We have defined dha as an angular value.
All values are in decimal degrees.
The HA encoder will not be mounted on the shaft so that its zero points correspond exactly with the meridian. The center of the telescope focal plane may not be well aligned with the mechanical center of the telescope. The North component of this error is indistinguishable from Dec Dial error so we will ignore it. However, the hour angle component leads to constant angular error at all declinations and so differs from HA dial error. Though we assume that the observations have been corrected for refraction.