STAR, A. Falcone et al.

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STAR Very Large Aperture Air Cherenkov
Telescopes Using Small Telescope ARrays

• Abe Falcone
• (Penn State University)
• Henric Krawczynski, James Buckley
• (Washington University)

2
Motivation

• reduction of energy threshold to 40 GeV with
  sensitivity 10x VERITAS
• preserve option for WFOV
• very fast slewing possible
• Observations of gamma ray objects out to
  cosmological redshifts of z 1.0-2.0 ? Probe
  star formation era and increase source count for
  statistical study
• Increased range of source types

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The Concept

• Make equivalent of one large aperture (30-50 m)
  telescope from many small telescopes (144
  telescopes with 2.5 m diameter)
• Then do it again 4 times -- stereo

Assuming Price of Mount ? d2.5
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The Concept

• Combine the signals from the detectors of each of
  the small telescopes after pre-amplification and
  prior to forming trigger, thus the total gathered
  light contributes to one trigger

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The Concept

• Combine several of these telescope sub-arrays
  into an array of several large aperture
  telescopes that can be operated in stereo mode

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Binary Signals
Expected Number of PE In Maximum Pixel
0.07. Most Likely Signals 0 PE or 1 PE.
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2
7
Consequences Detectors Single Photon
Counters Simple Multiplexing Delays and
Signal-Transmission Digital.
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4
2
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Technical Implementation

• Design pointing angle dependent delays for
  individual telescopes prior to trigger generation
• Design/Obtain cheap and reliable mounts and
  positioners
• Identify/Design a suitable focal plane detector.
  We are considering
• avalanche photodiodes
• microchannel Plate with silicon detector (this is
  the most promising candidate at the moment due to
  low noise, low cross-talk levels, and potential
  single PE resolution)
• Multianode PMTs
• Hybrid CMOS Si detector

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Multi-Anode PMTs
Hamamatsu-H8500

• However
• NO single PE resolution with off the shelf
  base
• (can be remedied)
• Potentially high crosstalk
• Expensive

• Size 5cm x 5cm,
• 8 x 8 Pixel,
• Gain 1 Million,
• Rise Time 0.8 nsec,
• Negligible Deadtime.

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MCPSi Detectors
Price/Camera 15,000
Price/Telescope 2.2M

• Development of hybrid MCPSi detectors at
  Washington University and Burle Ind.
• 15 cm diameter or 5 cm diameter
• High Open Area Ratio
• High quantum efficiency
• Excellent single PE resolution
• Fast, low noise readout of pixellated Si detector

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Signal Delay and Multiplexing

• On-Board Multiplexing and Optical Coupling!
• Prototype

Prototype FPGA Board (Buckley et al.)
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Mounts and Positioners

• Positioner design must be robust to minimize
  maintenance of many telescopes
• Pointing accuracy should be within 0.02o
• Slew speeds of several degrees per second should
  be achievable
• Initial investigations indicate that
  specifications should be achievable for
  10k/telescope for the 2.5 m scopes

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Pros Cons of Telescope Arrays

• Advantages
• Achievement of very low threshold due to huge
  total light collecting surface
• Cheap relative to large telescopes due to small
  mounts required for 2.5 meter single dishes
• Potential for fast slewing to look at prompt
  emission since small telescopes can be moved
  quickly
• Ability to use modern detector technology since
  focal plane detector is small
• High sensitivity allowing study of timing at
  unprecedented scales
• Array and sub-array dimensions easily scaled
• Reduction of time-spread seen in traditional
  Davies-Cotton design

• Disadvantages
• Increased of telescopes to maintain (and build)
• Pointing angle dependent delay required
• Many channels required
• Performance Availability of detectors still
  being evaluated

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Cost Trade Offs
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A More Moderate Approach

• Alleviate some disadvantages with a similar array
  strategy, but with telescopes that are large
  enough to get 1 PE/pixel (6 meter)
• Gives options of new camera types or standard
  PMTs
• Less detector noise issues
• Less telescopes to maintain/build
• Camera upgrade easier
• Still avoid technical problems of avoiding
  bohemoths 30 m scopes, such as timing, huge
  cameras, slow slewing, ....

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Open Questions and Work in Progress

• Use Monte Carlo to Compare Sources/ and
  Spectroscopic Capabilities of STAR in combination
  with various VERITAS configurations
• Use Monte Carlo to determine ideal STAR
  configuration
• Optimize primary dish size/configuration
• Technical R D
• Further development testing various detector
  optionsMCPSi Detectors, CMOS Si, multianode
  PMTs
• Reliable Low-Cost Mount/positioner
• Delay and DAQ electronics

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Conclusions

• The STAR approach of building large aperture
  IACTs has potential to bring the threshold of
  IACTs down to 40 GeV, thus opening the field to
  more scientific studies of previously undetected
  sources. STAR is still in the exploratory phase,
  and there are several tasks left to complete,
  including
• Use Monte Carlo to simulate detector response and
  sensitivity in combination with VERITAS
• Optimize configuration of array, based upon
  Monte Carlo studies
• Design pointing angle dependent delay
• Evaluate/Develop detectors and readout
  electronics
• Evaluate telescope mount options

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Monte Carlo Simulations
Y o
0.4o
Parabolic f1.2, Ø 25 m Tiles 60 cm
STAR Dav. Cot. f1.2, Ø 2.5 m Tiles 60 cm
X o