2. The IACT system of HEGRA
The HEGRA telescope system consists presently of 4, in the near future of 5, identical IACTs - one at the center and 3 (in future 4) at the corners of a 100 m by 100 m square area. The multi-mirror reflector of each telescope has an area of 8.5 m2. Thirty front aluminized and quartz coated spherical mirrors of 60 cm diameter and of 4.9 m focal length are independently mounted on an almost spherical frame of an alt-azimuth mount, following the design of Davies and Cotton. Each telescope is equipped with a 271 channel camera of pixel size resulting in an effective field of view of . The PMT pulses are fed into trigger electronics and into shapers followed by 120 MHz flash analog-to-digital converters (FADCs). A multilevel trigger demands at least two adjacent pixels in each of at least 2 telescopes (Bulian et al. 1998). The topological "next-neighbor" condition of two adjacent pixels reduces the number of night sky background triggers. In the following analysis we use the software trigger condition of at least two telescopes with images with more than 40 photoelectrons.
At the beginning of each night, the camera is flat-fielded using an UV laser at each telescope to illuminate a scintillator via an optical cable. The scintillator emits a spectrum with peak emission in the near-UV and blue, similar to atmospheric Cherenkov light. An absolute calibration of the system has been performed with a direct laser measurement and a calibrated low-power photon detector (Fraß et al. 1997). This measurement has determined the conversion factor from photons to FADC counts with an accuracy of 10%.
The pointing of the telescopes is checked on a regular basis with so called "point runs" (Pühlhofer et al. 1997), where a section of the sky surrounding a bright star is scanned. The pointing of each telescope is inferred from the currents measured in the PMTs surrounding the image of the star. After applying the resulting pointing correction function, an effective pointing accuracy of better than is achieved. This accuracy has been experimentally confirmed with -ray data from the Crab Nebula and Mkn 501 (Pühlhofer et al. 1997).
The first system telescope, called CT3, was installed in December 1995. Subsequently CT4 started operation in July 1996, CT5 in September 1996, and CT6 in November 1996. The array of 4 telescopes is operational since end of November 1996. Since then, minor changes of the hardware were carried out. These are summarized in Table 1. In the following, HEGRA data from March 1997 to October 1997 are used. The relevant hardware changes are (i) a mirror adjustment of CT3 and CT4 on May 12th, 1997 and (ii) the incorporation of the topological next-neighbour trigger condition on hardware level for CT3 and CT6 as well, and a reduction of the single pixel trigger threshold for all IACTs from 10 to 8 photoelectrons on June 24th, 1997. CT4 and CT5 had been operated with the next-neighbour trigger condition from the very beginning. These changes divide the Mkn 501 1997 data into 3 groups (period I - period III). In the next subsection it will be shown how the data is corrected for these changes by the use of detailed Monte Carlo simulations.
© European Southern Observatory (ESO) 1999
Online publication: December 22, 1998