Second GLAST Tower Installed
By Matthew Early Wright
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.
Integration, Testing and Calibration, Quality Assurance and
Tracker teams shown at the LAT GRID on April 11, shortly after
(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
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
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
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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