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Polar Alignment Methods

The following was developed for a discussion of polar alignment at a meeting of the Orange County Astronomers' AstroImaging group. Comments and corrections are welcome and may be sent to kodama@alumni.caltech.edu.

Why Polar Align?

  • For visual use, being polar aligned means simplified tracking - a constant speed on one axis is all that is required. High power viewing virtually requires tracking.
  • For long film exposures or even a sequence of CCD images taken over a long period, polar-aligned tracking avoids rotation of the image. High accuracy alignment is required for long focal lengths, large image areas (e.g. 4x5 film), high resolution, or long exposures.
 
Basic Alignment Methods:
  • Alignment Using Compass & Bubble Level
    • Limited to accuracy of compass and bubble level readings and altitude markings on your mount.
    • Be sure to correct for magnetic deviation from true north when using a magnetic compass.
  • Align on Pole Using Main Scope
    • Requires a clear (and reasonably dark) view of the sky near the pole.
    • Mostly useful to northern hemisphere observers.
    • Requires attention to accurate alignment between the scope's optical axis and the mount's mechanical axis.
    • Watch out for optical "flop" in movable mirror systems such as SCT's. Use a mirror lock if possible.
    • Take refraction into account at low latitudes.
    • Calculate celestial pole's offset from reference stars or use a special polar finder reticle.
  • Polar Finder Scope
    • May not be available in some mounts.
    • Requires good alignment between the optical axis of the polar finder scope and the mount's mechanical axis.
    • Requires a clear (and reasonably dark) view of the sky near the pole.
    • Mostly useful to northern hemisphere observers.
    • Take refraction into account at low latitudes.
    • Calculate celestial pole's offset from reference stars or use a special polar finder reticle.
    • See the detailed writeup on the use of the Kenko/Losmandy polar finder.
  • Reference Star Iteration
    • Depends on setting circle accuracy (highest accuracy on computer-driven scopes with motor encoders)
    • Watch out for optical "flop" in movable mirror systems such as SCT's. Use a mirror lock if possible.
Precision Alignment Methods:
  • Drift Alignment - for maximum accuracy in imaging applications or permanent telescope installations.

Copyright 2001 by David A. Kodama