Bokeh Mirror Alignment for Cherenkov Telescopes

TL;DR

This paper introduces a novel Bokeh-based alignment method for segmented Cherenkov telescope reflectors, enabling daytime alignment without star observations, improving image quality and reducing calibration complexity.

Contribution

The paper presents a simple, extendable Bokeh alignment technique for segmented reflectors that does not require clear night skies or star observations, facilitating easier and more flexible telescope calibration.

Findings

Bokeh alignment effectively aligns segmented reflectors during daytime.

The method improves the image quality of Cherenkov telescopes.

Demonstrated successfully on the FACT telescope in La Palma.

Abstract

Imaging Atmospheric Cherenkov Telescopes (IACTs) need imaging optics with large apertures and high image intensities to map the faint Cherenkov light emitted from cosmic ray air showers onto their image sensors. Segmented reflectors fulfill these needs, and composed from mass production mirror facets they are inexpensive and lightweight. However, as the overall image is a superposition of the individual facet images, alignment remains a challenge. Here we present a simple, yet extendable method, to align a segmented reflector using its Bokeh. Bokeh alignment does not need a star or good weather nights but can be done even during daytime. Bokeh alignment optimizes the facet orientations by comparing the segmented reflectors Bokeh to a predefined template. The optimal Bokeh template is highly constricted by the reflector's aperture and is easy accessible. The Bokeh is observed using the out of focus image of a near by point like light source in a distance of about 10 focal lengths. We introduce Bokeh alignment on segmented reflectors and demonstrate it on the First Geiger-mode Avalanche Cherenkov Telescope (FACT) on La Palma, Spain.