1. Future Imaging Atmospheric Cherenkov Telescopes:
Performance of Possible Array Configurations for g-photons in the GeV-TeV Range
S. Sajjad, A. Falvard and G. Vasileiadis
LPTA, Université Montpellier 2, CNRS/IN2P3,
Montpellier, France
Now, the current generation of IACT has shown us how these instruments are possibly the best tools around for probing the gamma-ray universe in a large part of the GeV-TeV domain. They have brought us a wealth of information, opened up new questions during the past few years. This also means that the next generation of IACT will be expected have better capabilities in order to move further in answering current questions in this domain. I am going to present a study we carried out concerning the a couple of possible configurations at two different altitudes.
ICRC’07- Merida

2. Choices and assumptions
Focus on gamma shower simulation and reconstruction:
Source, shower core, energy and effective area
gamma-hadron identification issues have not been touched
Parabolic mirrors
In order to focus on the issues concerning the showers themselves, we have ignored:
geomagnetic field
night sky background
image cleaning

3. The Tools: Package developed for
Simulating the response of IACT arrays to atmospheric showers
Reconstructing the various shower parameters
 will be made public
Before I come to that let me introduce the tools we used to study these configurations.

4. The Tools: Simulation Shower simulation done through CORSIKA
(version currently used - will be updated)

5. The Tools: Simulation Shower simulation done through CORSIKA
(version currently used will be updated)
Telescope simulation
Uses CORSIKA output

6. The Tools: Simulation Shower simulation done through CORSIKA
(version currently used will be updated)
Telescope simulation
Uses CORSIKA output
Reflection of Cherenkov photon by a parabolic mirror onto camera plane

7. The Tools: Simulation Shower simulation done through CORSIKA
(version currently used will be updated)
Telescope simulation
Uses CORSIKA output
Reflection of Cherenkov photon by a mirror onto camera plane
Variable diametre, focal length, camera size, pixel size, camera position

8. The Tools: Simulation Shower simulation done through CORSIKA
(version currently used will be updated)
Telescope simulation
Uses CORSIKA output
Reflection of Cherenkov photon by a mirror onto camera plane
Variable diametre, focal length, camera size, pixel size, camera position
Position and orientation of individual telescopes

9. The Tools: Simulation Shower simulation done through CORSIKA
(version currently used will be updated)
Telescope simulation
Uses CORSIKA output
Reflection of Cherenkov photon by a mirror onto camera plane
Variable diametre, focal length, camera size, pixel size, camera position
Position and orientation of individual telescopes
Array simulation (simulate up to 100 telescopes)

10. The Tools: Shower reconstruction
Source reconstruction
Superposed images from all tels in the camera frame of ref.:
The axes from all images should meet at one point: the source
Each axis should pass from the centroid of corresponding image
The distance of the pixels from the corresponding axis should be minimum
The transverse profile of images is considered to be Gaussian
source position

11. The Tools: Shower reconstruction
Source reconstruction
Superposed images from all tels in the camera frame of ref.:
The axes from all images should meet at one point: the source
Each axis should pass from the centroid of corresponding image
The distance of the pixels from the corresponding axis should be minimum
The transverse profile of images is considered to be Gaussian
Likelihood function
source position

12. The Tools: Shower reconstruction
Source reconstruction
Superposed images from all tels in the camera frame of ref.:
The axes from all images should meet at one point: the source
Each axis should pass from the centroid of corresponding image
The distance of the pixels from the corresponding axis should be minimum
The transverse profile of images is considered to be Gaussian
Likelihood function
-ln(Lall) map
source position

13. The Tools: Shower reconstruction
Source reconstruction
Superposed images from all tels in the camera frame of ref.:
The axes from all images should meet at one point: the source
Each axis should pass from the centroid of corresponding image
The distance of the pixels from the corresponding axis should be minimum
The transverse profile of images is considered to be Gaussian
Likelihood function
-ln(Lall) map
source position

14. The Tools: Shower reconstruction
Shower core reconstruction
Same principle as for source reconstruction
Calculations in the ground frame of reference
-ln(Lall) map
core position N1 N3 N2 d1 d2 d3 d4

15. The Tools: Shower reconstruction
Shower core reconstruction
Same principle as for source reconstruction
Calculations in the ground frame of reference
-ln(Lall) map
core position N1 N3 N2 d1 d2 d3 d4 N4
Energy reconstruction
Uses the linear relationship between shower energy and the average number of photo-electrons in the image

16. Energy domains and choices for telescopes
The observational issues and physics goals depend on the energy domain
Different parts of the arrays could be optimised for observations in different energy domains

17. Energy domains and choices for telescopes
The observational issues and physics goals depend on the energy domain
Different parts of the arrays could be optimised for observations in different energy domains
~300 GeV to a few tens of TeV
Domain where IACT show best performance
Good reconstruction and gamma-hadron discrimination with10-15m diam. telescopes
Need to improve sensitivity
Large number of medium sized tels over a large surface
 12.5 m telescopes

18. Energy domains and choices for telescopes
The observational issues and physics goals depend on the energy domain
Different parts of the arrays could be optimised for observations in different energy domains
~Below 300 GeV
Showers are smaller, more fluctuations, poorer reconstruction and gamma-hadron separation
Fluxes from sources tend to be higher
Need to collect more Cherenkov light from showers
A few telescopes with large diam. telescopes
 30 m telescopes

19. Optimum telescope separation studied in terms of shower reconstruction
Selection of array parameters
Optimum telescope separation studied in terms of shower reconstruction
Square unit of
4 telescopes
preliminary
Below 300 GeV
50 GeV
30 m
300 GeV- few TeV
300 GeV
12.5 m
Trigger: at least two telescopes with 50 p. e.

20. Optimum telescope separation studied in terms of shower reconstruction
Selection of array parameters
Optimum telescope separation studied in terms of shower reconstruction
1200 m
400 m
preliminary
Below 300 GeV
50 GeV
30 m
300 GeV- few TeV
300 GeV
12.5 m
Trigger: at least two telescopes with 50 p. e.

21. Optimum telescope separation studied in terms of shower reconstruction
Selection of array parameters
Optimum telescope separation studied in terms of shower reconstruction
1200 m
400 m
preliminary
Below 300 GeV
50 GeV
30 m
300 GeV- few TeV
300 GeV
12.5 m
Trigger: at least two telescopes with 50 p. e.

22. Optimum for 50 GeV(tel. diam. 30m) ~200m
30 m diam.
4 tels
200 m
For 1800 m a. s. l.
Optimum for 50 GeV(tel. diam. 30m)
~200m

23. Optimum for 300 GeV(tel. diam. 12.5m) ~140m For 1800 m a. s. l.
30 m diam.
4 tels
12.5 m diam.
33 tels
400 m
100 m
140 m
Optimum for 300 GeV(tel. diam. 12.5m)
~140m
For 1800 m a. s. l.
Optimum for 50 GeV(tel. diam. 30m)
~200m

24. Two telescope configurations
420 m
400 m
100 m
140 m
Two telescope configurations
30 m diam.
4 tels
12.5 m diam.
33 tels
30 m diam.
5 tels
12.5 m diam.
49 tels

25. Two telescope configurations
360 m
At 3600 m
 System rescaled
350 m
87 m
120 m
Two telescope configurations
30 m diam.
4 tels
12.5 m diam.
33 tels
30 m diam.
5 tels
12.5 m diam.
49 tels

26. Results Effective area 1800 m 1800 m 3600 m ~1.8X106 m2 ~1.5X106 m2
1 TeV
~1.8X106 m2
~1.5X106 m2
300 GeV
~1.6X106 m2
~1.2X106 m2
50 GeV
~0.8X106 m2
~0.6X106 m2
Typical effective areas of 4 tel systems: X106 m2

27. Results Four telescope array (FTA) diam=12.5m
Source reconstruction precision LA
FTA
1 TeV
0.05°
~0.06°
50 GeV
~0.17~°
~0.25~°
Large arrays (LA)
Reconstruction done for cores within an 800X800 m2 region (after trigger)

28. Core reconstruction precision
Results
1 TeV
~5 m
300 GeV
~8 m
50 GeV
20-25 m
Typical precision on shower core for 1 TeV for a 4 tel array
core
100,100
~2m
200,200
20 m
Reconstruction done for cores within an 800X800 m2 region (after trigger)

29. Results Energy resolution ~5% ~8 % ~15 % ~5% ~25% 1 TeV 300 GeV 50 GeV
Typical precision on shower core for 1 TeV for a 4 tel array
core
100,100
~5%
200,200
~25%

30. Summary
A public package capable of simulating IACT system response to atmospheric showers and reconstructing gamma-ray parameters has been realised.
It was used to study the effect of telescope separation on various reconstruction parameters.
Two arrays with 37 and 54 telescopes have been studied at 1800 m and 3600 m for gamma parameter reconstruction and show considerable improvement with respect to the current generation.