Factors Affecting Apparent Encoder Polarity

The following factors impact the resulting apparent polarity of an encoder.  Each factor is titled in a "[normal] or [reversed]" manner (the first-listed possibility is "normal," the second "reversed").   These are additive, so that an even number of reversing factors will result in normal polarity.  Additional factors may exist.

1. R.A./Azm or Dec./Alt Axis.  Most astronomical CPSs (computerized pointing systems) interpret each axis in an opposing manner.  It is also worth noting that the R.A./Azm axis' polarity is referred to in an azimuthal sense, which runs opposite to R.A. measurement.

2. Inward or Outward Facing.  The encoder shaft can be made to face inward (toward the telescope) or outward (away from the telescope).

3. West or East Bearing.  The Dec./Alt encoder can oftentimes be mounted on either the West or East side of the mount.

4. Odd or Even Gears.  When gear-driven, an even number of gears causes the encoder to rotate in a direction opposite that of a direct or pulley drive (all other factors being equal).

5. Rotating or Static Shaft.  Normally the encoder body is stationary, and the encoder shaft rotates. Sometimes the mounting is designed to keep the encoder shaft stationary, while the body rotates.

6. A-B or B-A Wiring.  The encoder can be wired with Channel A leading Channel B, or Channel B leading Channel A.  If the encoders are wired with opposite sense, swapping them (axis-for-axis) will reverse the resultant polarity.

7. Normal or Reversed Configuration.  The decoder (e.g. BBox or NGC-MAX) can usually be configured for normal (positive/clockwise) or reversed (negative/counter-clockwise) sensing of each axis.  This can be used to "normalize" an otherwise reversed encoder polarity.

8. North or South Hemisphere.  Southern Hemisphere operation results in an apparent polarity reversal of both encoders.

The encoder mounting kits from JMI are designed to be "normal," such that all encoders have positive/clockwise polarity provided installation is followed correctly.  Only Southern Hemisphere observers should need to configure the NGC-MAX or BBox for reversed (negative/counter-clockwise) polarity.

JMI Telescopes Jim's Mobile, Inc. 1960 Kerr Gulch Rd Evergreen, CO  80439 USA
Phone:  (303) 233-5353
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