Justification of VERITAS parameters

To investigate the optimum array configuration, several parameters were varied: the number of telescopes, the spacing between the telescopes, the focal length of the telescope, the aperture of the telescopes, and the field of view of the cameras. The number of channels in the camera was fixed before the optimization for cost reasons. Thus, our investigations of different camera fields of view are simultaneously investigations of changes in the spacing of the PMTs. In the central energy range of 200GeV to 1TeV, the sensitivity of the array to point sources is not significantly affected by small changes in the array's characteristics. Thus, the choices of telescope parameters are driven mostly by their effects on the performance of VERITAS at the low and high ends of its sensitive energy range.

We have chosen the stereoscopic route to advance the atmospheric Cherenkov technique due to its excellent angular resolution and background rejection. At a minimum, three telescopes are necessary to utilize the stereoscopic imaging technique. Two telescopes can be used for bright images, but faint cascades can only be accurately reconstructed with images in three telescopes because the individual image axes are not well-defined. There are three primary reasons for choosing a seven telescope array over one with three to four telescopes. First, a seven telescope array is significantly more flexible because it can be split into sub-arrays halving the time required for surveys, making it possible to more effectively monitor variable AGN and to more efficiently participate in correlated observational campaigns, e.g., with GLAST, X-ray telescopes and other Cherenkov telescopes. Previous multiwavelength campaigns with the Whipple telescope have been highly successful in probing physical mechanisms in AGN. Second, when used as a single instrument, a seven telescope array better measures spectra over a broad energy range due to larger collection area and sensitivity at low energies. Determination of accurate spectra over broad energy ranges bears directly upon models of gamma-ray production mechanisms for SNR, AGN and perhaps models of intergalactic background radiation. Finally, some cosmic sources (e.g., gamma-ray pulsars or gamma-ray bursts) may be detectable only in the lowest energy range which is accessible only with the full seven-telescope array.

The separation between the telescopes was chosen to be 80m. This provides the best combination of flux sensitivity and energy threshold. Increasing the spacing does not improve flux sensitivity in the 200GeV - 1TeV energy range but it does increase the energy threshold. Decreasing the spacing can reduce the energy threshold somewhat, but the sensitivity is significantly impaired over the entire energy range because the stereoscopic capabilities become less efficient and the effective area of the array decreases.

The chosen focal length for the telescopes is 12m, making this an f/1.2 system. Increasing the focal length from the current f/0.7 system significantly improves the optical quality of the telescopes by reducing the effects of aberrations (see § 3). This improvement is necessary to match the angular size of the PMTs in the VERITAS cameras ( $\sim 0.15^\circ$). However, increasing the focal length beyond 12m does not improve the performance of VERITAS enough to justify the increased cost and complexity needed to achieve the required alignment precision and stability for the pixel size chosen. Also, the reduction in energy threshold for increasing the focal length beyond 12m is outweighed by the less efficient light collection caused by increasing the pixel separation to maintain the required field of view. Thus, 12m provides the best combination of improved optical quality, sufficient field of view and small pixel spacing.

The chosen field of view of the telescope is 3.5$^\circ$. From experience operating the Whipple telescope, this field of view is required for individual telescopes to accurately reconstruct images from point sources and it also gives significant sensitivity to off-axis or extended sources. Increasing the field of view beyond this will improve the high energy response of the telescope, but also increases the energy threshold for the fixed number of channels. The chosen field of view is a compromise between these competing priorities.