FRAM for CTA

P Janeček,M Prouza,J Ebr,M Doro

doi:10.1051/epjconf/20158903006

Abstract

The Cherenkov Telescope Array (CTA) is a project to build a new generation ground-based gamma-ray observatory. Among other goals, the project aims to achieve high precision of gamma-ray energy measurement while maximizing the use of observation time. These objectives require detailed and fast information about atmospheric conditions, particularly the transparency which includes cloudiness (including thin high-altitude clouds) and aerosol content. This knowledge is required not only to select and calibrate data after observation, but also to make on-the-fly scheduling decisions. To provide such data without interfering with the observation (as would be the case when using laser-based methods), we propose to use the “FRAM” (F(/Ph)otometric Robotic Atmospheric Monitor) device, which is a small robotic astronomical telescope with a large field of view and a sensitive CCD camera. FRAM will use stellar photometry to measure atmospheric extinction across the field of view of the CTA. The fast robotic mount of the telescope allows quick observation of multiple fields when the array is split and even a check of the conditions in the directions of the upcoming observations. The FRAM concept is built upon experience gained with a similar device operated at the Pierre Auger Observatory.

Highlights

When finished, the Cherenkov Telescope Array (CTA) will be by far the largest ground-based IACT (Imaging Atmospheric Cherenkov Telescope) gamma-ray observatory ever built [1], with two sites equipped by about a hundred telescopes of different sizes in total
As the CTA aims for maximally efficient use of its observation time, scheduling decisions will be taken on-the-fly, requiring accurate knowledge of immediate atmospheric conditions in various directions across the sky
To bridge the gap between these two kinds of instruments, we propose to install on both CTA sites a device (which we call FRAM – F(/Ph)otometric Robotic Atmospheric Monitor) which consists of an astronomical CCD camera attached to a suitable photographic lens on an equatorial mount

Summary

Introduction

The Cherenkov Telescope Array (CTA) will be by far the largest ground-based IACT (Imaging Atmospheric Cherenkov Telescope) gamma-ray observatory ever built [1], with two sites (one in each hemisphere) equipped by about a hundred telescopes of different sizes in total. The maximal allowed error for Cherenkov light intensity has been set at 8% with an aim to lower the error to as little as 5% if possible. As there are several sources of uncertainties alongside the atmospheric conditions (such as the calibration of various parts of the detector), the error budget for each individual contribution is extremely thin and the uncertainty of the Cherenkov light intensity due to aerosol content of the atmosphere must be lower than 2%. As the CTA aims for maximally efficient use of its observation time, scheduling decisions will be taken on-the-fly, requiring accurate knowledge of immediate atmospheric conditions in various directions across the sky

The FRAM concept

FRAM at the Pierre Auger Observatory

Hardware and operation

FRAM data and their use within CTA

Findings

Conclusions and outlook