On Polar Axis Alignment

I have a portable setup. I do not, and probably never will have an observatory at my home site due primarily to the crummy horizon. This means having to set up each and every time I go out. And, since the light pollution is horrid and getting worse all the time, I image with a CCD camera, and that means having to polar align the mount.

I hate polar alignment! It is just one of those tedious exercises you do because you know you need too, but wish you didn't. I have tried various methods over the years but nothing I had tried seemed to fill my requirements for high precision and speed. It has been an obsessive quest for me, kind of like the search for the holy grail of polar alignment procedures.

For me, the ideal polar alignment procedure would be:

  1. Simple - if the procedure is too complex it will be prone to error
  2. Deterministic - based on quantifiable values, no estimates
  3. Precise - one or two iterations to arrive at the required tolerance
  4. Automated - the procedure should be automated as much as possible
  5. Fast - the entire procedure completed 30 minutes before astronomical twilight
  6. Repeatable - the procedure should work reliably every time

On this web page I plan to consolidate the information I have uncovered on this topic.


Survey of Polar Alignment Methods

This online article is basically my views of the various polar alignment methods I have used in the past and how they rate according to my ideal view of a polar alignment procedure. I am happy to report that I have found a procedure that meets and exceeds my criteria as set forth above!

Determining Polar Axis Alignment Accuracy

This article is foundational as I get right down to the brass tacks of how to determine, via the drift method, just how far off the mount is from the true celestial pole. It is based primarily on some work by Richard Hook, with a few of my own thoughts thrown in. It is primarily a mathematical model (mothers, hide your children), but has some very practical applications. Along with knowing the alignment error comes the all important question of just how good does the alignment need to be. This question is answered in gory detail as well. Enjoy.

For those of you who may not be mathematically inclined, I want no bleeding from the ears, so I have created several javascript calculators for most of the equations in the article as follows:
Polar Alignment Error (Equation 1)
Image Scale and Field of View (Equation 2)
Maximum Polar Alignment Error (Equation 3)
Drift Rate (Equation 4)
Total Alignment Error and Error Angle (Equations 5, 7)
Maximum Focal Length (Equation 9)
Angular Distance Between Two Stars (Equation 10)
Maximum Guide Star Angle (Equation 11)
Maximum Exposure Time (Equation 12)
Field Rotation (Equation 13)
Maximum Declination (Equation 14)

Measuring Polar Axis Alignment Error

In this article I expound on the geometry of misalignment. Many illustrations are given which make the alignment problem easier to grasp. Insight is given into how the drift alignment method works. The article concludes by discussing various ways to measure alignment error.

Star Offset Positioning for Polar Axis Alignment

What good is measuring alignment error if we have no way to correct it reliably? This article reveals a very simple technique I call star offset positioning. It is a technique where an alignment error is translated into an offset from a star such that centering the star by means of the altitude and azimuth adjustments brings the mount into close alignment. I have done much testing with this technique and regard it as one of the most accurate and productive tasks in the whole polar alignment process.

Star Offset Position Calculator