Improving Polar Alignment

When using the Three-Point Polar Alignment (TPPA) routine, many users encounter persistent polar alignment errors. This guide aims to help you understand and resolve these issues effectively.

Common Issue:

After using the TPPA routine on the left side of the pier to zero the azimuth and altitude errors, users often notice a significant polar alignment error (3-4 arcminutes) when switching to the other side. Correcting the error on the opposite side and switching back results in the same discrepancy.

This issue is not limited to a specific mount. It has been observed across various models, including:

• Strain Wave mounts
• Worm gear mounts.
• Direct Drive mounts

Understanding and Addressing Polar Alignment Errors When Switching Pier Sides.

When switching from one side of the pier to the other, users often encounter alignment errors due to various factors. Here’s a detailed look at why these errors occur and how to manage them.

1. Mechanical Shifts:

• Moving the telescope from one side of the pier to the other involves significant mechanical motion. Even though 3 arcminutes might seem small, any movement on a soft ground or minor flexure can introduce shifts in the mount’s position.
• Mechanical imperfections in your telescope gear / setup can contribute to alignment discrepancies.

2. Refraction:

Atmospheric refraction is the bending of light as it passes through the Earth’s atmosphere, which affects the apparent position of celestial objects. This refraction can distort the observed position of celestial objects, introducing additional errors into the polar alignment process.

Calculation of Refraction Error at key altitudes:

Assuming a latitude of 45 degrees and observing a star near the celestial equator, the altitude range will be from approximately 0 degrees (near the horizon) to 90 degrees (zenith).

For simplicity, we will calculate refraction at 0°, 15°, 30°, 45°, 60°, 90° altitudes and then average these values.

• At 0° altitude: 35 arcminutes error
• At 15° altitude: 10 arcminutes error
• At 30° altitude: 3 arcminutes error
• At 45° altitude: 1.2 arcminutes error
• At 60° altitude: 0.6 arcminutes error
• At 90° altitude: close to 0 arcminutes error

Please note that atmospheric refraction can vary with changes in temperature, pressure, and humidity, so this calculation is an approximation based on standard atmospheric conditions.

You can read more here: https://astrocamera.net/equipmnt/p-align/refract.htm

An average refraction error when moving the telescope from the west pier to the east pier, considering the above altitudes and assuming standard conditions, is approximately 424 arcseconds, close to 7 arcminutes!

NINA’s TTPA includes an experimental option to adjust its polar alignment calculations for atmospheric refraction. While this feature aims to improve accuracy, its reliability has not been fully validated.

 

If you are not familiar with the TPPA plugin and how to setup it please watch the video below.

 

3. Flexure:

Flexure within the telescope rings, focuser parts and mount setup can cause slight misalignments as the orientation changes.

Practical Considerations:

Autoguiding:

• When using autoguiding, a polar alignment error of under 5 arcminutes is generally sufficient. Autoguiding can correct minor misalignments, making field rotation negligible.

Note: Do not try to reach zero arcseconds, as you are simply wasting your time.

Steps to Minimize Alignment Errors:

1. Ensure a Stable Tripod / Pier:
   Place the mount on a firm, stable surface to minimize shifts caused by ground softness.
2. Check for Flexure:
   Inspect and tighten all mount and telescope components to reduce flexure. (Focuser, Telescope Rings, Mirror etc)
3. Consider Atmospheric Conditions:
   Be aware of atmospheric refraction effects, especially when observing near the horizon.
4. Iterative Alignment:
   Perform multiple iterations of TPPA, on one side of pier only and making small adjustments each time. This iterative process can help average out minor errors and achieve better alignment.

By understanding these factors and applying the suggested steps, you can significantly reduce polar alignment errors and enhance the quality of your astrophotography.