Cherenkov Telescope Array Mirror Alignment Report


The Cherenkov Telescope Array (CTA) is

a groundbased observatory for gamma rays with

very high energy characteristics within the range

of 10 GeV to 100 TeV. It is currently still under

development. As the title of the project suggests,

it takes advantage of the Cherenkov radiation that

is produced by the gamma ray’s interaction with

the earth’s upper atmosphere, similar to the

guiding physical principles of VERITAS. The

primary differences are that CTA will feature

more telescopes and possess a larger energy range.

Cherenkov radiation will be collected by

an array of dozens of optical telescopes of various

sizes. These optical telescopes will be composed

of hexagonal mirrors that must be aligned relative

to each other so as to represent the shape of the

telescope as a whole properly. It was in this area

of research that my efforts were focused. The

most recent proposed system with which to align

the mirrors consists primarily of a laser and a

webcam. Sixty to three hundred snapshots are

taken of the laser and the position of the laser’s

centroid in each frame is measured and analyzed

by a custom-written computer program executed

via ROOT. One to ten microns was the ideal

resolution that was required of the system to meet

the needs of this project.

Since the position of the laser’s centroid

was given by pixel number, it was necessary to

determine the ratio of pixels to millimeters via

another ROOT program described in more detail

on the following pages. It was also necessary to

ensure by using an OPENCV analysis program

that the pixels were not saturated. As will be

shown, the consequences of saturation are

bothersome edge effects that produce erroneous