Terry Schalk

Using SysML for MBSE analysis of the LSST system

The Large Synoptic Survey Telescope is a complex hardware - software system of systems, making up a highly automated observatory in the form of an 8.4m wide-field telescope, a 3.2 billion pixel camera, and a peta-scale data processing and archiving system. As a project, the LSST is using model based systems engineering (MBSE) methodology for developing the overall system architecture coded with the Systems Modeling Language (SysML). With SysML we use a recursive process to establish three-fold relationships between requirements, logical & physical structural component definitions, and overall behavior (activities and sequences) at successively deeper levels of abstraction and detail. Using this process we have analyzed and refined the LSST system design, ensuring the consistency and completeness of the full set of requirements and their match to associated system structure and behavior. As the recursion process proceeds to deeper levels we derive more detailed requirements and specifications, and ensure their traceability. We also expose, define, and specify critical system interfaces, physical and information flows, and clarify the logic and control flows governing system behavior. The resulting integrated model database is used to generate documentation and specifications and will evolve to support activities from construction through final integration, test, and commissioning, serving as a living representation of the LSST as designed and built. We discuss the methodology and present several examples of its application to specific systems engineering challenges in the LSST design.

The DECam Data Acquisition and Control System

In this paper we describe the data acquisition and control system of the Dark Energy Camera (DECam), which will be the primary instrument used in the Dark Energy Survey (DES). DES is a high precision multibandpath wide area survey of 5000 square degrees of the southern sky. DECam currently under construction at Fermilab will be a 3 square degree mosaic camera mounted at the prime focus of the Blanco 4m telescope at the Cerro-Tololo International Observatory (CTIO). The DECam data acquisition system (SISPI) is implemented as a distributed multi-processor system with a software architecture built on the Client-Server and Publish-Subscribe design patterns. The underlying message passing protocol is based on PYRO, a powerful distributed object technology system written entirely in Python. A distributed shared variable system was added to support exchange of telemetry data and other information between different components of the system. In this paper we discuss the SISPI infrastructure software, the image pipeline, the observer interface and quality monitoring system, and the instrument control system.

The LSST camera overview: design and performance

The LSST camera is a wide-field optical (0.35-1μm) imager designed to provide a 3.5 degree FOV with 0.2 arcsecond/pixel sampling. The detector format will be a circular mosaic providing approximately 3.2 Gigapixels per image. The camera includes a filter mechanism and shuttering capability. It is positioned in the middle of the telescope where cross-sectional area is constrained by optical vignetting and where heat dissipation must be controlled to limit thermal gradients in the optical beam. The fast f/1.2 beam will require tight tolerances on the focal plane mechanical assembly. The focal plane array operates at a temperature of approximately -100°C to achieve desired detector performance. The focal plane array is contained within a cryostat which incorporates detector front-end electronics and thermal control. The cryostat lens serves as an entrance window and vacuum seal for the cryostat. Similarly, the camera body lens serves as an entrance window and gas seal for the camera housing, which is filled with a suitable gas to provide the operating environment for the shutter and filter change mechanisms. The filter carousel accommodates 5 filters, each 75 cm in diameter, for rapid exchange without external intervention.

The LSST camera system overview

The LSST camera is a wide-field optical (0.35-1um) imager designed to provide a 3.5 degree FOV with better than 0.2 arcsecond sampling. The detector format will be a circular mosaic providing approximately 3.2 Gigapixels per image. The camera includes a filter mechanism and, shuttering capability. It is positioned in the middle of the telescope where cross-sectional area is constrained by optical vignetting and heat dissipation must be controlled to limit thermal gradients in the optical beam. The fast, f/1.2 beam will require tight tolerances on the focal plane mechanical assembly. The focal plane array operates at a temperature of approximately -100°C to achieve desired detector performance. The focal plane array is contained within an evacuated cryostat, which incorporates detector front-end electronics and thermal control. The cryostat lens serves as an entrance window and vacuum seal for the cryostat. Similarly, the camera body lens serves as an entrance window and gas seal for the camera housing, which is filled with a suitable gas to provide the operating environment for the shutter and filter change mechanisms. The filter carousel can accommodate 5 filters, each 75 cm in diameter, for rapid exchange without external intervention.

Graphical User Interfaces of the Dark Energy Survey

The Dark Energy Survey (DES) is a 5000 square degree survey of the southern galactic cap set to take place on the Blanco 4-m telescope at Cerra Tololo Inter-American Observatory. A new 500 MP camera and control system are being developed for this survey. To facilitate the data acquisition and control, a new user interface is being designed that utilizes the massive improvements in web based technologies in the past year. The work being done on DES shows that these new technologies provide the functionality and performance required to provide a productive and enjoyable user experience in the browser.