Star Trails and Moiré Patterns in Night-Sky Photography

January 30, 2020

Star Trails On Desert

Photographer: Nima Asadzadeh 
Summary Author: Nima Asadzadeh 

The star trails shown above were viewed over the Maranjab Desert in Iran. You may be able to notice that some of the star trails, particularly at the center of the image, form rather unusual symmetrical patterns. These patterns result from an optical phenomenon that’s called the moiré effect. Whenever we observe repetitive shapes, the light rays that reach our eyes are interfered with in such a way to form rather symmetrical patterns. This seems to be the case no matter the detector, human eyes, CCD cameras, etc. To avoid seeing these shapes, better resolution is needed to separate out all the tiny sections of the whole image so other paths are available for the light rays to reach our eyes without interference. For instance, looking through the holes of mesh fencing from up close rather than from a dozen feet away.

In this image, the interval between consecutive shots is about 1 second more than the appropriate shutter speed for an 18 mm focal length. This extra second causes a gap between each of the shots required to make the image and the repetitive trails of the stars, causing the moiré patterns to appear. A zoom-viewed image is used to compare with the full-size image. Note that the brightest star trail is that of the brightest star in the sky, Sirius. Image taken on December 14, 2019.