Fr.: seconde d'arc
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.
mili sâniye-ye kamâni
Fr.: milliseconde d'arc
A unit of angle equal to one thousandth of an → arcsecond, or 1/3 600 000 degree.
A measure of angle smaller than 1 arcsecond, usually until 0.1 arcsecond.
Imaging in excellent seeing conditions using an adequate detector to obtain stellar images whose profile lies in the sub-arcsecond range.