Introduction to Astronomical Photometry, Edwin Budding, Osman Demircan, pp. 15, Cambridge University Press, 2007, ISBN 978-0-521-84711-7, ... Also benefitting from the Almagest, Abd al Rahman Sufi, at Isfahan in the tenth century, decided that not only the positional determinations of the stars as given in the Almagest, but also their magnitudes, could be checked, or reassessed ...
