- Email: [email protected]
The fluorescence detector prototype for the Auger project: optical system
Nuclear Instruments and Methods in Physics Research A 461 (2001) 577–578
The fluorescence detector prototype for the Auger project: optical system G. Borreania, N. Cartigliab, R. Cestera, F. Daudob, A. de Capoaa,*, F. Marchettob, D. Mauriziob, E. Menichettia, N. Pastroneb a
Dipartimento di Fisica Sperimentale, Universita’ di Torino, Torino, Italy b INFN, Sezione di Torino, Torino, Italy
Abstract We present the characteristics and performance of the optical system for the fluorescence detector prototype of the Pierre Auger Project. The system adopts a Schmidt camera design with a mirror collecting, on a spherical surface, the light incoming through a limited aperture diaphragm. An absorption filter is installed in the diaphragm to preferentially trasmit the nitrogen fluorescence light. # 2001 Elsevier Science B.V. All rights reserved. PACS: 96.40; 42.15.E; 42.79.C Keywords: Auger project; Fluorescence detector; Optical system; Absorption filter
1. Optical design The Pierre Auger Observatory detects cosmic rays above 1019 eV. It uses two techniques: a Surface Array detects at the ground level particles from air showers; a Fluorescence Detector, composed of telescopes, tracks light produced from the interaction of the shower particles with the atmosphere. The fluorescence light is collected by the telescopes with a Schmidt optics consisting of: *
*
a spherical converging mirror (radius=3.47 m; angular dimensions=628 in azimuth and elevation), a circular diaphragm (radius=0.85 m.) centered on the mirror center of curvature, *Corresponding author. E-mail address: [email protected] (A. de Capoa).
*
*
a detector array (pixel camera) positioned on a spherical surface (radius=1.778 m.) with center in the center of curvature of the mirror (field of view=308 in azimuth and elevation), an absorption filter, located in the plane of the diaphragm.
Incoming light traverses the filter at the diaphragm and is focussed onto the pixel camera. Image characterisctics depend on the ratio between the diaphragm diameter and the mirror radius of curvature. The linear dimensions of the telescope determine the amount of collected light and have no effect on the image shape. The angular dimensions of the pixel camera determine the observed region of the sky, while the mirror angular dimensions are calculated to collect all the light accepted by the diaphragm.
0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 1 3 0 4 - 8
SECTION IX.
578
G. Borreani et al. / Nuclear Instruments and Methods in Physics Research A 461 (2001) 577–578
Fig. 1. Images of a distant light source.
A ray-tracing program was used to study the image formed by light emitted from a distant point source. A set of parallel rays coming from a given direction is traced through the system from the diaphragm entrance to the pixel camera. In Fig. 1 we show four different images corresponding to four different entering directions. The image we see is due to the combined effect of the spherical aberration of the mirror and the shadow made by the pixel camera. The Schmidt diaphragm almost completely removes coma aberration. Light entering the system from a given direction, illuminates a limited region of the mirror. The illumination of the mirror is not uniform: the center and the border are seldom illuminated due to the camera shadow and to the presence of the diaphragm, respectively.
2. The optical filter The optical filter1 transmits the nitrogen fluorescence light and cuts the background from night sky light. Different types of glass filters were evaluated: the best ones are Ug11 (manifactured by Schott), Mug2 and Mug6 (manifactured by Schott-Desag). Tests performed show that Mug2 (that is similar to Mug6) has the best performance for fluorescence light differentially attenuated by the atmosphere. We also found that image distortions, due to geometrical imperfections of Mug2 filter, exist, but they are negligible compared the ones due to the mirror imperfections.
1 See Auger technical notes: GAP-1999_31, R. Cester et al.; and GAP-2000_11, G. Borreani et al.
The fluorescence detector prototype for the Auger project: optical system G. Borreania, N. Cartigliab, R. Cestera, F. Daudob, A. de Capoaa,*, F. Marchettob, D. Mauriziob, E. Menichettia, N. Pastroneb a
Dipartimento di Fisica Sperimentale, Universita’ di Torino, Torino, Italy b INFN, Sezione di Torino, Torino, Italy
Abstract We present the characteristics and performance of the optical system for the fluorescence detector prototype of the Pierre Auger Project. The system adopts a Schmidt camera design with a mirror collecting, on a spherical surface, the light incoming through a limited aperture diaphragm. An absorption filter is installed in the diaphragm to preferentially trasmit the nitrogen fluorescence light. # 2001 Elsevier Science B.V. All rights reserved. PACS: 96.40; 42.15.E; 42.79.C Keywords: Auger project; Fluorescence detector; Optical system; Absorption filter
1. Optical design The Pierre Auger Observatory detects cosmic rays above 1019 eV. It uses two techniques: a Surface Array detects at the ground level particles from air showers; a Fluorescence Detector, composed of telescopes, tracks light produced from the interaction of the shower particles with the atmosphere. The fluorescence light is collected by the telescopes with a Schmidt optics consisting of: *
*
a spherical converging mirror (radius=3.47 m; angular dimensions=628 in azimuth and elevation), a circular diaphragm (radius=0.85 m.) centered on the mirror center of curvature, *Corresponding author. E-mail address: [email protected] (A. de Capoa).
*
*
a detector array (pixel camera) positioned on a spherical surface (radius=1.778 m.) with center in the center of curvature of the mirror (field of view=308 in azimuth and elevation), an absorption filter, located in the plane of the diaphragm.
Incoming light traverses the filter at the diaphragm and is focussed onto the pixel camera. Image characterisctics depend on the ratio between the diaphragm diameter and the mirror radius of curvature. The linear dimensions of the telescope determine the amount of collected light and have no effect on the image shape. The angular dimensions of the pixel camera determine the observed region of the sky, while the mirror angular dimensions are calculated to collect all the light accepted by the diaphragm.
0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 1 3 0 4 - 8
SECTION IX.
578
G. Borreani et al. / Nuclear Instruments and Methods in Physics Research A 461 (2001) 577–578
Fig. 1. Images of a distant light source.
A ray-tracing program was used to study the image formed by light emitted from a distant point source. A set of parallel rays coming from a given direction is traced through the system from the diaphragm entrance to the pixel camera. In Fig. 1 we show four different images corresponding to four different entering directions. The image we see is due to the combined effect of the spherical aberration of the mirror and the shadow made by the pixel camera. The Schmidt diaphragm almost completely removes coma aberration. Light entering the system from a given direction, illuminates a limited region of the mirror. The illumination of the mirror is not uniform: the center and the border are seldom illuminated due to the camera shadow and to the presence of the diaphragm, respectively.
2. The optical filter The optical filter1 transmits the nitrogen fluorescence light and cuts the background from night sky light. Different types of glass filters were evaluated: the best ones are Ug11 (manifactured by Schott), Mug2 and Mug6 (manifactured by Schott-Desag). Tests performed show that Mug2 (that is similar to Mug6) has the best performance for fluorescence light differentially attenuated by the atmosphere. We also found that image distortions, due to geometrical imperfections of Mug2 filter, exist, but they are negligible compared the ones due to the mirror imperfections.
1 See Auger technical notes: GAP-1999_31, R. Cester et al.; and GAP-2000_11, G. Borreani et al.