Angular resolution

This tells you the smallest object that you would be able to see through the telescope. E$.$g$.$ whether you can distinguish two stars that are close together or whether they appear to be only one star.

E$.$g$.$ two lines on a wall $2$ mm apart can be distinguished by the naked eye from $2$ m away.

$tanA=\frac{2}{2000}$

Which gives angle $A$ as $3\mathrm{.}5$ minutes of arc. $(60$ minutes of arc in every degree$)$

The easiest way to calculate the resolution of the telescope is to look at the Moon and estimate the smallest thing you can see $-$ e$.$g$.$ the sea of Crises $-$ estimate its size in comparison to the size of the Moon $-$ e$.$g$.\frac{1}{{10}^{th}}$ the diameter of the Moon. You know that the Moon is $30$ minutes of arc across so you can estimate the angular resolution of the telescope. You could also test it on stars $-$ Mizar and Alcor in the Big Dipper: Mizar and Alcor are approx $4$ minutes of Arc apart. Betelgeuse in Orion has a companion star only $1$ minute of arc away $-$ can you see it$?$