[Erasmus]

The lack of sweep would cause a brief centering of the beam, and that coupled with the persistence of the phosphors would result in the slowly-fading central “dot”.

The persistance of the phosphors was below 1/30 of a second. The persistence of the "dot" was due to the slow cooling of the electron gun filament and the comparably slow bleedoff of the high voltage ("ultor anode") supply.

[chimera]

Makes sense. Persistence would have to be short otherwise you’d have a lot of “ghost image” overlay. Sorry, too much time with the old-style graphics terminals, which for vector stroke graphics depended on high-persistence phosphors (although there was an “erase” key, how did that work?).