Rotator
Limits
The Mount PA has a range of -163.37 to 193.63, so there is a blind spot
of 3 degrees. It is useful to be able to determine the time at which a
rotator limit would be reached for a new field, without having to move
the telescope and read the information off the TCS. This can help to
plan the observations during the night.
To calculate when a rotator limit will be hit
- Use Staralt to plot the Sky PA during the night for
the proposed field
- If the parallactic angle is increasing throughout the
observations, the rotator will be moving towards a negative limit
- If the parallactic angle is decreasing throughout the
observations, the rotator will be moving towards a positive limit
- Use the formula:
Parallactic Angle = Sky PA - Mount PA
- Given the Sky PA of the observations, and the Mount
PA of either the positive or negative limit, obtain the parallactic
angle for the time at which the rotator limit will be reached
- Use the staralt plot to read off the time for this
parallactic angle
To determine if a rotator limit will be hit
for fixed time observations
For a planned observation with fixed Sky PA and for a definite start
and end time, we can determine in advance if a rotator limit will be
reached during the observations:
- Use Staralt to plot the Sky PA during the night
- Read off the parallactic angle for the start and end
of observations
- Use the formula:
Mount PA = Sky PA - Parallactic Angle
- Make 2 calculations, one using the parallactic angle
at the start of observations, and one at the end
- Obtain the Mount PA for both these times: if it falls
within the permitted rotator range then no limit will be reached
