Far-eye

A far-eye is an instrument used in navigation, consisting of glass lenses arranged inside a metal cylinder in such a way that far-away objects are visually magnified when looking through it.

Season 8
When Arya Stark sails west of Westeros to explore the Sunset Sea, she is seen picking up a far-eye from the table in her cabin.

In the books
"Far-eye" is the term used in the A Song of Ice and Fire novels for a telescope - the word "telescope" never appears in the text.

The best far-eyes are made in the Free City of Myr, by its master lens-crafts, and these telescopes are called Myrish eyes.

Maesters often possess far-eyes, to study astronomy - both Maester Luwin and Maester Aemon possessed their own. Jon Snow used Aemon's far-eyes to spy on the wildling camp from the top of the Wall in the leadup to the Battle of Castle Black.