Compass Roses

A compass rose displays a (N,E) or an (Az,El) rosette close to the guider or instrument display. Consider the guider display: the TV coordinate system is defined to be a conventional (origin at bottom-left) coordinate system when looking at the guider display. Now referring to See Telescope coordinate systems looking towards sky (Right Nasmyth focus) (Right Nasmyth), it is clear that the anti-clockwise (clockwise if there's a Y flip) rotation from

• the TVX axis to N = t + (90 - PA) = t + 90 - (p + a + r)
• the TVX axis to E = t + (90 - PA) + 90 = t + 180 - (p + a + r)
• the TVX axis to El = t + (90 - V) = t + 90 - (a + r)
• the TVX axis to Az = t + (90 - V) - 90 = t - (a + r)

and this defines the guider compass rose orientations as a function of TV angle, TV flip, parallactic angle, elevation angle and rotator angle (independent of rotator tracking mode). The instrument compass roses are analogous (but note that whereas the TV coordinate system is defined in relationship to the guider display, the instrument coordinate system is typically defined in relationship to a slit).

The above formulae are generalized to all focal stations using the same "sign(foc)" elevation sign that was used in the previous section.