Re: Field Rotation

Howard Ritter

Field rotation occurs whenever, and only when, motion occurs around two axes during tracking. Ideally, this wouldn’t happen with an equatorial mount except when the polar axis is imperfectly aligned. In practice, it can happen when atmospheric refraction moves the object higher or lower as its elevation changes and a correction in Dec is needed, but I think this is important only for long exposures at low elevations.

Field rotation due to an imperfectly aligned polar axis necessitating corrections in Dec is a consequence of spherical trigonometry and is not affected by what star in the field is chosen; only the axis of rotation depends on that, since the rotation will be centered on the guide star, or on the average position of the stars used in multiple-star guiding.

Absolute encoders don’t affect it. They only tell the control computer exactly where the telescope is pointed, factoring out things like slop and backlash in the mount between the motor shafts and the axis shafts.


On May 28, 2022, at 12:57 PM, M Hambrick <mhambrick563@...> wrote:

Am I correct in my thinking that field rotation is inevitable even on a perfectly polar aligned mount ? I seem to recall reading that somewhere on this forum.

Regardless, are there techniques besides good polar alignment that will minimize the amount of field rotation that occurs during a long series of guided exposures ? A couple possible things come to mind: 

  • Guide star selection (farther away from the axis of rotation)
  • Multiple star guiding
  • Having a mount with absolute encoders.

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