Drift Method Polar Alignment

For astrophotographers and those collecting scientific data with a telescope equipped with an equatorial mount, a precise polar alignment is very important in order to get the best results. In this document, we will cover the method called “Drift Alignment”. The procedure below results in very accurate polar alignment, and minimizes the need for tracking corrections during astrophotography. The advantages of this type of polar alignment method are: 

  • It can be performed through visual means or with a digital camera (CCD or CMOS Science Cameras or DSLR will work).
  • A precise polar alignment can be achieved even if the celestial pole is hidden from view.
  • A precise polar alignment can be achieved even if the telescope has “cone error” with relationship to the mount.
  • This method can be performed in the Northern or Southern Hemisphere.
  • This method works on all types of equatorial mounts.
  • You can start Drift Method Polar Alignment during the evening twilight, and see target stars through your telescope before it gets truly dark. This gives a good chance to finish the final polar alignment adjustment before full darkness sets in. 

What Is Needed for Drift Alignment?

Eyepiece or Camera?

Drift Alignment requires the use of a crosshair eyepiece or a digital camera taking brief exposures where the display can mark the target star’s position and the star can be seen to update in near real time. Some image capture software can place a crosshair graphic over the “live view” of the star. You can also create a cross hair on a clear plastic overlay on your monitor for this purpose. If a Drift Alignment is to be done visually, an Illuminated Reticle Eyepiece (9mm focal length or less) is well suited in this application. 

Determining the Test Duration

Note that the longer the target star stays stationary in declination, the more precise your polar alignment is. For many applications a testing duration of 10 to 15 minutes with no detectable drift in the North/South axis is sufficient. Longer testing durations to 30 to 60 minutes or more, can be used to obtain the best precision polar alignment (recommended for telescopes that are permanently set up in an observatory). 

Getting Started

  • Roughly rotate the polar axis of the mount towards the celestial pole of the sky. Use a compass to determine North or South if the celestial pole is hidden from view. 
  • Level your tripod.
  • Adjust the latitude adjustment of the mount to your local latitude as closely as you can.
  • Make a more refined adjustment in azimuth polar axis of the mount towards the celestial pole of the sky. 
  • Place the illuminated reticle eyepiece into the eyepiece holder of the telescope.
  • Point the telescope, with the tracking drive running, at a moderately bright star near where the meridian (the North/South line passing through your local zenith) and the celestial equator intersect. 

Tracking a Meridian Target Star on the Celestial Equator

For best results, the target star, is any star should be located within ±30 minutes in R.A. of the meridian and within ±5° of the celestial equator. Pointing the telescope at stars that are crossing the meridian, with the Declination set to 0°, will point the telescope in the right direction.

Note the extent of the star’s drift in Declination (*disregard drift in Right Ascension): 

  • If the star drifts South , the telescope’s polar axis is pointing too far East. 
  • If the star drifts North, the telescope’s polar axis is pointing too far West. 

Move the equatorial mount in azimuth (horizontally) to effect the appropriate change in polar alignment. Reposition the telescope’s East-West polar axis orientation until there is no further North-South drift by the star. Note that the star that you were tracking on will move across the field. You can either recenter the target star or you can choose a new one. 

Track the star for the period of time that you want to test for, to be certain that the target star remains stationary in Declination. Note that the longer time period the target star stays stationary, the more precise the polar alignment. 

Tracking an Eastern Sky Target Star on the Celestial Equator

Next, point the telescope at another moderately bright star near the horizon, but still within ±5° of the celestial equator. For best results, the star should be about 20° or 30° above the horizon and within ± 5° of the celestial equator. Again note the extent of the star’s drift in Declination: 

Once again, note the extent of the star’s drift in Declination (*disregard drift in Right Ascension): 

  • If the star drifts South, the telescope’s polar axis is pointing too low. 
  • If the star drifts North, the telescope’s polar axis is pointing too high.

Move the equatorial mount in latitude (vertically) to effect the appropriate change in polar alignment. Reposition the telescope’s North-South polar axis orientation until there is no further North-South drift by the star. 

Note that the star that you were tracking on will move across the field. You can either recenter the target star or you can choose a new one. 

Track the star for the period of time that you want to test for, to be certain that the target star remains stationary in Declination. Note that the longer time period the target star stays stationary, the more precise the polar alignment.

Once you have completed this step, and the target star no longer drifts within the test time period that you have selected, the Drift Alignment is complete and you have a precise polar alignment. 

*When making a Drift Alignment, errors in polar alignment are seen if the target star drifts either North or South. Drifting East or West is ignored because those errors are due to tracking errors (e.g. periodic error). 

Glossary of Terms

Azimuth: The direction of a celestial object from the observer, expressed as the angular distance from the north or south point of the horizon to the point at which a vertical circle passing through the object intersects the horizon.

Celestial Equator: The projection into space of the earth's equator; an imaginary circle equidistant from the celestial poles.

Declination:
The angular distance of a point north or south of the celestial equator, usually expressed in degrees, and minutes.

Equatorial Mount: A telescope mounting with one axis aligned to the celestial pole, which allows the movement of celestial objects to be followed by motion about this axis alone.

Latitude: The angular distance of a place north or south of the earth's equator, or of a celestial object north or south of the celestial equator, usually expressed in degrees and minutes.

Horizon: The line at which the earth's surface and the sky appear to meet.

Meridian: A circle of constant longitude passing through a given place on the earth's surface and the terrestrial poles.

Polar Alignment: The act of aligning the polar rotational axis of a telescope’s equatorial mount with the celestial pole. 

Polar Axis: The East/West axis of an equatorially mounted telescope which is at right angles to the North/South declination axis and parallel to the earth's axis of rotation. The polar axis is the axis which the telescope is turned to follow the apparent movement of celestial objects as they rise in the East and set in the West as a result from the earth's rotation.

Right Ascension: Right ascension (or RA), in astronomy, the east–west coordinate by which the position of a celestial body is ordinarily measured; more precisely, it is the angular distance of a body's hour circle east of the vernal equinox, measured along the celestial equator.