An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

1) Optics: An image defect, one of a large number of bright and dark spots, that appears when an object is illuminated by monochromatic, highly → coherent light. This phenomenon results from the → interference of a number of randomly phased complex contributions of electromagnetic → wavefronts scattered from an object with rough structure, such as a piece of paper, a display screen, or a metallic surface. In particular, whenever the object is rough on the scale of an optical wavelength, the image has a grainy appearance. Also called speckle noise.
2) Astro.: The pattern produced by a short-exposure image of a → point source, such as a star, when the → wavefront is torn apart under the effect of the → atmospheric turbulence. Speckles change very rapidly with time as a function of the atmospheric turbulence. → speckle lifetime. Long exposure images of these changing speckle patterns result in a blurred image of the star, called a → seeing disk. → Fried parameter.

Speckle "a speck or small spot, as a natural dot of color on skin, plumage, or foliage," from M.E.speck (from O.E. specca "small spot, stain," of unknown origin; probably related to Du. speckel "speck, speckle") + -le a noun suffix having originally a diminutive meaning.

Pakâl, from pak "spot" (Lâri, Gerâši), pašy "mingled, confused" (Tâleši), probably related to pisé "dappled, variegated," pis, pisi "leprosy," neveštan "to write," pišé "profession," → professional astronomer; Mid.Pers. parš "speckled, spotted," pēsīdan "to color, adorn," pēsit "adorned;" O.Pers. pais- "to adorn, cut, engrave;" Av. paēs- "to paint, adorn," paēsa- "adornment;" cf. Skt. peś- "to adorn, hew out, decorate," piśáti "adorns; cuts;" Gk. poikilos "multicolored;" L. pingit "embroiders, paints;" O.C.S. pisati "to write;" O.H.G. fēh "multicolored;" Lith. piēšti "to draw, adorn;" PIE base *peik- "colored, speckled."

A technique for generating a clear composite image of a celestial object blurred by → atmospheric turbulence in which a large number of short-exposure photographs are mathematically correlated by a computer. By comparing the behavior of the → speckles in a series of images it is possible to approach the theoretical resolution of the telescope.

→ speckle; → interferometry.

The time scale on which a stellar image changes significantly due to → atmospheric turbulence. It is proportional to the ratio r₀/Δv, where r₀ is the → Fried parameter and Δv the standard deviation of the distribution of wind velocities weighted by the turbulence structure coefficient. Typical lifetimes in the visible range from about 3 to 30 milliseconds.

→ speckle; → life; → time.

Omr "life-time;" from Ar. 'umr; pakâl, → speckle.

An image defect associated with the → speckle phenomenon.

→ speckle; → noise.