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## 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

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