The Telescope Array Project

By Stan Thomas, University of Utah – Physics Department

The Telescope Array Project ( is an experiment to study ultra-high energy cosmic rays located near Delta, Utah. The cosmic rays are ultra-high energy particles that hit the upper atmosphere. The original particle creates a cascade of millions of daughter particles that eventually reaches the ground.

At ground level we have an array of over 500 particle detector stations covering an area roughly 30 km by 30 km. Overlooking the array are three telescope stations. The telescope stations have large mirrors and cameras made up of 16x16 arrays of photomultiplier tubes. The telescopes record an image of the cosmic particle shower as it passes through the atmosphere and the particle detector stations record the number of particles hitting the ground.

To accurately measure the amount of light produced by the cosmic ray particle shower we need to calibrate the telescopes. One method is to use a laser with accurately measured energy to create a simulated cosmic ray shower in the atmosphere. We want to locate the laser around 4km from the telescopes to minimize the uncertainty in the amount of light reaching the telescope due to atmospheric variations.

The plan is to set up a remote, solar powered laser station. The laser would be operated for 5-10 minutes twice per night during moonless nights. The laser station needs to be operated remotely throughout the year, winter and summer. The plan is to use one radiometer to measure a pick off from the main beam. The absolute calibration of that radiometer is not important.It's calibration just needs to be stable over long periods of time (1-2 months). A second radiometer would be used to measure the absolute energy of the laser. Once or twice per month we would use the absolutely calibrated radiometer to measure the ratio between the pick of beam energy and the absolute beam energy. Then we use the pick of beam energy to compute the energy of the primary beam which is sent skyward.

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