Linux, Talon and Astronomy
We've been fascinated by astronomy since the ancient Chinese first charted the skies. The computerization of astronomy, some would argue, is its greatest leap forward yet. Today, unattended robotic telescopes scan skies that have been charted over centuries, recording their findings in modern databases. CCD cameras capture images impossible to define on film. It's an exciting time to be an astronomer, whether amateur or professional.
The revolution in astronomy doesn't stop at the hardware. Research-grade telescopes in observatories from Spain to Korea are under the control of open-source software and Linux-based computers. Under the open-source model, scientists are free to modify the control software, creating a trickle-down effect that benefits amateurs. Open source and Linux even have changed the scientific method. With source code freely available, peer review now occurs not only on the data, but on the data gathering methods as well.
At the forefront of this open-source astronomy revolution is Talon. Talon was originally developed by Ellwood Downey as the Observatory Control and Astronomical Analysis Software (OCAAS). In 2001, the software was purchased by Torus Technologies of Iowa City, Iowa. In late 2002, Torus was purchased by Optical Mechanics, Inc., and the updated OCAAS package was released as Talon under the GPL.
During the past two years, I've had the daily pleasure of working with Talon. I've installed and configured the software on multiple telescope packages, and I've followed these telescopes to destinations around the world for installation and on-site configuration. It's my pleasure to share with you some of the broad points of Talon installation, configuration and use.
Talon can be downloaded at observatory.sourceforge.net. The software interacts with integrated motion control boards, available from Optical Mechanics, Inc. (Optical Mechanics Motion Controllers) or Oregon Microsystems (PC39 Motion Controllers). Object acquisition and tracking, scheduled operations, environmental monitoring, dome control, image analysis and processing all fall under the control of Talon. Networked operations also are possible using a remote X session.
The Talon package contains a full installation script; install.sh creates a talon user, compiles the binaries and creates a set of text configuration files for initial operation of the telescope.
Talon contains a full compliment of astronomy applications designed specifically for use as a suite of tools. The main Talon interface utilizes the Motif toolset, producing a familiar and unified look and feel throughout the application set. Although the toolset is rich, the following four tools should be of use to most observers.
xobs is the main Talon control window and is launched with the terminal command startTel. It contains all the monitoring and calibration tools necessary for operation. This window provides manual control of the telescope and any attached peripherals, such as a filter wheel or dome control. It also provides a constant display of the current position of the telescope, as calculated by feedback from the motor encoders. This feedback is provided in a set of text boxes within the xobs window.
When using Talon, the first important task is to find the home position of the various encoders located throughout the system. These encoders close the loop on the operation of the axes, providing a static count for the full travel of each axis. Movement of the telescope is calculated in part by the motion of the chosen axis in relation to the zero position on the encoder. Decrementing the Declination encoder, for example, generally moves the telescope to the north. The operation to find homes in the xobs window hunts for and establishes the zero positions on each axis encoder.
Using the software paddle command in the xobs window (Figure 2), the user can position the telescope, filter wheel and focus position manually. The motion of the telescope to the east and west is referred to as the right ascension (RA) or hour angle (HA) of the telescope. To travel north and south is referred to as Declination (Dec). Using positive and negative encoder counts, moving the telescope axes is a simple matter of moving the axis positive or negative x (RA) or y (Dec). These coordinates are in relation to the North Pole.
Additionally, Talon provides data on the weather conditions at the observing site with an attached Davis weather station. This feature ensures that the telescope is not exposed to adverse weather conditions during unattended operations. When conditions fall within a predetermined range, the observatory dome or roll-off roof closes, the telescope moves to a stowed position and operations cease. As with the position data, this information is provided in text boxes within the xobs interface.
Finally, xobs provides a search function that allows the user to enter the name of a celestial object, search an internal database and automatically slew the telescope into position to observe and photograph the requested object.
telsched is the element of Talon that makes robotic unattended observing sessions possible. This can be a critical function for institutions conducting research from remote locations or those requiring repeated observations of particular objects over a given period of time.
The telsched command opens a scheduler for these unattended observing sessions. The scheduler automatically calculates images to be taken during the session based on the size (in degrees) of the chunk of sky the user selects. In general, the tighter the area of the sky (fewer degrees), the more images taken. Images taken by telsched during an unattended session are stored in a directory of the user's choice. All instructions created by the telsched program are stored in a flat file. These instructions are referenced by xobs when the telescope is slaved off to robotic control from the xobs interface.
Camera is another terminal-launched application in the Talon suite. It provides complete control over the functions of a CCD camera attached to the telescope.
The camera application includes tools for exposure time, image size, software image filtering and image analysis. Camera also contains tools for adjusting the brightness and contrast of images, determining the area of interest (AOI) of the image and automatically labeling objects by comparison to the World Coordinates System (WCS). The latter tool is, in fact, a pattern-matching algorithm that allows the system to compare known patterns of objects to the WCS database.
xephem provides a software ephemeris, or sky charting interface, for the rest of the Talon suite. As with other ephemerides, it relies heavily on correct geographical and time coordinates; this information can be configured manually by the user. xephem also can be configured to poll an attached GPS at regular intervals, adjusting the system time to account for internal clock drift.
The xephem program, launched from the command line with xephem, provides a granular view of the current sky. Data on each object is provided in a right-click pop-up screen. The user also can point the telescope using this pop-up, a feature used extensively for calibration. Magnification can be increased, effectively looking deeper and deeper into the sky. As an alternative to zooming, the user may select a minimum magnitude (apparent brightness) threshold. This allows brighter stars to be filtered in the ephemeris view, leaving only the dimmer objects in the window. The sky view also may be rotated, and object type filtering is provided. For example, globular clusters can be selected, eliminating the view of all other object types.
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