Calibration strategies for the Cherenkov Telescope Array

Abstract

The Central Calibration Facilities workpackage of the Cherenkov Telescope Array (CTA) observatory for very high energy gamma ray astronomy defines the overall calibration strategy of the array, develops dedicated hardware and software for the overall array calibration and coordinates the calibration efforts of the different telescopes. The latter include LED-based light pulsers, and various methods and instruments to achieve a calibration of the overall optical throughput. On the array level, methods for the inter-telescope calibration and the absolute calibration of the entire observatory are being developed. Additionally, the atmosphere above the telescopes, used as a calorimeter, will be monitored constantly with state-of-the-art instruments to obtain a full molecular and aerosol profile up to the stratosphere. The aim is to provide a maximal uncertainty of 10% on the reconstructed energy-scale, obtained through various independent methods. Different types of LIDAR in combination with all-sky-cameras will provide the observatory with an online, intelligent scheduling system, which, if the the sky is partially covered by clouds, gives preference to sources observable under good atmospheric conditions. Wide-field optical telescopes and Raman Lidars will provide online information about the heightresolved atmospheric extinction, throughout the field-of-view of the cameras, allowing for the correction of the reconstructed energy of each gamma-ray event. The aim is to maximize the duty cycle of the observatory, in terms of usable data, while reducing the dead time introduced by calibration activities to an absolute minimum.

Cross calibration of telescope optical throughput efficiencies using reconstructed shower energies for the Cherenkov Telescope Array

“…From figure 4, it can clearly be seen that all telescopes are individually calibrated to < 10% with this method, the majority to < 5%. This meets the CTA performance goal of 5% systematic uncertainty on the individual telescope measurement of the absolute Cherenkov light intensity [13], whilst the SSTs may struggle to achieve this with alternatives such as the muon calibration.…”

“…and MAGIC arrays [11,12]. Therefore, this approach has the potential to calibrate the telescope optical efficiencies at the few percent level, fulfilling the CTA performance goal of 5% systematic uncertainty on the absolute intensity of Cherenkov light at each telescope [13]. Whilst comparison of image sizes directly is possible for telescopes of identical specifications, in the case of CTA with multiple telescope types, the variation in image size with hardware cannot be corrected for, especially near trigger threshold.…”

Using Muon Rings for the Calibration of the Cherenkov Telescope Array: A Systematic Review of the Method and Its Potential Accuracy

“…Just as for the overall image (Eq. 7), a pixel-wise optical bandwidth parameter B pix can be derived that should correlate with the flat-fielding constants from the light flasher calibration (Aharonian et al 2004;Gaug 2006;Hanna et al 2010;Gaug et al 2014;Brown et al 2015). Bolz (2004) finds a good correlation for the H.E.S.S.-I telescopes with a spread of 5% standard deviation.…”

Capabilities beyond Gamma Rays

“…A proper understanding of atmospheric variations is the key to such an anisotropy measurement, and again CTA will improve dramatically on the situation for current IACTs. A complete monitoring and correction scheme is under development for CTA [647], aiming to greatly reduce systematic variations in measured flux. Once cosmic-ray electron anisotropy is measured or constrained, cosmic-ray electrons are expected to play an important role in the calibration and monitoring of CTA [648].…”