What Is Field Rotation

What is field rotation? How does it affect my scope’s viewing and imaging?

The human fascination for celestial objects led to the discovery of astronomical observation aids. Many concepts in astronomy make the endeavour fruitful.

Field rotation is one of the essential abstractions which requires more familiarity. The awareness will help you fare better as a beginner in astronomy.

In this section, we dive deeper into what field rotation means for celestial observation. We will also give you bonus insights into how you can overcome the same.

What is field rotation?

Field rotation is an inconsistency for long-exposure imaging. When you use an Alt-Az mount to point at a celestial object for Astro imaging, you might often come across field rotation.

Field rotation is also commonly called frame rotation. This is because the image of the celestial object seems to rotate after it undergoes field rotation.

You should not confuse field rotation with what is experienced while imaging the Polaris for long exposures. Field rotation is not selective. It will affect any and every object in the sky.

The image rotates around the centre, which blurs the details of stretched objects. This is seen in an Astro-image of the nebulae.

As mentioned earlier, field rotation is quite common in Alt-Az scopes. But it can occur in improperly aligned EQ mounts as well.

How does field rotation affect my scopes’ viewing and imaging?

The typical direction of the south, north, east, and west changes in the image due to this disparity. If your goal is simply observing the celestial objects, field rotation can be ignored.

However, for Astro imaging or photography, you need to find a method of overcoming it. Even with precise tracking skills, if the mount alignment is not perfect, there will be a minimal field of rotation.

In such an image, the centre is sharp and clear, showing all the details of the objects you captured. However, moving towards the edge, arc- concentric patterns can be seen.

Field rotation makes it is impossible to use an Alt-Az mount for Astro-imaging. You can, however, use an EQ mount (polar aligned) for eliminating this disparity.

How can I avoid the field of rotation?

Field rotation is an undesirable effect caused during long-exposure photography. But the field rotation is not persistent. You can overcome it using the methods mentioned down below.

If you wish to stick by your Alt-Az telescope, the situation is not as hopeless. You need to use an EQ wedge.

The wedge is a perfect solution for field rotation. This converts your Alt-Az mount is transformed into an EQ mount.

Another robust solution for field rotation is using a de-rotator. You need to place this device in between the camera and the telescope.

If the field rotation causes the image to rotate along the centre, the de-rotator reverses that rotation.

Can I further remove the field of rotation in my Astro images?

For short exposure photographs, a few images editing software can remove the field of rotation. The program de-rotates the series of images.

What causes a field of rotation?

The Alt-Az telescope has a localized alignment. They are present at the mounted camera level, unable to follow an object’s path in the sky.

This can be better understood with our own eyes. When we look up at the sky, our eyes remain parallel to the ground.

We do not experience a field of rotation with our vision. This is because the image is held only for a short duration.

For longer duration astrophotography, there is a significant accumulation of light energy. This leads to the image to undergo rotation starting from the centre to the edges.

Field of rotation in an Alt-Az mount

When you have a camera attached to your Alt-Az telescope, the field of view is remarkably broad. With perfect alignment, the object remains centred for long.

But as long exposure imaging progress, there is a slight rotation. This is the essence of the field of rotation in an Alt-Az mount.

The subjected celestial object remains perfectly centred. However, the stars around the objects show up as arc in the image.

Why does an EQ mount not have a field of rotation?

With accurate polar alignment, and EQ telescope does not undergo the field of rotation. This argument applies to conventional EQ mounts as well as the advanced versions.

This ability to overcome the field of rotation in an EQ mount is simple to understand. While tracking objects in the sky, the telescope is never horizontal.

The tube of the EQ mounted scope rotates from top to bottom.

Recommended Reading:


  1. themcdonalds.net
  2. celestron.com
  3. rasc.ca