Diffraction spikes are lines radiating from bright light sources in reflecting telescope

Reflecting telescope

A reflecting telescope is an optical telescope which uses a single or combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century as an alternative to the refracting telescope which, at that time, was a design that suffered from...

 images. They are artifacts

Artifact (error)

In natural science and signal processing, an artifact is any error in the perception or representation of any visual or aural information introduced by the involved equipment or technique....

 caused by light diffracting

Diffraction

Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...

 around the support vanes of the secondary mirror. Refracting telescope

Refracting telescope

A refracting or refractor telescope is a type of optical telescope that uses a lens as its objective to form an image . The refracting telescope design was originally used in spy glasses and astronomical telescopes but is also used for long focus camera lenses...

s and their photographic images do not have the same problem.

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The secondary mirror in a reflecting telescope has to be positioned at the central axis of the telescope and so has to be held by struts within the telescopes tube. No matter how fine these support rods are they diffract

Diffraction

Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...

 the incoming light from a subject star and this appears as diffraction spikes which are the Fourier transform

Fourier transform

In mathematics, Fourier analysis is a subject area which grew from the study of Fourier series. The subject began with the study of the way general functions may be represented by sums of simpler trigonometric functions...

 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

Amateur astronomy

Amateur astronomy, also called backyard astronomy and stargazing, is a hobby whose participants enjoy watching the night sky , and the plethora of objects found in it, mainly with portable telescopes and binoculars...

 like the visual effect they give to bright stars - the "Star of Bethlehem

Star of Bethlehem

In Christian tradition, the Star of Bethlehem, also called the Christmas Star, revealed the birth of Jesus to the magi, or "wise men", and later led them to Bethlehem. The star appears in the nativity story of the Gospel of Matthew, where magi "from the east" are inspired by the star to travel to...

" appearance - and even modify their refractors to exhibit the same effect, or to assist with focusing when using a CCD

Charge-coupled device

A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...

.

External links

• Diffraction spikes explained by Astronomy Picture of the Day
  Astronomy Picture of the Day
  Astronomy Picture of the Day is a website provided by NASA and Michigan Technological University . According to the website, "Each day a different image or photograph of our universe is featured, along with a brief explanation written by a professional astronomer."The photograph is not necessarily...
  
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