May 6, 2005  


Second GLAST Tower Installed

By Matthew Early Wright

GLAST has started to take physical shape with the installation of the second tracking tower last month. Now the team can perform the first tests ever of multiple towers in the grid array. Researchers expect to learn much about how the towers and the analysis software will cooperate to eventually produce sharp images of the gamma-ray sky.

GLAST Integration, Testing and Calibration, Quality Assurance and Tracker teams shown at the LAT GRID on April 11, shortly after its completion.
(Photo by Diana Rogers)

This is the second major milestone in the instrument’s construction since the first tracker arrived from Italy in late January (see First GLAST Tracker, TIP, February 4, 2005). Eventually, 16 such towers will constitute the main array of the instrument, known as the Large Area Telescope (LAT) grid.

“When the hardware finally comes together, that’s an exciting time,” said Lowell Klaisner (GLAST), LAT project manager. “It gives a whole different feeling than working through technical details.”

The GLAST team celebrated the installation after the All Hands meeting on April 13.

Each tower consists of a silicon strip tracker detector, a cesium iodide calorimeter, an electronics unit and an individual power supply. The complex design of the LAT towers makes installing them a delicate task. Now that the first two have been installed successfully, team members expect that the rest will go in just as smoothly.

“The mechanical assembly went together flawlessly, with only minor issues that were easily addressed,” said Elliott Bloom (EK), team manager for Integration, Testing and Calibration (I&T). “It’s a big sigh of relief, a big celebration.”

Though the towers function as individual units, it is also important that they work well together. Depending on the angle at which a gamma ray strikes, a single event may leave particle traces across two or more towers. In this situation, it is important for the trackers to yield consistent information and for the software to reconstruct an accurate picture of the event.

As the LAT takes shape, the team will perform more involved tests to see how the LAT functions as a unit. These tests will examine not only how the towers function together, but also how well the system software can integrate data from separate towers.

The I&T team observed the first tracks through both towers last week. A graphic of the muon tracks through both towers has been posted to the Web site at

“This is the first time we’ve seen a track in more than one tower – ever,” Bloom said. Further analysis of these two-tower tests will let the researchers know what areas need work. “We will see if there are any differences in the individual towers, and how their geometry reacts.”

The type of high-energy gamma rays that GLAST will image in space do not penetrate Earth’s atmosphere, so the team has to rely on other sources to test the LAT array here on Earth. Cosmic rays can be used to test the tracker units. But the team needs a different strategy to test data processing hardware and software, since cosmic rays do not strike frequently or with enough energy to reflect the high rate of information GLAST will collect in space.

“We do simulations in the test bed to reflect the full event rate,” Klaisner said. “This will show that the computers can handle the data processing.”

Collaborators at Ohio State have developed software that can generate a large number of random events. This information is fed directly into the tower electronics module, producing an accurate simulation of actual gamma-ray data.

“The signals from the simulation are identical to what you’d get from a tracker,” Klaisner said. “And real electronics and real software are processing the signal.”

Another major milestone will come with the installation of the next two towers into the grid by the end of May. Testing four towers together can give enhanced three-dimensional resolution over that offered by only two towers. It will also present I&T team members with challenges they have not yet faced.

“Four trackers form a quadrant, which means new issues,” Bloom said. “We will have to install cable trays, flight cables, and additional support structures. But from there, each quadrant is identical.”

Klaisner is proud of the team and their latest accomplishment. “It’s a credit to the people involved,” he said. “The installation went very smoothly, and I expect it will go just as smoothly 14 more times. Then we’ll be done.”

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The Stanford Linear Accelerator Center is managed by Stanford University for the US Department of Energy

Last update Friday May 06, 2005 by Topher White