[Robh (Rob)]

Double stars are obviously harder to split when they are closer together.
A separation of 10 arcseconds is fairly easy at 100 magnification but 4 arcseconds requires 150 magnification to be seen easily.
However, this assumes the stars are around similar visual magnitude. Where one star is much brighter than the other, the brighter star's glare can drown out the secondary. In the case of Antares, the red primary is of visual magnitude one but the blue-white secondary is of magnitude 5.5. The separation is only about 2.9 arcseconds, so the primary overwhelms its companion. Difficult to see even in larger telescopes as the primary's brightness is enhanced even more.

I find the magnitudes and separations for multiple stars useful but the position angles are not much use for just two stars (binary). The PA of the secondary is measured counter-clockwise from the primary and a line to celestial north. The problem is that in a high power view you don't know where the direction of the NCP is and constellations are continually on the move. Consequently, establishing your orientation is difficult except on lower powers where you can establish the position of a clearly defined nearby star. However, where there are more than two stars in a system, the separations, magnitudes, and position angles can all be used to try to establish which stars are which relative to each other.

Regards, Rob