I. K. Kaul

Drive-Control System for the TACTIC Gamma-Ray Telescope

A PC-based drive-control system has been developed for the altitude-azimuth mounted TACTIC γ-ray telescope to control the speed and direction of motion of its 2-axes. Details of various hardware components chosen for the telescope, including hybrid-stepper motor, 16-bit absolute encoder and CAMAC-based programmable stepper motor controller, are discussed in this paper. The telescope-control strategy, based on the position-loop with a proportional type control for the source-seek mode and the on/off type control for the tracking mode, is explained in detail. Some important performance features of the telescope, including its blind-spot size, drive-system backlash and encoder-error compensation, are also presented. The drive system has been extensively field-tested and has been operating satisfactorily during observation campaigns carried out since March 1997 with the TACTIC Imaging Element. A tracking accuracy of ±3 arc-minutes has been achieved. A test report of its performance, with regard to its tracking accuracy on the basis of successful detection of TeV γ-rays from the active galaxy Markarian 501 in April–May 1997, is also presented.

On a single-counts rate stabilization scheme employed in the imaging camera of the TACTIC γ-ray telescope

A cost-effective methodology for operating the 349-pixel Cerenkov light imaging camera of the imaging element of the TACTIC γ-ray telescope array at stable single-counts rate and safe anode current values has successfully been tried out despite variations in the light of the night sky experienced by these pixels from time to time. Important details of this unconventional scheme are discussed and reference is made to the methods evolved for in situ absolute and relative gain calibrations of the camera pixels, which are particularly required for this mode of operation.

Hybrid Zener-based VDN - a better choice for Cerenkov light imaging cameras

A Zener-based voltage divider network has been used for the photomultiplier tubes (PMTs) employed in the multi-pixel camera of a Cherenkov imaging telescope. This hybrid network ensures a linear photomultiplier response at a chain current nearly an order of magnitude less than that required for a purely resistive voltage divider network of conventional design. The network has been validated for the Thorn-EMI photomultiplier tube 9135B and the results discussed here are also representative of other PMT makes.