FIND THE POLAR AXIS OF ROTATION

Here are two methods to precisely find the polar axis of rotation of the Earth:

  1. With the help of software and a CCD camera
  2. The star hopping method

1- With the help of software and a CCD camera

For an investment of around $ 400 Canadian, it is possible to align the mount to the Celestial North Pole (PNC) with an accuracy of up to 30 arc seconds (30 ″ arc). She asks to use a German equatorial mount. We use the software Pole Master with the CCD camera dedicated to it. This precision will allow exposure times of 10 minutes or more per photo. Usually, polar alignment can be achieved in just 2-3 minutes after starting the software. This is the revolutionary method to align the mount on the axis of rotation of the Earth in record time!

Click on this lien to consult the user guide.

2- The star hopping method

The stars hopping method does not require any monetary investment. It is valid as much for German equatorial mounts as for azimuthal mounts fitted with a equatorial table. If necessary, to print an image, with the right mouse button, click on the image and then choose Print image.

As we can see, visually the polar axis of rotation or PNC points towards the pan of the Little Dipper. From the North Star, we can draw a straight line towards the star of the tip of the Ursa Minor pan (see the red line). Bring the telescope finder in this direction (using the mount's azimuth and latitude buttons), right next to the Pole Star.

Using the telescope finder, search for the 3 guide stars connected by the red triangle above. 

  • The image seen above is as seen in the finder (North-South and East-West inverted image). Rotate the image to match the current view.
  • Be careful with the image to the researcher being inverted, the Cabin Crew Member appears opposite the star of the Little Dipper.
  • Starting from the bright star near the North Star, connect the three bright stars from 1er triangle in blue. This first location will help to find the 3 guide stars of the red triangle.
  • Look for the 3 guide stars which are connected by the red triangle in the image above.
  • Still using the mount's azimuth and latitude buttons, position the finder's reticle (the X) towards the North Celestial Pole as shown in the image above.
  • Put on an eyepiece or install your CCD camera offering a field of view of at least 35 ′ of arc. To know the field of view of your eyepiece or CCD camera (or other digital cameras), click on this lien. The image shown above is with a 40mm eyepiece.
  • The image seen above is as seen through the eyepiece through the Edge HD 800 telescope through the elbow (East-West inverted image).
  • Rotate the image to match the current view. Find the 3 bright stars near the North Star (see red). Note: Be careful, the Pole Star revolves around the PNC in 24 hours so that its position varies continuously.
  • Inside the triangle formed by the 3 stars, find the star that is twice the distance from the axis of rotation. Center the axis of rotation (the imaginary red dot) at the center of the 40mm eyepiece view.

The polar axis of rotation for the years 2020 and 2021

Due to the movement of precession of the equinoxes, the axis of rotation of the Earth (or Celestial North Pole) slowly moves with the time of 50,29 ″ of arc per year. Currently, this displacement brings a rapprochement towards the North Star. Here is its position for the years 2020 and 2021:

The image seen above is as seen through the 40mm eyepiece through the Edge HD 800 telescope through the elbow (East-West inverted image). Rotate the image to match the current view.

Using the mount azimuth and latitude buttons, move the mount in that direction. For more precision, you can use a illuminated reticulated eyepiece. You will thus obtain an alignment precision very close to 1 ′ of the PNC arc and even a little less depending on your ability, which will allow you to perform exposure times of 10 minutes or more per photo.

If you are using a CCD camera to center the PNC, you can display a reticle in your favorite imaging software as in the following example:

Maxim DL software crosshair

With the CCD camera, use the following image as a reference:

The reference image seen above is as seen with a CCD camera through the Edge HD 800 telescope (without elbow). The image is reversed East-West and North-South. Rotate the image to match the current view. Please note that the field of vision varies depending on your equipment.


Richard Beauregard
Sky Astro - CCD

Revised 2021/11/05