Diffraction spikes are lines radiating from bright light sources, causing what is known as thestarburst effect[1] orsunstars[2] in photographs and in vision. They areartifacts caused by lightdiffracting around the support vanes of the secondary mirror inreflecting telescopes, or edges of non-circular cameraapertures, 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 acharge-coupled device (CCD) image sensor.
In the vast majority ofreflecting telescope 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 theydiffract the incoming light from a subject star and this appears as diffraction spikes which are theFourier transform of the support struts. The spikes represent a loss of light that could have been used to image the star.[3][4]
Although diffraction spikes can obscure parts of a photograph and are undesired in professional contexts, someamateur astronomers like the visual effect they give to bright stars – the "Star of Bethlehem" appearance – and even modify their refractors to exhibit the same effect,[5] or to assist with focusing when using aCCD.[6]
A small number of reflecting telescopes designs avoid diffraction spikes by placing the secondary mirror off-axis. Early off-axis designs such as theHerschelian and theSchiefspiegler telescopes have serious limitations such asastigmatism and long focal ratios, which make them useless for research. The brachymedial design byLudwig Schupmann, which uses a combination of mirrors and lenses, is able to correctchromatic aberration perfectly over a small area and designs based on the Schupmann brachymedial are currently used for research ofdouble stars.
There are also a small number of off-axis unobstructed all-reflectinganastigmats which give optically perfect images.
Refracting telescopes and their photographic images do not have the same problem as their lenses are not supported with spider vanes.
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 theaperture circular for a pleasingbokeh, when stopped down to highf-numbers (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.[7] 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 withn blades yieldsn spikes ifn is even, and 2n spikes ifn is odd.[8]
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Images fromtelescopes withsegmented 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 theJames Webb Space Telescope.[9]
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An improperly cleaned lens or cover glass, or one with a fingerprint may have parallel lines which diffract light similarly to support vanes.[10] They can be distinguished fromspikes due to non-circular aperture as they form a prominent smear in a single direction, and fromCCD bloom by their oblique angle.
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In normal vision, diffraction through eyelashes – and due to the edges of the eyelids if one is squinting – produce 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 anentoptic phenomenon.
Diffraction spike in normal human vision can also be caused by some fibers in the eye lens sometimes calledsuture lines.[11]
Across screen filter, also known as a star filter, creates a star pattern using a very finediffraction grating 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.[7]
In amateurastrophotography, 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 theprimary mirror 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.
