1. Coordinate system

Positions $(c_x, c_y)$, deflections and sizes are in arcseconds (″). Position angles $\varphi$ are in radians, counter-clockwise from $+x$, and the ruler reports angles in degrees. The image panel shows a square patch of sky FOV arcsec across (default 4″, set under View → Field), centred on $(0, 0)$, with $x$ increasing to the right and $y$ upward.

The scale bar at the bottom right shows 1″ when the FOV is at most 6″, 2″ up to 10″, 5″ up to 30″ and 10″ beyond. Turn it on or off under View → Display → Scale bar. It is included in saved images.

Measuring with the ruler

The ruler measures the distance and direction between two points on the image. Turn it on or off with its button at the bottom of the tool column on the left, or press L. On phones, first turn on View → Display → Rulers; the ruler button then appears at the bottom left of the image. While it is on, drag between two points. The line is labelled distance″ · angle°, with the angle measured counter-clockwise from the $+x$ direction, from 0° to 360°.

• Each drag adds a measurement. Click one to select it, drag its line to move it, or drag an end to reshape it. Selecting a measurement deselects any object, and the reverse.
• The ends are fixed on the sky, so a measurement keeps its place and length when you change the FOV.
• Delete the selected measurement with a trash button (in the tool column or on the image) or Backspace. The × on the image deletes all of them. Esc first deselects, then turns the ruler off.
• View → Display → Rulers shows or hides the measurements. Turning the ruler on also turns this on, and turning it off also turns the ruler off.
• Measurements stay when you turn the ruler off. They appear in saved images and recordings; the ruler’s buttons do not.
• Measurements are not saved in configs, and loading a config or preset removes them.