Preliminary optical design of a polychromator for a Raman LIDAR for atmospheric calibration of the Cherenkov Telescope Array

The preliminary design of a polychromator unit for a Raman lidar (Light Detection And Ranging) for atmospheric calibration in the framework of the Cherenkov Telescope Array (CTA) project is presented. To obtain high quality data from CTA, a precise monitoring of the atmospheric transmission is needed. Remote-sensing instruments, like elastic/Raman lidars, have already been proven a powerful tool in environmental studies, and a lidar installed and operated at the CTA site is foreseen for correcting systematic biases on the energy and flux. The lidar we discuss here consists of a powerful laser that emits light pulses into the atmosphere, a mirror of 1.8 m diameter that collects the backscattered light and a polychromator unit where the light is analyzed. The laser is a pulsed Nd:YAG with the first two harmonics available at 355 and 532 nm and the polychromator has 4 read-out channels: two to analyze the elastic backscattering at 355 and 532 nm and two for the Raman Nitrogen back-scattered light, at 387 and 607 nm, respectively. The polychromator module needs to collect the majority of the light coming from the telescope, to split the different wavelengths and to focus the beams onto photomultiplier detectors. The collection and focalization of the beams are done by means of simple lens-couples and the separation with custom-made dichroic mirrors and narrow-band filters. The performance of the conceived optical design, the adopted design choices for the glass, surface figure and size of the lenses, and the expected throughput for the different channels are hereafter described. © 2012 SPIE.