Center for Cosmological Physics
Enrico Fermi Institute
Mini-Symposium on the Auger
Observatory
October 4, 2002
Giorgio Matthiae
University of Roma II and INFN

4 peripheral stations (eyes)
6 fluorescence telescopes / station
Azimuthal angle of view 180 0
Los Leones, Coihueco, Los Morados, Norte

Installation of the mirror supports

Diaphragm
PMT camera
Spherical mirror
Shutter
UV Filter, corrector ring

Spherical aberration Coma aberration
C
F
C spot
C
Diaphragm
C
Spherical focal surface
Coma suppressed

Design of the telescopes
Basic parameters defined from the requirement of accurate measurement of the shower profile
Aperture: 1.5 m 2 effective area
Pixel size: 1.5 degrees
Schmidt optics:
- coma aberration eliminated, circle of least confusion (spot) independent of the incident direction
- aperture defined by the diaphragm
- mirror size larger than for classical design
Spot size from spherical aberration :
Δs ~ h (h/R) 2 , Δθ = Δs/R ~ (h/R) 3 f/1 optics is a good compromise: R = 3.4 m
Diaphragm diameter = 1.7 m
Spot size : 0.5 degree ( 15 mm diameter)
Pixel size: 1.5 degrees (45 mm )
(the spot size is 1/3 of the pixel size)
Field of view: 30 degrees azimuth
28.6 degrees elevation

Shape nearly square due to square field of view.
Size: 3.5 m x 3.5 m in order to avoid vignetting.
Tesselation: 6 x 6 elements
• Aluminum
Reflectivity: 88.0% (with Al
2
•
Polished Glass
O
3 coating)
Reflectivity: 86.3% (with SiO
2 coating)
The mirror elements are mounted on a rigid support structure.
Each element can be accurately aligned independently.
Quality tests:
reflectivity at 370 nm
- spot size obtained with point light source at the center of curvature

PMT camera
The FD telescope at
Los Leones mirror
Front end / read-out electronics
HV + LV

The ring lenses (aspherical profile) correct the additional spherical aberration, keeping the spot size within the design value of
15 mm diameter factor 2 gain in light collection

• The diameter of the spot is 15 mm as calculated.
• Good check of the alignment of the mirror elements

Fluorescence spectrum of nitrogen

The UV filter
•
The UV filter (M-UG6) matches the fluorescence spectrum of N
2.
• Transmission: about 85 % at 350 nm, down to
20 % at 300 nm and 400 nm.
Reduction of “dark sky background” by nearly a factor of 8.

• Array of 440 hexagonal pixels placed on the spherical focal surface.
(22 rows x 20 columns)
•
Pixel: PMT XP3062 with light collectors
(45 mm wide)

The camera light collectors
•
Light collectors to recuperate light incident between the
PMTs or at the very edge of the photocathode.
• Plastic elements covered by aluminized mylar.
• Test with light source simulating the spot created by the mirror shows recuperation of light.

440 PMTs
90 cm

PMT active divider
Better gain stability passive
Dark sky background active

The PMT signal is sampled at a rate of 10 MHz by FADC with 12 bits.
100 ns
First Level Trigger : Threshold regulated to keep single pixel rate at a given value, around 100 Hz.
Second Level Trigger : pattern recognition algorithm
5 adjacent pixels
Third Level Software Trigger : time – space correlation

GPS time (hybrid operation) data

Xe lamp + optical fibers
• Equalization of PMT gain
• Stability of gain

Direct measurement of the response of each channel to a given flux of incident photons.
Wide light beam of uniform intensity provided by a UV LED (375 nm) and a flat cylinder (“drum”) with diffusing walls mounted outside the telescope aperture (ideally a “dome”).
The number of photons is obtained from Si photodetector calibrated at NIST

The drum mounted at Los Leones
Preliminary result gives about 5 photons / FADC count as average over all pixels of the camera
Another method : remote laser of known intensity shot vertically in the atmosphere.
Calculation of Rayleigh and aerosol scattering allows predicting flux of photons at the telescope.
Similar result.

Hybrid vs. Surface Detector
10 19 eV 10 20 eV
Surface Hybrid Surface Hybrid
Δθ
2.0
0 0.4
0 1.0
0 0.4
0
Δ core 80 m
30 m 40 m 30 m
ΔE/E 18 %
4.2 % 7.0% 2.5 %
ΔX max
17 g/cm 2
15 g/cm 2

Fraction of stereo FD
100
80
60
40
20
0
2
3
4
18 19 20
Log Energy (eV)
2,3,4

Shower geometry reconstruction
First step: reconstruct the
Shower – Detector Plane (SDP)
Shower
χ i
R
P
ψ
Telescope t i
(
χ i
) = t
0
R
+ c tg
χ
0
- χ i
2
χ
0
3 parameter fit : t
0
, R
P and χ
0

First hybrid event
FD on line display

FD - SD matching

Triggered pixels
FADC traces
100 ns time bin
Cosmic passing through PMTs

crossing telescopes boundary

ψ
Laser degrees
Laser shot axis

R
P
(Km) degrees
Ψ (degrees)

•
Pixel calibration
•
Atmospheric corrections
•
Fluorescence yield
•
Estimate of Cherenkov light
•
Reconstruction of the longitudinal profile
•
Fit with Gaisser-Hillas form
•
Estimate of the energy and of the depth of maximum X max
•
Geometrical reconstruction from correlation of time vs. elevation angle χ i

A “low-energy” shower fully contained in the atmosphere

Longitudinal profile and geometrical reconstruction
R
P
~ 13 km
Θ ~ 57 0
χ
0
~ 82 0

Time vs. angle correlation for a laser shot at R
P
= 25 km
Very useful to understand the analysis of the real cosmic ray events !

•
All components of the 24 FD telescopes are financed.
They are ready or ordered.
•
Installation and commissioning of the telescopes in the two buildings
(Leones and Coihueco) will be completed in 2003 .
This makes
½ of the overall FD.
•
Some problem of funding for the construction of the remaining two buildings Morados and Norte but, good reasons for optimism !