Chadwick, P., Lyons, K., McComb, T., Orford, K., Osborne, J., Rayner, S., …Turver, K. (1999). Observations of TeV gamma rays from Markarian 501 at large zenith angles. Journal of Physics G: Nuclear and Particle Physics, 25, 1749-1753. https://doi.org/10.1088/0954-3899/25/6/312