Stars can trail in images for a variety of reasons listed below.

The Earth's Rotation

Stars will naturally trail in a long time exposure on a fixed tripod. Since the Earth is rotating constantly, objects in the sky seem to move and rotate around the north and south celestial poles. You will get more trailing near the celestial equator compared to near the poles.

Stars trail on a fixed tripod during long exposures due to the Earth's rotation. This image is a 60-second exposure at ISO 1600 at f/4 with a zoom lens at 25mm of focal length.

• Solution:
  • For fixed tripod shots, use a higher ISO with a shorter exposure, or a wider angle lens.

  • For longer exposures, or a long focal length camera lens, or a telescope, use an equatorial tracking mount that has been correctly polar aligned.

An equatorial tracking mount must be properly polar aligned. If it is not, you might get trailing in the north-south direction.

Stars will trail in a north-south direction due to inaccurate polar alignment.

• Solution: Properly polar align your mount so that the polar axis of the scope is parallel with the rotation axis of the Earth by pointing it at the north or south celestial pole.

Almost all mounts suffer from some tracking errors due to periodic error. Stars will trail in an east-west direction in an image from periodic error.

Trailing in the east-west direction is usually caused by periodic error in the mount's drive gears.

• Solution:
  • Shoot shorter exposures if possible (but not so short that the faint detail is lost in the camera noise).

  • Shoot lots of exposures and throw away the ones that are trailed, keeping the good ones.

  • Use a guidescope or off-axis guider to manually guide or use an autoguider.

  • If your mount offer's Periodic Error Correction (PEC), spend the time to carefully train the PEC and then use it.

Telescopes with mirrors, such as Newtonians, and in particular Schmidt-Cassegrains, can produce trailed stars in images because the mirror can move during the exposure. This becomes even more of a problem when a mirrored scope is used as a guidescope on top of another scope with a mirror in it. Then you have two mirrors that can move independently of where the mount is tracking. Mirror shift can often result in "double stars" where each star in the image is double. The scope tracked accurately for a while forming one star image, and then the mirror shifted and continued to track after the shift, forming another star image. It is also possible to get trailed stars from a mirror that shifts gradually over time.

Double Stars are a common problem resulting from mirror flop in a Schmidt-Cassegrain telescope.

• Solution:
  • Use an off-axis guider.

  • Lock the mirror down. If you can permanently lock the mirror down in an Schmidt-Cassegrain, you can switch to an external focuser.

  • Don't try to guide with a mirror scope on top of another mirror scope that is imaging. At least use a refractor as a guidescope, but you will still probably have problems with the mirror moving in the main scope.

Flexure is usually mechanical in nature and is caused when the optical axis of the telescope moves slightly while the mount is tracking. Flexure is usually more of a problem when a separate guidescope is used with an imaging scope, especially on a tandem-bar setup. With flexure, stars can trail in any direction.

Stars trail randomly due to flexure somewhere in the mechanical or optical system of the telescope.

• Solution:
  • Use an off-axis guider. Using the imaging scope's optical system for a guidestar eliminates the problem of flexure completely. Off-axis guiders can be more inconvenient to use however.

  • Get rid of a side-by-side tandem-bar setup and mount the guidescope directly on top of the imaging scope.

  • Beef up the rigidity of the entire mounting assembly by getting more substantial dovetail bars and rings.

Field rotation causes stars at the edges of the field to be trailed radially. It can be caused by inaccurate polar alignment and use of a guidescope. In this particular case, the stars at the edge of the field will be trailed around the guidestar.

The other main cause of field rotation is the use of a computerized altazimuth mount. Because the scope tracks the stars by moving the mount in both altazimuth and altitude, the field will rotate around the center of the frame. In this case, there is no polar alignment to correct, and no guidescope. The amount of field rotation in an altazimuth mount will be different in different parts of the sky for different exposure lengths.

Stars trail around the center of the frame from field rotation in an altazimuth mount.

• Solution:
  • For inaccurate polar alignment - correctly polar align your mount.

  • For an altazimuth mount -
    • Put it on an equatorial wedge.

    • Use a field de-rotator. These are expensive and rare.

    • Switch from an altazimuth mount to a German-equatorial mount.

It is possible to have combinations of these problems, which can lead to weirdly trailed stars. The most common is usually periodic error combined with flexure.

If you experience trailing, examine your star field and compare it to a star chart or planetarium program to determine the direction of trailing. North-south trailing usually indicates incorrect polar alignment. East-west trailing usually indicates periodic error. Note that these directions in the frame will be determined by the orientation of the camera.

When you get more advanced, you will reach the point where you realize that every frame lost to poor tracking is precious clear dark-sky time that was wasted, and that it is a shame to throw away those photons that came from so far away. If you are really committed to doing the absolute best that you can in astrophotography, this is when you will want to start guiding so that your yield goes up to a much higher percentage.

With some mounts, both expensive and inexpensive, even guiding won't solve problems with erratic errors. This is when it is time to get a better mount.

Problem: Trailing - The Bottom Line Stars that are trailed in an image usually indicate a problem with either polar alignment, flexure, or tracking accuracy. Other things can also caused trailed images, but they are usually the result of mechanical problems or operator error.