Eye examiner

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As eyes auditors in to astronomy two star designated that are close together and the sky with normal vision can be seen eyes as separate objects.

Mizar and Alkor of the Big Dipper in the constellation Big Bear are eye checkers

With daylight and healthy eyes, the limit of the resolution , the angular visual acuity , is around one angular minute (1 '- corresponding to a visual acuity = 1). The individual differences are in the range of about 0.5 'to 2.0' or between 30 "and 120" ( arc seconds ). These values ​​apply to the point of sharpest vision, the foveola , where the receptors are particularly close. But these are exclusively the less light-sensitive cone cells that also allow color vision. Very low luminance levels can only be perceived with the rod cells, for example at night and eyes adapted to dark, with a lower resolution.

Mizar (A and B; bottom left) and Alkor (top right), as well as Sidus Ludoviciana

When examining the eyes with regard to angular visual acuity through tests to determine whether stars are seen as one or two, not only their angular distance is decisive, but also their respective apparent brightness . With some stars it is sufficient for a (foveal) color perception, with others not. Therefore the resolvable angular distance can be a factor of 2 to 5 less favorable than the individual visual acuity would suggest. In addition, the respective viewing conditions must be taken into account.

The two stars Mizar (ζ UMa) and Alkor (g UMa) in the constellation Great Bear (Ursa Maior) are considered to be the most famous eye checkers . They can be found in the star course of the Big Dipper , Mizar with an apparent brightness of 2.0 m as the bend of the drawbar, Alkor with 4.0 m as the "little rider" above. These stars have an angle of separation of 705 ″, just under 12 ′ or 0.19 °, so that in good visibility conditions they can be seen separately by most people. With technical aids, not free-eyed, they are double stars that can be separated into further components (Mizar A and B; Alkor A and B). It is not finally clear whether these are gravitationally bound and form a multiple star system.

A test star for "eagle eyes", however, is Epsilon Lyrae in the constellation Lyra (Lyra), as a double star with sharp eyes that can occasionally be separated. The two components ε¹ and ε² have a brightness of 4.7 m and 4.6 m , respectively , their angular distance is 207 ″, a good 4 ′ or 0.06 °. Telescopically, the two are in turn separable, so that a four- fold star is present.

Theta Tauri is easier to separate , a relatively bright 3rd magnitude double star in the constellation Taurus (Taurus) with an angular separation of 337 ", just under 6 'or 0.09 °. About half of the observers see it separately. The two components θ¹ and θ² are 3.8 m and 3.4 m bright.

Other pairs of stars that are suitable as "eye checkers" are Algiedi , My Scorpii , Sigma Tauri and Delta Gruis .


Individual evidence

  1. ^ Oswald Thomas : Astronomy - Facts and Problems , p. 18ff, p. 415ff. Bergland-Buch, Graz-Vienna-Leipzig-Berlin 1934.
  2. protocols of measurement exercises , H.Streble, Univ.Stuttgart 1980th
  3. Observation exercises for astronomy, TU Wien 2010–12.