In a physical/technical context, I (being not a native speaker) am looking for an adjective that describes the absence of tilt and found “untilted”, which seems however not widely used. More precisely, I want to describe an array of tiltable mirrors in the state where the individual mirrors are not tilted, i.e. lying in the plane of the array.
I feel that “plane” and “flat” are not equivalent synonyms, since they refer to the surface shape, not its orientation. For example, each mirror in that array could be spherically shaped (i.e. not plane/flat), but be untilted at the same time, and vice versa.
“Horizontal” also does not seem applicable, because the overall orientation of the array is not fixed and thus a reference to the horizon/earth makes little sense.
Another idea is to say “all mirrors have zero tilt”, if that is correct English?
Is my understanding of the meaning of these words correct? What might be other precise and comprehensible synonyms for “untilted”?