Dear Friends! By chance I came across this old thread today. And I am surprised that no one has mentioned the fundamental work by Peter Weiss, which was already published in 1993: Peter Weiss, The Vision of Constantine, in J. Bleicken (ed.), Colloquium aus Anlaß des 80. Geburtstags von Alfred Heuß (Frankfurter Althistorische Studien 13, Kallmünz 1993) pp.143-69. You can get a copy of this work from the 'Journal of Roman Archives' via jra@journalofromanarch.com for $10 +$6 for postage +fees (2003!) In this work it is shown that the first vision Constantine had (He had one vision and two dreams!) took place before the war against Maxentius (Eusebius, Vita Constantini. Told to him by Constantine himself!) and can be traced back to a solar event. This solar event must have been a solar halo. Peter Weiss shows photographic images of various solar halos which are almost identical to the reverse of this coin type. They too show a large circle in the sky with a large cross inside. This first vision of Constantine is not described as a dream image, but as an event during the day! I observed a halo myself in October 2018 and was impressed by this apparition with 2 parhelia. Unfortunately, I did not have my camera with me and had to get it first to take pictures. By that time the halo had faded considerably, so my photos do not do justice to this impressive phenomenon in any way. Best regards Jochen