the survey.
The initial real-time display and data archive tools are the first application of the EPICS toolkit methodology at Apache Point Observatory and the SDSS.
As the observers and engineering staff become more familiar with the features and the modularity of this approach, we expect to see an increase in the number user-written applications and displays as well as the use of EPICS on other subsystems besides the Telescope Performance Monitor.
Gunn, J. E., et al. 1999, http://www.astro.princeton.edu/PBOOK/
Thuot, M. E., et al. 1996, in Proc. LINAC96
EPICS consortium 1999, http://mesa53.lanl.gov/lansce8/epics/