Svarog/shu_hunter,
I would guess, in the light of new information
that a closed iris would be useful on set to reduce the CGI/optical demands in post-, particularly with people moving backwards and forwards in front of it the whole time.
I.e. CGI/optical of the iris actually closing, then cut to the prop iris, closed, for pro-longed gateroom (or "embarkation room - we're walking..."
) footage.