All angular positions are measured in arcseconds (″): object coordinates $(c_x, c_y)$, deflection angles, and size parameters all use this unit. The image panel shows a square patch of sky of side field of view fov (default 4″), centred on the optical axis. A scale bar at the bottom right of the image shows a round angular length (1″ for fov ≤ 6″, 2″ up to 10″, 5″ up to 30″, 10″ beyond), togglable in the View tab and included in captures. Radians appear only in intermediate formulae and are converted back to arcseconds throughout.
To read an angular separation directly off the image, enable the ruler (it is off by default) with the Ruler toggle in the View tab, or press L, which enables and arms it in one step; once enabled, the button at the bottom-left of the image panel (just above the colorbar, when one is shown; or L) toggles arming. With the tool armed, click and drag between any two points: the line’s length is reported in arcseconds and its position angle, measured counter-clockwise from the positive $x$-axis (rightward) with $y$ increasing upward, as distance″ · angle°.
Endpoints are stored in sky coordinates, so a measurement stays anchored to the same points (and its readout stays constant) as you change the field of view. Measurements are additive: each drag adds another line, and the newest one becomes selected. A committed measurement is a first-class object: click it to select it, drag its line to move it, or drag either endpoint to reshape it. Only one thing (a measurement or an object) is selected at a time. Delete the selected measurement with the trash button next to the ruler or with Backspace; the × button clears all of them. Measurements persist when you toggle the ruler off (Esc first deselects the current measurement, then disarms the tool). The ruler lines are drawn on the same overlay as the markers and critical curves, so they appear in saved PNGs and recordings; the ruler buttons, being ordinary UI chrome, are excluded from both.