3.1 Rotator Control

Rotator Control

A rotator can be in any one of three tracking modes:

position angle mode, in which the YIM axis is maintained at a fixed "position angle" (PA) relative to North,
vertical angle mode, in which the YIM axis is maintained at a fixed "vertical angle" (V) relative to the vertical (Elevation axis), and
stationary mode, in which the rotator is stationary at a fixed "rotator angle" (r).

See Position Angles and Rotator User Angles discusses position angles in more detail, taking NIRC and LRIS as examples.

The rotator therefore requires just two parameters, a "user angle" (ROTDEST keyword) and a tracking mode" (ROTMODE keyword). The interpretation of ROTDEST is a function of the mode. Referring to See Telescope coordinate systems looking towards sky (Right Nasmyth focus) , it is clear that at Right Nasmyth the following are always true:

PA = p + a + r
V = a + r

Therefore, taking each tracking mode in turn:

TABLE 1. User and rotator angles for rotator tracking modes (Right Nasmyth only)
tracking mode	user angle (UA)	rotator angle (r)
position angle	PA	UA - p - a
vertical angle	V	UA - a
stationary	r	UA

It should also be clear that at Left Nasmyth the sign of elevation (a) has to be changed and that at all other focal stations elevation is simply ignored. Thus we have:

TABLE 2. User and rotator angles for rotator tracking modes (all focal-stations)
tracking mode	user angle (UA)	rotator angle (r)
position angle	PA	UA - p - sign(foc) 1 * a
vertical angle	V	UA - sign(foc) * a
stationary	r	UA

1. sign(foc) is the elevation sign as a function of focal-station: +1 for Right Nasmyth, -1 for Left Nasmyth, 0 for all others.

Rotator Control

TABLE 1. User and rotator angles for rotator tracking modes (Right Nasmyth only)

TABLE 2. User and rotator angles for rotator tracking modes (all focal-stations)