Abstract
The IFAE/UAB Raman LIDAR project aims to develop a Raman LIDAR suitable for the online atmospheric calibration of the CTA. Requirements for such a solution include the ability to characterize aerosol extinction to distances of more than 20 km with an accuracy better than 5%, within time scales of less than one minute. The Raman LIDAR consists therefore of a large 1.8 m mirror and a powerful pulsed Nd-YAG laser. A liquid light-guide collects the light at the focal plane and transports it to the readout system. An in-house built polychromator has been characterized thoroughly with respect to its capability to separate effciently the different wavelengths (355 nm, 387 nm, 532 nm and 607 nm). It was found to operate according to specifications, particularly that light leakage from the elastic channels (532 nm and 355 nm) into the much dimmer Raman channels (387 nm and 607 nm) could be excluded to less than 2 × 10-7. We present here the status of the integration and commissioning of this solution and plans for the near future. After a one-year test period at the Observatorio del Roque de los Muchachos, an in-depth evaluation of this and the solutions adopted by a similar project developed by the LUPM, Montpellier, will lead to a final Raman LIDAR proposed to be built for both CTA sites.
Highlights
The Cherenkov Telescope Array (CTA) [1] will observe cosmic γ-rays from few tens of GeV to several hundreds of TeV with greatly improved sensitivity, angular and energy resolution over current experiments [2]
After a one-year test period at the Observatorio del Roque de los Muchachos, an in-depth evaluation of this and the solutions adopted by a similar project developed by the LUPM, Montpellier, will lead to a final Raman LIDAR proposed to be built for both CTA sites
The CTA has opted for a combination of two complementary instruments to achieve this goal: A wide-field telescope optimized for stellar photometry [5, 6] across a field-of-view as large as that of CTA and a powerful Raman LIDAR [7] capable of characterizing aerosol extinction from ground to about 20 km a.s.l. during the short time scales on which the CTA telescopes either change observation target or Wobble position [8]
Summary
The Cherenkov Telescope Array (CTA) [1] will observe cosmic γ-rays from few tens of GeV to several hundreds of TeV with greatly improved sensitivity, angular and energy resolution over current experiments [2]. Such improvements must come along with a significant reduction of systematic uncertainties in energy and flux reconstruction [3], dominated by atmospheric effects [4]. The polychromator separates the four wavelengths and detects the light with four Hamamatsu R11920 photomultiplier tube (PMT) units, identical to the ones used in the CTA Large Size Telescopes (LSTs) cameras [13]. ∗ does not yet include a dedicated near-range optics and readout
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