A flaw that turns a defocused star into an ellipse or a cross whose orientation swings by 90° from one side of focus to the other. It comes from pinched or badly figured optics, poor collimation, or the eye itself. If it rotates when you turn your head, it is your eye.
Related terms
- Star test
You point at a moderately bright star at high power and defocus it slightly on each side of focus. With good, well collimated optics you see identical concentric rings on both sides. It is the final judge of collimation and optical quality.
- Collimation
The adjustment that lines up a telescope’s mirrors on a single axis. A Newtonian drifts a little each time it is moved and should be checked before every session, especially if it is fast (f/5 or below). A poorly collimated telescope gives soft images, even with excellent optics.
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