Understanding Diffraction Spikes and their Impacts in Astrophotography

Stars appear to have spikes when we look at them, right? From a young age, we're accustomed to seeing them depicted with these sharp rays emerging from their bodies, and we even draw them that way!

But did you know that these spikes are actually optical artifacts caused by diffraction, and are most commonly associated with reflector telescopes? To the naked eye or through other types of telescopes, such as refractors, stars appear as points of light - no spikes!

Some astrophotographers embrace diffraction spikes as an artistic element that adds character to their images, while others see them as an unwanted distraction and strive to avoid them.

In this blog post, we’ll explore the causes of diffraction spikes, the factors that influence their appearance, and some tips for achieving sharp, well-defined spikes.

The Origin of Diffraction Spikes

Diffraction, the phenomenon of waves spreading out as they pass through an aperture or around obstacles, plays a crucial role in shaping the star images we observe through telescopes. Ideally, our view would be unobstructed, but sometimes the obstacle is within the telescope itself! This is the case with Newtonian telescopes, where the secondary mirror and its supporting vanes, known as spider vanes, are situated in the optical path.

Due to their placement, spider vanes create a diffraction effect known as diffraction spikes, and each vane produces a pair of spikes perpendicular to its orientation. The brighter the star, the more prominent these spikes appear, with the vanes' thickness influencing the spikes width and contrast.

In other cases, diffraction spikes can be produced by obstacles from the environment. For instance, while imaging, you might inadvertently shoot with a telephone cable or electricity transmission line that crossed the field of view. Any obstructions can diffract light, and can produce spike patterns!

It's also important to note that the shape of the telescope's aperture affects the diffraction pattern. In telescopes with a circular aperture, like all commercially available telescopes, light diffracts into an Airy disk, which presents as a round star. On the other hand, the James Webb Space Telescope (JWST) uses hexagonal mirrors along with three supports for its secondary mirror, both contributing to distinctive diffraction patterns in the star images.

While many of us have grown accustomed to seeing diffraction spikes in star images from childhood, opinions among astrophotographers are mixed. Some dislike them for the way they can detract from the image's clarity, while others love them so much that they recreate these effects with telescopes like refractors, which don't naturally produce them. This can be achieved by adding a 3D-printed spider or by stretching two strands of fishing line across the telescope's front. Alternatively, spikes can be added in some image-processing softwares.

Diffraction Spike Variations Across Telescope Designs

We previously noted that each spider vane generates a pair of diffraction spikes. However, in Newtonian reflectors, which typically employ four vanes, we observe only four spikes in the star images. This apparent discrepancy is due to the overlapping of spikes. Specifically, each horizontal vane in these telescopes produces a vertical spike both above and below the star. Consequently, the two spikes at the top overlap to form a single brighter spike, and the same occurs with the two spikes at the bottom. The same principle applies to the vertical vanes, which each produce a pair of horizontal spikes. Thus, despite each vane generating two spikes, the overlapping results in only four distinct spikes being visible.

In contrast to Newtonian reflectors with four spider vanes, those employing three vanes result in a different visual outcome. With three vanes, each vane still produces a pair of diffraction spikes, leading to six distinct spikes in total. The arrangement of the vanes in a triangular formation means there is no overlap similar to what occurs in four-vane configurations. Consequently, each of the six spikes produced by the three vanes remains visible and separate, creating a symmetrical six-spiked pattern around bright stars.

In the case of the James Webb Space Telescope (JWST), the diffraction spikes observed in its imagery result from two distinct structural features. Firstly, the primary mirror's hexagonal segments produce six prominent spikes. Additionally, the three support struts of the secondary mirror generate six smaller spikes. Due to some overlap in these configurations, images captured by JWST typically display eight diffraction spikes. Six of these are brighter and result from the overlapping effects of both the primary mirror's geometry and the secondary mirror supports. The remaining two are solely attributed to the secondary supports. This overlap and interaction between different structural elements create a unique visual signature in the telescope's observations.

Maximizing Diffraction Spikes' Quality

To achieve crisp and symmetric diffraction spikes, it is crucial that the spider vanes in your telescope are perfectly aligned and straight. Even minor imperfections—such as slight bends, twists, or misalignments—can significantly alter the appearance of the spikes. An example of this can be seen in the image below, taken with a Sky-Watcher Quattro 150P, where the vanes were slightly bent. Ensure your vanes are securely tightened to maintain proper alignment. Some telescope manufacturers use solid spiders made from a single piece of aluminum. This solid construction prevents the vanes from bending, enhancing the durability and stability of the secondary mirror. Such improvements are particularly beneficial for astrophotography, as they ensure that the diffraction spikes remain sharp and well-defined over time, allowing astrophotographers to focus on capturing good images without worrying about mechanical imperfections.

Furthermore, other elements within the optical path, such as mirror clips, can introduce unwanted artifacts or spike-like flares around bright stars. Many astrophotographers address this issue by employing an aperture mask. This device is a circular ring, slightly smaller than the telescope's mirror, designed to obscure any components that might disrupt the light path, such as clips.

Finally, during multi-night astrophotography sessions, it's crucial to maintain the orientation of your telescope in its tubes. Rotating the telescope between sessions will result in misalignment of diffraction spikes from one night to the next. This misalignment leads to images where stars appear with an excessive number of spikes, and this will ruin your images. Keeping the telescope fixed in the same position ensures that the diffraction spikes produced are uniform across different nights, preserving the quality of your captures.