Diffraction spikes are lines radiating from bright light sources, causing what is known as the starburst effect or sunstars in photographs and in vision. They are artifacts caused by light diffracting around the support vanes of the secondary mirror in

reflecting telescope A reflecting telescope (also called a reflector) is a telescope that uses a single or a combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century by Isaac Newton as an alternati ...

s, or edges of non-circular camera

aperture In optics, the aperture of an optical system (including a system consisting of a single lens) is the hole or opening that primarily limits light propagated through the system. More specifically, the entrance pupil as the front side image o ...

s, and around eyelashes and eyelids in the eye. While similar in appearance, this is a different effect to "vertical smear" or "blooming" that appears when bright light sources are captured by a charge-coupled device (CCD) image sensor.

Causes

Support vanes

In the vast majority of

designs, the secondary mirror has to be positioned at the central axis of the telescope and so has to be held by struts within the telescope tube. No matter how fine these support rods are they diffract the incoming light from a subject star and this appears as diffraction spikes which are the

Fourier transform In mathematics, the Fourier transform (FT) is an integral transform that takes a function as input then outputs another function that describes the extent to which various frequencies are present in the original function. The output of the tr ...

of the support struts. The spikes represent a loss of light that could have been used to image the star. Although diffraction spikes can obscure parts of a photograph and are undesired in professional contexts, some amateur astronomers like the visual effect they give to bright starsthe "

Star of Bethlehem The Star of Bethlehem, or Christmas Star, appears in the nativity of Jesus, nativity story of the Gospel of Matthew Matthew 2, chapter 2 where "wise men from the East" (biblical Magi, Magi) are inspired by the star to travel to Jerusalem. There, ...

" appearanceand even modify their refractors to exhibit the same effect, or to assist with focusing when using a CCD. A small number of reflecting telescopes designs avoid diffraction spikes by placing the secondary mirror off-axis. Early off-axis designs such as the Herschelian and the Schiefspiegler telescopes have serious limitations such as

astigmatism Astigmatism is a type of refractive error due to rotational asymmetry in the eye's refractive power. The lens and cornea of an eye without astigmatism are nearly spherical, with only a single radius of curvature, and any refractive errors ...

and long focal ratios, which make them useless for research. The brachymedial design by Ludwig Schupmann, which uses a combination of mirrors and lenses, is able to correct

chromatic aberration In optics, chromatic aberration (CA), also called chromatic distortion, color aberration, color fringing, or purple fringing, is a failure of a lens to focus all colors to the same point. It is caused by dispersion: the refractive index of the ...

perfectly over a small area and designs based on the Schupmann brachymedial are currently used for research of

double star In observational astronomy, a double star or visual double is a pair of stars that appear close to each other as viewed from Earth, especially with the aid of optical telescopes. This occurs because the pair either forms a binary star (i.e. a ...

s. There are also a small number of off-axis unobstructed all-reflecting anastigmats which give optically perfect images.

Refracting telescope A refracting telescope (also called a refractor) is a type of optical telescope that uses a lens (optics), lens as its objective (optics), objective to form an image (also referred to a dioptrics, dioptric telescope). The refracting telescope d ...

s and their photographic images do not have the same problem as their lenses are not supported with spider vanes.

Non-circular aperture

Iris diaphragms with moving blades are used in most modern camera lenses to restrict the light received by the film or sensor. While manufacturers attempt to make the

circular for a pleasing

bokeh In photography, bokeh ( or ; ) is the aesthetic quality of the blur produced in out-of-focus parts of an image, whether foreground or background or both. It is created by using a wide aperture lens. Some photographers incorrectly restr ...

, when stopped down to high

f-number An f-number is a measure of the light-gathering ability of an optical system such as a camera lens. It is calculated by dividing the system's focal length by the diameter of the entrance pupil ("clear aperture").Smith, Warren ''Modern Optical ...

s (small apertures), its shape tends towards a polygon with the same number of sides as blades. Diffraction spreads out light waves passing through the aperture perpendicular to the roughly-straight edge, each edge yielding two spikes 180° apart. As the blades are uniformly distributed around the circle, on a diaphragm with an even number of blades, the diffraction spikes from blades on opposite sides overlap. Consequently, a diaphragm with ''n'' blades yields ''n'' spikes if ''n'' is even, and 2''n'' spikes if ''n'' is odd. File:Path near floating restaurant, with moon, Infosys Mysore.JPG, 5 blades giving 10 spikes File:Night London Panorama with Full Moon.jpg, 6 blades giving 6 spikes File:US Navy 061220-N-4965F-001 The Pearl Harbor Memorial fountain illuminates the night as holiday lights shine from the Ticonderoga-class guided-missile cruiser USS Lake Erie (CG 70).jpg, 7 blades giving 14 spikes File:Sydneyoperahouse at night.jpg, 8 blades giving 8 spikes File:US Army 52253 Best Warrior At Night.jpg, 9 blades giving 18 spikes File:Speyer - Altstadt - Gedächtniskirche der Protestation - Gewölbe der Vierung quer.jpg, 10 blades giving 10 spikes File:Squarelens.jpg, 4 blades giving 4 spikes

Segmented mirrors

Images from

telescope A telescope is a device used to observe distant objects by their emission, Absorption (electromagnetic radiation), absorption, or Reflection (physics), reflection of electromagnetic radiation. Originally, it was an optical instrument using len ...

s with segmented mirrors also exhibit diffraction spikes due to diffraction from the mirrors' edges. As before, two spikes are perpendicular to each edge orientation, resulting in six spikes (plus two fainter ones due to the spider supporting the secondary mirror) in photographs taken by the

James Webb Space Telescope The James Webb Space Telescope (JWST) is a space telescope designed to conduct infrared astronomy. As the largest telescope in space, it is equipped with high-resolution and high-sensitivity instruments, allowing it to view objects too old, Lis ...

. Webb's_First_Deep_Field.jpg, The first JWST deep field with diffraction spikes The exotic stellar population of Westerlund 1 (potm2409a).jpg, JWST image of star cluster Westerlund 1 with diffraction spikes A Wreath of Star Formation in NGC 7469 (potm2212a).jpeg, JWST image of the spiral galaxy NGC 7469 with diffraction spikes JWST_diffraction_spikes.svg, Edges of the JWST primary mirror segments and spider colour-coded with their corresponding diffraction spikes

Dirty optics

An improperly cleaned lens or cover glass, or one with a fingerprint may have parallel lines which diffract light similarly to support vanes. They can be distinguished from spikes due to non-circular aperture as they form a prominent smear in a single direction, and from CCD bloom by their oblique angle.

In vision

In normal vision, diffraction through eyelashesand due to the edges of the eyelids if one is squintingproduce many diffraction spikes. If it is windy, then the motion of the eyelashes cause spikes that move around and scintillate. After a blink, the eyelashes may come back in a different position and cause the diffraction spikes to jump around. This is classified as an

entoptic phenomenon Entoptic phenomena (), occasionally and incorrectly referred to as entopic phenomena, are visual effects whose source is within the human eye itself. In Helmholtz's words: "Under suitable conditions, light falling on the eye may render visible, ...

. Diffraction spike in normal human vision can also be caused by some fibers in the eye lens sometimes called suture lines.

Other uses

Special effects

20240920 53rd Street Hyde Park Chicago near equinox stacked rotated and recropped 4x3

A cross screen filter, also known as a star filter, creates a star pattern using a very fine

diffraction grating In optics, a diffraction grating is an optical grating with a periodic structure that diffraction, diffracts light, or another type of electromagnetic radiation, into several beams traveling in different directions (i.e., different diffractio ...

embedded in the filter, or sometimes by the use of prisms in the filter. The number of stars varies by the construction of the filter, as does the number of points each star has. A similar effect is achieved by photographing bright lights through a window screen with vertical and horizontal wires. The angles of the bars of the cross depend on the orientation of the screen relative to the camera.

Bahtinov mask

In amateur

astrophotography Astrophotography, also known as astronomical imaging, is the photography or imaging of astronomical objects, celestial events, or areas of the night sky. The first photograph of an astronomical object (the Moon) was taken in 1839, but it was no ...

, a Bahtinov mask can be used to focus small astronomical telescopes accurately. Light from a bright point such as an isolated bright star reaching different quadrants of the

primary mirror A primary mirror (or primary) is the principal light-gathering surface (the objective) of a reflecting telescope. Description The primary mirror of a reflecting telescope is a spherical, parabolic, or hyperbolic shaped disks of polished ...

or lens is first passed through grilles at three different orientations. Half of the mask generates a narrow "X" shape from four diffraction spikes (blue and green in the illustration); the other half generates a straight line from two spikes (red). Changing the focus causes the shapes to move with respect to each other. When the line passes exactly through the middle of the "X", the telescope is in focus and the mask can be removed.

References

External links

Diffraction spikes explained
by

Astronomy Picture of the Day Astronomy is a natural science that studies celestial objects and the phenomena that occur in the cosmos. It uses mathematics, physics, and chemistry in order to explain their origin and their overall evolution. Objects of interest include ...

. * * {{cite web, last=Kratzke, first=Bastian, title=Best lenses for Sunstars, url=https://phillipreeve.net/blog/best-lenses-for-sunstars/, work=phillipreeve.net, date=15 July 2020 Astrophotography Science of photography Diffraction