3.3 Present status
4.1.2 Fluorescence Detector
The Fluorescence Detector comprises four sites located on the top of small hills in the array's boundaries:
Los Leones, Los Morados, Loma Amarilla and Coihueco. Each site houses a six telescopes uorescence eye overviewing the array 180◦ in azimuth and 28.6◦ in elevation [42]. The communication with the CDAS and the SD is established through the antenna tower built on each uorescence site. Data taking began at Los Leones and Coihueco eyes in January 2004 and at Los Morados in March 2005, while the Loma Amarilla eye was only nished by February 2007 [42, 38]. Besides the optical telescopes, the atmosphere is an integral part of the Fluorescence Detector as well. Since it was characterised in 3.2.2, this section will only present a detailed description of this telescopes.
The optical devices used in the FD, protected from rain and wind by individual shutters, are Schmidt telescopes with a eld of view of 30◦ in azimuth and 28.6◦ in elevation [24]. As exemplied in gure 4.3, each telescope consists of several separate parts.
First of all, an 80 cm x 40 cm ultraviolet lter is installed after the shutter in order to transmit in the 280-430 nm band, thus blocking visible light. The lter reduces the night background light and ensures an acceptable signal-to-noise ratio. Moreover, it acts as a window protecting the devices from dust and rain.
Then, the ultraviolet radiation crosses a circular diaphragm with a radius of 0.85 m that is surrounded by a corrector ring with outer radius 1.10 m. The corrector ring, composed of 24 ultraviolet transmitting glass segments, almost doubles the eective area and is constructed in such a manner that it allows a good quality of the optics [43, 44].
Between the diaphragm and the mirror there is the 440 PMT camera (described below) that allows the imaging of the shower prole. The light is collected by a 3.5 m x 3.5 m spherical mirror with 3.4 m of radius [43, 42] presenting a mean reectivity around 90% in the 300-400 nm band [44]. The large dimensions of the mirror require that it consists of several segments − hexagonal and square-shaped segments were chosen in order to maximise light collection and sky coverage.
The PMT signals are sampled in 100 ns time bins and are then subjected to two basic levels of trigger− see, for instance, [44]−that rule out random coincidences and select potentially physical events. Besides,
(a) Schematic representation [43]. (b) Detailed view of the PMT camera on the right and the mirror with square- shaped segments on the left [24].
Figure 4.3: The Schmidt telescope used in the FD.
the telescopes must undergo several calibration processes so that the energy reconstruction accuracy is better than 15% [42] − this requirement is of extreme importance in the study of the far end of the cosmic ray spectrum as discussed in section 3.3.
PMT camera
The PMT camera lies on the spherical focal surface of the telescope and consists of a 6 cm x 94 cm x 86 cm aluminum body supported by two legs and housing 22 rows x 20 columns of hexagonal PMTs [43]. Each PMT corresponds to a pixel so that the camera presents a total of 22x20=440 pixels. Light collection is maximised surrounding each PMT with six mercedes stars: these are inclined reecting surfaces designed to direct∼90% of the incident light into the centre of the PMT, thus smoothing the transition between pixels.
As the camera lies on a spherical surface, the pixels are not regular hexagons in spherical coordinates but have variable size in order to best cover the focal surface [44]. So, to ease the data analysis it is convenient to dene a coordinate system in which the camera is rectangular and the pixels regular. The new coordinates (β, α), proposed in [45], are represented in gure 4.4 and their relation to spherical coordinates (θ, φ)is given by the equations:
β =arcsin(sin(φt−φ)sinθ) (4.1)
α=αc−αm+arcsin cosθ
cosβ
(4.2) where φt andαm= 16◦ are respectively the azimuth and the elevation angles of the telescope axis and αc=
√ 3◦
2 is the oset angle between the camera centre and the telescope axis [46].
In these coordinates, each pixel of the camera is a regular hexagon of radiusrpix =
√3◦
2 , side length lpix=
√3◦
2 and side-to-side dimension dpix= 1.5◦ as shown in gure 4.5. The mercedes structures form a similar inner hexagon scaled by 0.8. For each telescope, the camera is a 22 rows x 20 columns grid of
Figure 4.4: The (β, α)coordinates and their relation to spherical coordinates (θ, φ). In this particular case,φt= 90◦ (adapted from [45]).
pixels whose central points have coordinates βij =
( 1.5◦·(10−i) ifj odd
1.5◦·(10−i) + 0.75◦ ifj even (4.3) αij = 1.5
√3◦
2 ·(j−11) (4.4)
beingi∈[1,20]the column number andj ∈[1,22]the row number. The whole camera is represented in gure 4.6 and, asαc=
√3◦
2 =rpix, the telescope axis centre coincides with the centre of a mercedes star.
Moreover, the camera limits in(β, α)are given by:
βmin= 1.5◦·(10−20)−dpix
2 =−15.75◦ βmax= 1.5◦·(10−1) + 0.75◦+dpix 2 = 15◦ αmin= 1.5
√3◦
2 ·(1−11)−rpix ' −13.86◦ αmax= 1.5
√3◦
2 ·(22−11) +rpix'15.16◦ The eld of view of the camera is then 30.75◦ inβ and∼29.01◦in α.
For practical reasons, each pixel must be labeled by a numberNpix ranging from 1 to 440. Usually the relationNpix = 22·(i−1) +j is used to dene the pixel number given the respective columni and row j. Inversely, there are also unambiguous relations that yield the column and row given the pixel number Npix: i=inthN
pix−1 22
i
+ 1 andj=mod[Npix−1,22] + 1.
Having the detailed camera description presented in the above paragraphs, one may convert the(θ, φ) direction of an incident photon into the number of the pixel it hit and, additionally, verify whether the photon undergone a mercedes reection or not. This is an important tool when taking into account the details of FD optics in order to analyse shower events, as done further ahead in this work.
Spot
A photon entering the diaphragm with a certain direction may be misplaced in the PMT camera. The so-called spot is precisely the circle of least confusion that measures the degree of this misplacement. The
Figure 4.5: The dimensions of an FD pixel and the mercedes structures in(β, α)coordinates.
main advantage in using Schmidt optics is that the spot is independent from the incident direction and is not worsened (on the contrary, it is improved) by the introduction of the correction rings. On the focal surface the spot radius is∼7.5 mm or∼0.25◦ which corresponds to∼ 16 of the pixel size [43, 42]. This eect is mainly due to mirror aberration and is expected to have radial behaviour or, in other words, depend on the distance to the camera centre.
Further detail on the spot is achieved with simulation of FD optics. The Karlsruhe group (KG) simulation [47] allows the study of the spot on dierent positions. Figure 4.7 shows the misplacement of the incident photons in a particular region of the focal surface. The knowledge of this kind of information is important in careful uorescence data analysis.