Did the Apollo Flight Director Attitude Indicator (FDAI) error needles relate in any direct way to the position of the ball under them, or were they only ever interpreted in relation to the white bars of the scale? ie, if the ball didn't exist, would the attitude error needles still function identically? (like the rate display)
For example, in KSP, the navball has markers for prograde and burn attitude projected directly onto the surface of the ball. It strikes me that the attitude error needles could be used in a similar fashion, to indicate a point on the ball, but this would require a non-linear movement towards the extremes of the scale as the desired point moves towards the edges of the balls visible area. I have not found any mention of the needles being used like this, so can you confirm that they were only ever referenced against the scale, and never the ball? And if that's the case, why put them on front of the ball at all?