Optical characteristics of the whipple observatory TeV gamma-ray imaging telescope

Abstract

In TeV γ-ray astronomy, large mirrors are used to collect Cerenkov light from electromagnetic cascades in the atmosphere in order to obtain low energy thresholds. The flux sensitivity of TeV γ-ray detectors is limited by background due to Cerenkov light bursts from isotropic, cosmic-ray showers which are much more numerous than γ-ray showers. It has recently been established that most of this background can be eliminated on the basis of the shapes of Cerenkov light images on the focal plane of a telescope. In order for this technique to work, the light collector must have adequate resolution over a relatively wide field of view. In this paper, the optical characteristics of the 10 m reflector used in the imaging detection of the Crab Nebula are examined and contrasted with those of a standard parabolic design. This 10 m reflector has a unique (Davies-Cotton) design with small spherical facet mirrors placed on spherical support structure with radius equal to exactly 1/2 the curvature radius of the facet mirrors. The off-axis focusing properties of this type of telescope have not been examined previously.