An Etymological Dictionary of Astronomy and Astrophysics
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An angular measurement equal to 1/3600 of a degree or 1/60 of an arcminute. It is denoted by the symbol ''. There are 206,264.5'' in a radian, so that 1'' = 4.848 × 10^-6 radians.

→ arc; → second.

A unit of angle equal to one thousandth of an → arcsecond, or 1/3 600 000 degree.

→ milli-; → arcsecond.

A measure of angle smaller than 1 arcsecond, usually until 0.1 arcsecond.

→ sub-; → arc second.

Imaging in excellent seeing conditions using an adequate detector to obtain stellar images whose profile lies in the sub-arcsecond range.

→ sub-arcsecond; → imaging.