Mechanical Performance

1
Introduction
Trigger System

• VERITAS is a major new ground-based observatory
  for studying nonthermal astrophysics in the
  gamma-ray band above 100 GeV. VERITAS is
  currently located at the Fred Lawrence Whipple
  Observatory Base Camp (111º W, 32º N) in southern
  Arizona, USA. It will have more than an order of
  magnitude greater sensitivity than the Whipple
  gamma-ray telescope, its predecessor and the
  pioneering instrument in the field.
• Stereo observations with the first two of four
  telescopes began in January, 2006. For this
  commissioning period, known TeV sources were
  observed. Results from one of these sources, the
  blazar Mrk421, are presented here to demonstrate
  the performance of the first two telescopes.

Single Telescope
Hardware Stereo Array Trigger
Stereoscopy
Composite Camera View

• Night-sky and muon events strongly suppressed at
  trigger level by two-telescope trigger
  requirement.
• Source position in field of view reconstructed
  using intersection of image axes.
• Improved angular and energy resolution
• Multiple views of the shower allow a more
  accurate determination of its core location and
  arrival direction, yielding an enhanced angular
  resolution and improved energy resolution.
• Lower energy threshold
• The background due to local muons is an important
  factor with regard to the lower limit on the
  energy threshold of a single telescope thus
  eliminating that background, together with
  increasing the total mirror area and improving
  the discrimination between gamma rays and cosmic
  rays all act to lower the energy threshold.

T1
T2
Stereo view of a large cosmic-ray event.
Individual Camera Views
T2
T1
Stereo view of a smaller event, along with summed
FADC traces (2-ns samples).
MechanicalPerformance

• Alt-Az mount
• Slew speed 1.0º/sec
• Tracking accuracy
• Raw pointing error RMS 20-30
• ?-ray source location good
• Precision continuous pointing monitor under
  development

Gamma/Hadron Separation
Optical Performance
MSW On (blue) and Off (black pts)

• Background rejection is based on differences in
  the air-shower development between gamma-ray and
  cosmic-ray primaries (and, for targets with a
  known position in the FoV, on the shower
  direction).
• Leads to morphological differences in camera
  images.
• Gamma-ray showers are primarily electromagnetic.
• compact, regular images.
• Cosmic-ray showers are primarily hadronic.
• images larger, often uneven light distribution
  (subshowers).
• Parameterize images as ellipses and use ellipse
  length and width (semi-major and semi-minor
  axes) to reject cosmic-ray primaries.
• Combine images from multiple telescopes with
  weightings determined via simulations mean
  scaled length (MSL) and width (MSW).

g-ray excess

• 350 mirror facets
• glass, aluminum coated on-site
• reflectivity gt90 at 320 nm

On - Off
Acknowledgements This research is supported by
grants from the U.S. Department of Energy, the
National Science Foundation, the Smithsonian
Institution, by NSERC in Canada, by Science
Foundation Ireland, and by PPARC in the UK.
MSL On (blue) and Off (black pts)
g-ray excess
On - Off