[en] This article describes the efforts made to constraint systematics of the energy reconstruction in the CELESTE experiment, a Cherenkov atmospheric detector. To do so, we significantly improved our understanding of the atmosphere using a LIDAR and of the optical throughput of the detector using stellar photometry.We have observed the blazars Markarian 501, 1ES1426+428 and detected Markarian 421, for which we present our lightcurve

[en] CELESTE is a γ-ray atmospheric Cherenkov telescope based on the sampling technique which has pioneered in lowering the energy threshold below 100 GeV. The improvements made in analysis and the most recent results are presented

[en] Celeste is an atmospheric Cherenkov telescope based on the reconversion of the Themis solar facility, located in the Eastern Pyrenees. The mirrors, named heliostats, recover the Cherenkov light emitted by the electromagnetic shower created by gamma-rays in the atmosphere. The Celeste experiment was designed during the 90's to cover the 30-300 GeV energy range and to fill the gap between satellites and imaging atmospheric Cherenkov telescopes. In 2000, we attained our goal with the detection of the Crab Nebula and those of the active galactic nucleus Markarian 421. This thesis presents the work accomplished since then to improve the sensitivity of our instrument by studying the detector and by developing a new analysis. Despite the very bad weather conditions, a new detection of the Crab Nebula is presented here which validates the principle of the new analysis. This manuscript ends with the study of the data sample taken on two Active Galactic Nuclei, the blazars Markarian 421 and 1ES1426+428. (author)

[en] CELESTE is an atmospheric Cherenkov telescope based on the sampling method which makes use of the de-commissioned THEMIS solar electrical plant in the French Pyrenees. A large (2000 m²) mirror surface area from 40 independent heliostats followed by a secondary optic, a trigger system using analog summing techniques and signal digitization with 1 GHz flash ADCs make possible the detection of cosmic γ-rays down to 30 GeV. This paper provides a detailed technical description of the CELESTE installation