GLAST Test Bed Complete
Peeking through the glass doors of a room
in Bldg. 84, the occasional passer-by puzzles at a giant, revolving
electronics contraption skewered on what looks like a cow-sized spit.
The imposing apparatus, completed last month, is the Large Area
Telescope (LAT) Test Bed, a hardware simulator part of the Gamma-Ray
Large Area Space Telescope (GLAST) development.
Sergio Maldonado (REG) at
the LAT’s test bed front-end electronics simulator (FES).
(Photo by Diana Rogers)
Scheduled to launch in 2007, GLAST will
scan the sky in our galaxy and elsewhere in the universe for such
gamma-ray sources as black holes and supernova explosions. With its
unprecedented energy range (20 MeV-300 GeV) and resolution, the four-ton
probe may also discover new and unexpected sources of gamma rays.
The LAT is GLAST’s instrument, an array
of 16 tower modules. Each tower module, assembled in collaboration with
Italy’s INFN, will consist of a silicon strip tracker (designed by UC Santa Cruz and Japanese physicists) and a cesium iodide
calorimeter (provided by the Naval Research Laboratory in Washington,
D.C., in collaboration with Swedish and French physicists).
Gamma rays hitting the LAT will cause showers of electrons. Based on
which silicon strips pick up the shower and on the energy absorbed by
the calorimeter, the read-out electronics will reconstruct the gamma
ray’s trajectory and energy.
LAT will also include an Anti-Coincidence Detector to pick up the
ricocheting particles, which indicate if an event is caused by a
background particle rather than a genuine gamma ray. “The read-out
electronics will have to decide whether all this data corresponds to an
actual cosmic gamma-ray event, and say: 'I think I see a gamma ray. We
should store this data, and send it to Earth,'” Jana Thayer (REG)
Designed to check if the on-board electronics work the way they are
supposed to, the test bed has two main parts. On the front end is a
four-by-four array of stacks of electronics boards, which simulate the
signals produced by the LAT tower modules. Each board is the same size
as the base of an actual tower module, so the whole simulator gives an
idea of the dimensions of the whole probe.
On the rear end—the side hidden from the gawker’s eye—is a full
prototype of the LAT’s read-out electronics. Gunther Haller (REG) headed
the design team. “Soon we will put fake data in—about 100 megabytes
per second—and read it from the other side, to see if what we get out
matches what we put in,” says Jana Thayer.
The simulator will also be useful once the real GLAST is in orbit,
assisting with the LAT’s calibration and helping diagnose any problems
that may come up.
The first, preliminary version of the entire system was powered up on
“I just put the last chip in,” says Gregg Thayer (REG) as, tethered to
his bench by an anti-static wrist strap, he finishes assembling a Data
Acquisition Board, a crucial component of the simulator. The rear end of
the simulator is functionally identical to the electronics of the actual
LAT, boasting over 200 custom made ASIC and programmable-logic (FPGA)
chips—though not the $8,000 apiece, flight-certified versions. Radically
different from your usual Pentium chip, an FPGA can be programmed,
allowing for extreme flexibility during the design and development of
Some of the boards fabricated at SLAC will be shipped to other GLAST
labs around the world, but this will be the only lab with a ‘fully
populated’ simulator, according to Thayer.
Both the tracker and the calorimeter are close relatives of instruments
found in BABAR,
which explains the , which explains the crucial role of high-energy
physicists in the project. “The LAT is a nice, little particle physics
detector—only it is going into space,” said Jana Thayer, whose previous
experience includes working at Cornell’s CLEO detector.
The amount of data GLAST will handle is not comparable to BABAR’s
terabytes, but is still impressive for a space probe. And GLAST’s
circuits will embed extra redundancies, since humans will not be on hand
to replace failed parts. “This is an unprecedented amount of electronics
going into space,” Thayer says.