A double's separation is the apparent distance of the companion, or secondary star, from the brighter star, the primary. Albireo (Beta Cygni), a beautiful double with colors described as gold and sapphire, has a separation of 34 arcseconds.
This is twice as wide as
Saturn's apparent disk size, so it is an easy object in a 3 inch
(75 mm) telescope. Separations of less than about 5 arcseconds can be challenging in telescopes smaller than 6 inches
(150 mm), especially when the seeing conditions are poor, the stars are ot unequal brightness, or the stars are faint.
The separation of double stars can be measured using a filar micrometer, a complex device that attaches to the telescope's eyepiece.